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llvm / llvm-project / lld / c43867b028f9011cb66eb07ed10a81625650806b / . / ELF / SyntheticSections.cpp
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//===- SyntheticSections.cpp ----------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file contains linker-synthesized sections. Currently,
// synthetic sections are created either output sections or input sections,
// but we are rewriting code so that all synthetic sections are created as
// input sections.
//
//===----------------------------------------------------------------------===//
#include "SyntheticSections.h"
#include "Config.h"
#include "InputFiles.h"
#include "LinkerScript.h"
#include "OutputSections.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "Target.h"
#include "Writer.h"
#include "lld/Common/DWARF.h"
#include "lld/Common/ErrorHandler.h"
#include "lld/Common/Memory.h"
#include "lld/Common/Strings.h"
#include "lld/Common/Version.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/BinaryFormat/Dwarf.h"
#include "llvm/DebugInfo/DWARF/DWARFDebugPubTable.h"
#include "llvm/Object/ELFObjectFile.h"
#include "llvm/Support/Compression.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/LEB128.h"
#include "llvm/Support/MD5.h"
#include "llvm/Support/Parallel.h"
#include "llvm/Support/TimeProfiler.h"
#include <cstdlib>
#include <thread>
using namespace llvm;
using namespace llvm::dwarf;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace llvm::support;
using namespace lld;
using namespace lld::elf;
using llvm::support::endian::read32le;
using llvm::support::endian::write32le;
using llvm::support::endian::write64le;
constexpr size_t MergeNoTailSection::numShards;
static uint64_t readUint(uint8_t *buf) {
return config->is64 ? read64(buf) : read32(buf);
}
static void writeUint(uint8_t *buf, uint64_t val) {
if (config->is64)
write64(buf, val);
else
write32(buf, val);
}
// Returns an LLD version string.
static ArrayRef<uint8_t> getVersion() {
// Check LLD_VERSION first for ease of testing.
// You can get consistent output by using the environment variable.
// This is only for testing.
StringRef s = getenv("LLD_VERSION");
if (s.empty())
s = saver.save(Twine("Linker: ") + getLLDVersion());
// +1 to include the terminating '\0'.
return {(const uint8_t *)s.data(), s.size() + 1};
}
// Creates a .comment section containing LLD version info.
// With this feature, you can identify LLD-generated binaries easily
// by "readelf --string-dump .comment <file>".
// The returned object is a mergeable string section.
MergeInputSection *elf::createCommentSection() {
return make<MergeInputSection>(SHF_MERGE | SHF_STRINGS, SHT_PROGBITS, 1,
getVersion(), ".comment");
}
// .MIPS.abiflags section.
template <class ELFT>
MipsAbiFlagsSection<ELFT>::MipsAbiFlagsSection(Elf_Mips_ABIFlags flags)
: SyntheticSection(SHF_ALLOC, SHT_MIPS_ABIFLAGS, 8, ".MIPS.abiflags"),
flags(flags) {
this->entsize = sizeof(Elf_Mips_ABIFlags);
}
template <class ELFT> void MipsAbiFlagsSection<ELFT>::writeTo(uint8_t *buf) {
memcpy(buf, &flags, sizeof(flags));
}
template <class ELFT>
MipsAbiFlagsSection<ELFT> *MipsAbiFlagsSection<ELFT>::create() {
Elf_Mips_ABIFlags flags = {};
bool create = false;
for (InputSectionBase *sec : inputSections) {
if (sec->type != SHT_MIPS_ABIFLAGS)
continue;
sec->markDead();
create = true;
std::string filename = toString(sec->file);
const size_t size = sec->data().size();
// Older version of BFD (such as the default FreeBSD linker) concatenate
// .MIPS.abiflags instead of merging. To allow for this case (or potential
// zero padding) we ignore everything after the first Elf_Mips_ABIFlags
if (size < sizeof(Elf_Mips_ABIFlags)) {
error(filename + ": invalid size of .MIPS.abiflags section: got " +
Twine(size) + " instead of " + Twine(sizeof(Elf_Mips_ABIFlags)));
return nullptr;
}
auto *s = reinterpret_cast<const Elf_Mips_ABIFlags *>(sec->data().data());
if (s->version != 0) {
error(filename + ": unexpected .MIPS.abiflags version " +
Twine(s->version));
return nullptr;
}
// LLD checks ISA compatibility in calcMipsEFlags(). Here we just
// select the highest number of ISA/Rev/Ext.
flags.isa_level = std::max(flags.isa_level, s->isa_level);
flags.isa_rev = std::max(flags.isa_rev, s->isa_rev);
flags.isa_ext = std::max(flags.isa_ext, s->isa_ext);
flags.gpr_size = std::max(flags.gpr_size, s->gpr_size);
flags.cpr1_size = std::max(flags.cpr1_size, s->cpr1_size);
flags.cpr2_size = std::max(flags.cpr2_size, s->cpr2_size);
flags.ases |= s->ases;
flags.flags1 |= s->flags1;
flags.flags2 |= s->flags2;
flags.fp_abi = elf::getMipsFpAbiFlag(flags.fp_abi, s->fp_abi, filename);
};
if (create)
return make<MipsAbiFlagsSection<ELFT>>(flags);
return nullptr;
}
// .MIPS.options section.
template <class ELFT>
MipsOptionsSection<ELFT>::MipsOptionsSection(Elf_Mips_RegInfo reginfo)
: SyntheticSection(SHF_ALLOC, SHT_MIPS_OPTIONS, 8, ".MIPS.options"),
reginfo(reginfo) {
this->entsize = sizeof(Elf_Mips_Options) + sizeof(Elf_Mips_RegInfo);
}
template <class ELFT> void MipsOptionsSection<ELFT>::writeTo(uint8_t *buf) {
auto *options = reinterpret_cast<Elf_Mips_Options *>(buf);
options->kind = ODK_REGINFO;
options->size = getSize();
if (!config->relocatable)
reginfo.ri_gp_value = in.mipsGot->getGp();
memcpy(buf + sizeof(Elf_Mips_Options), &reginfo, sizeof(reginfo));
}
template <class ELFT>
MipsOptionsSection<ELFT> *MipsOptionsSection<ELFT>::create() {
// N64 ABI only.
if (!ELFT::Is64Bits)
return nullptr;
std::vector<InputSectionBase *> sections;
for (InputSectionBase *sec : inputSections)
if (sec->type == SHT_MIPS_OPTIONS)
sections.push_back(sec);
if (sections.empty())
return nullptr;
Elf_Mips_RegInfo reginfo = {};
for (InputSectionBase *sec : sections) {
sec->markDead();
std::string filename = toString(sec->file);
ArrayRef<uint8_t> d = sec->data();
while (!d.empty()) {
if (d.size() < sizeof(Elf_Mips_Options)) {
error(filename + ": invalid size of .MIPS.options section");
break;
}
auto *opt = reinterpret_cast<const Elf_Mips_Options *>(d.data());
if (opt->kind == ODK_REGINFO) {
reginfo.ri_gprmask |= opt->getRegInfo().ri_gprmask;
sec->getFile<ELFT>()->mipsGp0 = opt->getRegInfo().ri_gp_value;
break;
}
if (!opt->size)
fatal(filename + ": zero option descriptor size");
d = d.slice(opt->size);
}
};
return make<MipsOptionsSection<ELFT>>(reginfo);
}
// MIPS .reginfo section.
template <class ELFT>
MipsReginfoSection<ELFT>::MipsReginfoSection(Elf_Mips_RegInfo reginfo)
: SyntheticSection(SHF_ALLOC, SHT_MIPS_REGINFO, 4, ".reginfo"),
reginfo(reginfo) {
this->entsize = sizeof(Elf_Mips_RegInfo);
}
template <class ELFT> void MipsReginfoSection<ELFT>::writeTo(uint8_t *buf) {
if (!config->relocatable)
reginfo.ri_gp_value = in.mipsGot->getGp();
memcpy(buf, &reginfo, sizeof(reginfo));
}
template <class ELFT>
MipsReginfoSection<ELFT> *MipsReginfoSection<ELFT>::create() {
// Section should be alive for O32 and N32 ABIs only.
if (ELFT::Is64Bits)
return nullptr;
std::vector<InputSectionBase *> sections;
for (InputSectionBase *sec : inputSections)
if (sec->type == SHT_MIPS_REGINFO)
sections.push_back(sec);
if (sections.empty())
return nullptr;
Elf_Mips_RegInfo reginfo = {};
for (InputSectionBase *sec : sections) {
sec->markDead();
if (sec->data().size() != sizeof(Elf_Mips_RegInfo)) {
error(toString(sec->file) + ": invalid size of .reginfo section");
return nullptr;
}
auto *r = reinterpret_cast<const Elf_Mips_RegInfo *>(sec->data().data());
reginfo.ri_gprmask |= r->ri_gprmask;
sec->getFile<ELFT>()->mipsGp0 = r->ri_gp_value;
};
return make<MipsReginfoSection<ELFT>>(reginfo);
}
InputSection *elf::createInterpSection() {
// StringSaver guarantees that the returned string ends with '\0'.
StringRef s = saver.save(config->dynamicLinker);
ArrayRef<uint8_t> contents = {(const uint8_t *)s.data(), s.size() + 1};
return make<InputSection>(nullptr, SHF_ALLOC, SHT_PROGBITS, 1, contents,
".interp");
}
Defined *elf::addSyntheticLocal(StringRef name, uint8_t type, uint64_t value,
uint64_t size, InputSectionBase &section) {
auto *s = make<Defined>(section.file, name, STB_LOCAL, STV_DEFAULT, type,
value, size, &section);
if (in.symTab)
in.symTab->addSymbol(s);
return s;
}
static size_t getHashSize() {
switch (config->buildId) {
case BuildIdKind::Fast:
return 8;
case BuildIdKind::Md5:
case BuildIdKind::Uuid:
return 16;
case BuildIdKind::Sha1:
return 20;
case BuildIdKind::Hexstring:
return config->buildIdVector.size();
default:
llvm_unreachable("unknown BuildIdKind");
}
}
// This class represents a linker-synthesized .note.gnu.property section.
//
// In x86 and AArch64, object files may contain feature flags indicating the
// features that they have used. The flags are stored in a .note.gnu.property
// section.
//
// lld reads the sections from input files and merges them by computing AND of
// the flags. The result is written as a new .note.gnu.property section.
//
// If the flag is zero (which indicates that the intersection of the feature
// sets is empty, or some input files didn't have .note.gnu.property sections),
// we don't create this section.
GnuPropertySection::GnuPropertySection()
: SyntheticSection(llvm::ELF::SHF_ALLOC, llvm::ELF::SHT_NOTE,
config->wordsize, ".note.gnu.property") {}
void GnuPropertySection::writeTo(uint8_t *buf) {
uint32_t featureAndType = config->emachine == EM_AARCH64
? GNU_PROPERTY_AARCH64_FEATURE_1_AND
: GNU_PROPERTY_X86_FEATURE_1_AND;
write32(buf, 4); // Name size
write32(buf + 4, config->is64 ? 16 : 12); // Content size
write32(buf + 8, NT_GNU_PROPERTY_TYPE_0); // Type
memcpy(buf + 12, "GNU", 4); // Name string
write32(buf + 16, featureAndType); // Feature type
write32(buf + 20, 4); // Feature size
write32(buf + 24, config->andFeatures); // Feature flags
if (config->is64)
write32(buf + 28, 0); // Padding
}
size_t GnuPropertySection::getSize() const { return config->is64 ? 32 : 28; }
BuildIdSection::BuildIdSection()
: SyntheticSection(SHF_ALLOC, SHT_NOTE, 4, ".note.gnu.build-id"),
hashSize(getHashSize()) {}
void BuildIdSection::writeTo(uint8_t *buf) {
write32(buf, 4); // Name size
write32(buf + 4, hashSize); // Content size
write32(buf + 8, NT_GNU_BUILD_ID); // Type
memcpy(buf + 12, "GNU", 4); // Name string
hashBuf = buf + 16;
}
void BuildIdSection::writeBuildId(ArrayRef<uint8_t> buf) {
assert(buf.size() == hashSize);
memcpy(hashBuf, buf.data(), hashSize);
}
BssSection::BssSection(StringRef name, uint64_t size, uint32_t alignment)
: SyntheticSection(SHF_ALLOC | SHF_WRITE, SHT_NOBITS, alignment, name) {
this->bss = true;
this->size = size;
}
EhFrameSection::EhFrameSection()
: SyntheticSection(SHF_ALLOC, SHT_PROGBITS, 1, ".eh_frame") {}
// Search for an existing CIE record or create a new one.
// CIE records from input object files are uniquified by their contents
// and where their relocations point to.
template <class ELFT, class RelTy>
CieRecord *EhFrameSection::addCie(EhSectionPiece &cie, ArrayRef<RelTy> rels) {
Symbol *personality = nullptr;
unsigned firstRelI = cie.firstRelocation;
if (firstRelI != (unsigned)-1)
personality =
&cie.sec->template getFile<ELFT>()->getRelocTargetSym(rels[firstRelI]);
// Search for an existing CIE by CIE contents/relocation target pair.
CieRecord *&rec = cieMap[{cie.data(), personality}];
// If not found, create a new one.
if (!rec) {
rec = make<CieRecord>();
rec->cie = &cie;
cieRecords.push_back(rec);
}
return rec;
}
// There is one FDE per function. Returns a non-null pointer to the function
// symbol if the given FDE points to a live function.
template <class ELFT, class RelTy>
Defined *EhFrameSection::isFdeLive(EhSectionPiece &fde, ArrayRef<RelTy> rels) {
auto *sec = cast<EhInputSection>(fde.sec);
unsigned firstRelI = fde.firstRelocation;
// An FDE should point to some function because FDEs are to describe
// functions. That's however not always the case due to an issue of
// ld.gold with -r. ld.gold may discard only functions and leave their
// corresponding FDEs, which results in creating bad .eh_frame sections.
// To deal with that, we ignore such FDEs.
if (firstRelI == (unsigned)-1)
return nullptr;
const RelTy &rel = rels[firstRelI];
Symbol &b = sec->template getFile<ELFT>()->getRelocTargetSym(rel);
// FDEs for garbage-collected or merged-by-ICF sections, or sections in
// another partition, are dead.
if (auto *d = dyn_cast<Defined>(&b))
if (d->section && d->section->partition == partition)
return d;
return nullptr;
}
// .eh_frame is a sequence of CIE or FDE records. In general, there
// is one CIE record per input object file which is followed by
// a list of FDEs. This function searches an existing CIE or create a new
// one and associates FDEs to the CIE.
template <class ELFT, class RelTy>
void EhFrameSection::addRecords(EhInputSection *sec, ArrayRef<RelTy> rels) {
offsetToCie.clear();
for (EhSectionPiece &piece : sec->pieces) {
// The empty record is the end marker.
if (piece.size == 4)
return;
size_t offset = piece.inputOff;
uint32_t id = read32(piece.data().data() + 4);
if (id == 0) {
offsetToCie[offset] = addCie<ELFT>(piece, rels);
continue;
}
uint32_t cieOffset = offset + 4 - id;
CieRecord *rec = offsetToCie[cieOffset];
if (!rec)
fatal(toString(sec) + ": invalid CIE reference");
if (!isFdeLive<ELFT>(piece, rels))
continue;
rec->fdes.push_back(&piece);
numFdes++;
}
}
template <class ELFT>
void EhFrameSection::addSectionAux(EhInputSection *sec) {
if (!sec->isLive())
return;
const RelsOrRelas<ELFT> rels = sec->template relsOrRelas<ELFT>();
if (rels.areRelocsRel())
addRecords<ELFT>(sec, rels.rels);
else
addRecords<ELFT>(sec, rels.relas);
}
void EhFrameSection::addSection(EhInputSection *sec) {
sec->parent = this;
alignment = std::max(alignment, sec->alignment);
sections.push_back(sec);
for (auto *ds : sec->dependentSections)
dependentSections.push_back(ds);
}
// Used by ICF<ELFT>::handleLSDA(). This function is very similar to
// EhFrameSection::addRecords().
template <class ELFT, class RelTy>
void EhFrameSection::iterateFDEWithLSDAAux(
EhInputSection &sec, ArrayRef<RelTy> rels, DenseSet<size_t> &ciesWithLSDA,
llvm::function_ref<void(InputSection &)> fn) {
for (EhSectionPiece &piece : sec.pieces) {
// Skip ZERO terminator.
if (piece.size == 4)
continue;
size_t offset = piece.inputOff;
uint32_t id =
endian::read32<ELFT::TargetEndianness>(piece.data().data() + 4);
if (id == 0) {
if (hasLSDA(piece))
ciesWithLSDA.insert(offset);
continue;
}
uint32_t cieOffset = offset + 4 - id;
if (ciesWithLSDA.count(cieOffset) == 0)
continue;
// The CIE has a LSDA argument. Call fn with d's section.
if (Defined *d = isFdeLive<ELFT>(piece, rels))
if (auto *s = dyn_cast_or_null<InputSection>(d->section))
fn(*s);
}
}
template <class ELFT>
void EhFrameSection::iterateFDEWithLSDA(
llvm::function_ref<void(InputSection &)> fn) {
DenseSet<size_t> ciesWithLSDA;
for (EhInputSection *sec : sections) {
ciesWithLSDA.clear();
const RelsOrRelas<ELFT> rels = sec->template relsOrRelas<ELFT>();
if (rels.areRelocsRel())
iterateFDEWithLSDAAux<ELFT>(*sec, rels.rels, ciesWithLSDA, fn);
else
iterateFDEWithLSDAAux<ELFT>(*sec, rels.relas, ciesWithLSDA, fn);
}
}
static void writeCieFde(uint8_t *buf, ArrayRef<uint8_t> d) {
memcpy(buf, d.data(), d.size());
size_t aligned = alignTo(d.size(), config->wordsize);
// Zero-clear trailing padding if it exists.
memset(buf + d.size(), 0, aligned - d.size());
// Fix the size field. -4 since size does not include the size field itself.
write32(buf, aligned - 4);
}
void EhFrameSection::finalizeContents() {
assert(!this->size); // Not finalized.
switch (config->ekind) {
case ELFNoneKind:
llvm_unreachable("invalid ekind");
case ELF32LEKind:
for (EhInputSection *sec : sections)
addSectionAux<ELF32LE>(sec);
break;
case ELF32BEKind:
for (EhInputSection *sec : sections)
addSectionAux<ELF32BE>(sec);
break;
case ELF64LEKind:
for (EhInputSection *sec : sections)
addSectionAux<ELF64LE>(sec);
break;
case ELF64BEKind:
for (EhInputSection *sec : sections)
addSectionAux<ELF64BE>(sec);
break;
}
size_t off = 0;
for (CieRecord *rec : cieRecords) {
rec->cie->outputOff = off;
off += alignTo(rec->cie->size, config->wordsize);
for (EhSectionPiece *fde : rec->fdes) {
fde->outputOff = off;
off += alignTo(fde->size, config->wordsize);
}
}
// The LSB standard does not allow a .eh_frame section with zero
// Call Frame Information records. glibc unwind-dw2-fde.c
// classify_object_over_fdes expects there is a CIE record length 0 as a
// terminator. Thus we add one unconditionally.
off += 4;
this->size = off;
}
// Returns data for .eh_frame_hdr. .eh_frame_hdr is a binary search table
// to get an FDE from an address to which FDE is applied. This function
// returns a list of such pairs.
std::vector<EhFrameSection::FdeData> EhFrameSection::getFdeData() const {
uint8_t *buf = Out::bufferStart + getParent()->offset + outSecOff;
std::vector<FdeData> ret;
uint64_t va = getPartition().ehFrameHdr->getVA();
for (CieRecord *rec : cieRecords) {
uint8_t enc = getFdeEncoding(rec->cie);
for (EhSectionPiece *fde : rec->fdes) {
uint64_t pc = getFdePc(buf, fde->outputOff, enc);
uint64_t fdeVA = getParent()->addr + fde->outputOff;
if (!isInt<32>(pc - va))
fatal(toString(fde->sec) + ": PC offset is too large: 0x" +
Twine::utohexstr(pc - va));
ret.push_back({uint32_t(pc - va), uint32_t(fdeVA - va)});
}
}
// Sort the FDE list by their PC and uniqueify. Usually there is only
// one FDE for a PC (i.e. function), but if ICF merges two functions
// into one, there can be more than one FDEs pointing to the address.
auto less = [](const FdeData &a, const FdeData &b) {
return a.pcRel < b.pcRel;
};
llvm::stable_sort(ret, less);
auto eq = [](const FdeData &a, const FdeData &b) {
return a.pcRel == b.pcRel;
};
ret.erase(std::unique(ret.begin(), ret.end(), eq), ret.end());
return ret;
}
static uint64_t readFdeAddr(uint8_t *buf, int size) {
switch (size) {
case DW_EH_PE_udata2:
return read16(buf);
case DW_EH_PE_sdata2:
return (int16_t)read16(buf);
case DW_EH_PE_udata4:
return read32(buf);
case DW_EH_PE_sdata4:
return (int32_t)read32(buf);
case DW_EH_PE_udata8:
case DW_EH_PE_sdata8:
return read64(buf);
case DW_EH_PE_absptr:
return readUint(buf);
}
fatal("unknown FDE size encoding");
}
// Returns the VA to which a given FDE (on a mmap'ed buffer) is applied to.
// We need it to create .eh_frame_hdr section.
uint64_t EhFrameSection::getFdePc(uint8_t *buf, size_t fdeOff,
uint8_t enc) const {
// The starting address to which this FDE applies is
// stored at FDE + 8 byte.
size_t off = fdeOff + 8;
uint64_t addr = readFdeAddr(buf + off, enc & 0xf);
if ((enc & 0x70) == DW_EH_PE_absptr)
return addr;
if ((enc & 0x70) == DW_EH_PE_pcrel)
return addr + getParent()->addr + off;
fatal("unknown FDE size relative encoding");
}
void EhFrameSection::writeTo(uint8_t *buf) {
// Write CIE and FDE records.
for (CieRecord *rec : cieRecords) {
size_t cieOffset = rec->cie->outputOff;
writeCieFde(buf + cieOffset, rec->cie->data());
for (EhSectionPiece *fde : rec->fdes) {
size_t off = fde->outputOff;
writeCieFde(buf + off, fde->data());
// FDE's second word should have the offset to an associated CIE.
// Write it.
write32(buf + off + 4, off + 4 - cieOffset);
}
}
// Apply relocations. .eh_frame section contents are not contiguous
// in the output buffer, but relocateAlloc() still works because
// getOffset() takes care of discontiguous section pieces.
for (EhInputSection *s : sections)
s->relocateAlloc(buf, nullptr);
if (getPartition().ehFrameHdr && getPartition().ehFrameHdr->getParent())
getPartition().ehFrameHdr->write();
}
GotSection::GotSection()
: SyntheticSection(SHF_ALLOC | SHF_WRITE, SHT_PROGBITS,
target->gotEntrySize, ".got") {
numEntries = target->gotHeaderEntriesNum;
}
void GotSection::addEntry(Symbol &sym) {
sym.gotIndex = numEntries;
++numEntries;
}
bool GotSection::addDynTlsEntry(Symbol &sym) {
if (sym.globalDynIndex != -1U)
return false;
sym.globalDynIndex = numEntries;
// Global Dynamic TLS entries take two GOT slots.
numEntries += 2;
return true;
}
// Reserves TLS entries for a TLS module ID and a TLS block offset.
// In total it takes two GOT slots.
bool GotSection::addTlsIndex() {
if (tlsIndexOff != uint32_t(-1))
return false;
tlsIndexOff = numEntries * config->wordsize;
numEntries += 2;
return true;
}
uint64_t GotSection::getGlobalDynAddr(const Symbol &b) const {
return this->getVA() + b.globalDynIndex * config->wordsize;
}
uint64_t GotSection::getGlobalDynOffset(const Symbol &b) const {
return b.globalDynIndex * config->wordsize;
}
void GotSection::finalizeContents() {
if (config->emachine == EM_PPC64 &&
numEntries <= target->gotHeaderEntriesNum && !ElfSym::globalOffsetTable)
size = 0;
else
size = numEntries * config->wordsize;
}
bool GotSection::isNeeded() const {
// Needed if the GOT symbol is used or the number of entries is more than just
// the header. A GOT with just the header may not be needed.
return hasGotOffRel || numEntries > target->gotHeaderEntriesNum;
}
void GotSection::writeTo(uint8_t *buf) {
target->writeGotHeader(buf);
relocateAlloc(buf, buf + size);
}
static uint64_t getMipsPageAddr(uint64_t addr) {
return (addr + 0x8000) & ~0xffff;
}
static uint64_t getMipsPageCount(uint64_t size) {
return (size + 0xfffe) / 0xffff + 1;
}
MipsGotSection::MipsGotSection()
: SyntheticSection(SHF_ALLOC | SHF_WRITE | SHF_MIPS_GPREL, SHT_PROGBITS, 16,
".got") {}
void MipsGotSection::addEntry(InputFile &file, Symbol &sym, int64_t addend,
RelExpr expr) {
FileGot &g = getGot(file);
if (expr == R_MIPS_GOT_LOCAL_PAGE) {
if (const OutputSection *os = sym.getOutputSection())
g.pagesMap.insert({os, {}});
else
g.local16.insert({{nullptr, getMipsPageAddr(sym.getVA(addend))}, 0});
} else if (sym.isTls())
g.tls.insert({&sym, 0});
else if (sym.isPreemptible && expr == R_ABS)
g.relocs.insert({&sym, 0});
else if (sym.isPreemptible)
g.global.insert({&sym, 0});
else if (expr == R_MIPS_GOT_OFF32)
g.local32.insert({{&sym, addend}, 0});
else
g.local16.insert({{&sym, addend}, 0});
}
void MipsGotSection::addDynTlsEntry(InputFile &file, Symbol &sym) {
getGot(file).dynTlsSymbols.insert({&sym, 0});
}
void MipsGotSection::addTlsIndex(InputFile &file) {
getGot(file).dynTlsSymbols.insert({nullptr, 0});
}
size_t MipsGotSection::FileGot::getEntriesNum() const {
return getPageEntriesNum() + local16.size() + global.size() + relocs.size() +
tls.size() + dynTlsSymbols.size() * 2;
}
size_t MipsGotSection::FileGot::getPageEntriesNum() const {
size_t num = 0;
for (const std::pair<const OutputSection *, FileGot::PageBlock> &p : pagesMap)
num += p.second.count;
return num;
}
size_t MipsGotSection::FileGot::getIndexedEntriesNum() const {
size_t count = getPageEntriesNum() + local16.size() + global.size();
// If there are relocation-only entries in the GOT, TLS entries
// are allocated after them. TLS entries should be addressable
// by 16-bit index so count both reloc-only and TLS entries.
if (!tls.empty() || !dynTlsSymbols.empty())
count += relocs.size() + tls.size() + dynTlsSymbols.size() * 2;
return count;
}
MipsGotSection::FileGot &MipsGotSection::getGot(InputFile &f) {
if (!f.mipsGotIndex.hasValue()) {
gots.emplace_back();
gots.back().file = &f;
f.mipsGotIndex = gots.size() - 1;
}
return gots[*f.mipsGotIndex];
}
uint64_t MipsGotSection::getPageEntryOffset(const InputFile *f,
const Symbol &sym,
int64_t addend) const {
const FileGot &g = gots[*f->mipsGotIndex];
uint64_t index = 0;
if (const OutputSection *outSec = sym.getOutputSection()) {
uint64_t secAddr = getMipsPageAddr(outSec->addr);
uint64_t symAddr = getMipsPageAddr(sym.getVA(addend));
index = g.pagesMap.lookup(outSec).firstIndex + (symAddr - secAddr) / 0xffff;
} else {
index = g.local16.lookup({nullptr, getMipsPageAddr(sym.getVA(addend))});
}
return index * config->wordsize;
}
uint64_t MipsGotSection::getSymEntryOffset(const InputFile *f, const Symbol &s,
int64_t addend) const {
const FileGot &g = gots[*f->mipsGotIndex];
Symbol *sym = const_cast<Symbol *>(&s);
if (sym->isTls())
return g.tls.lookup(sym) * config->wordsize;
if (sym->isPreemptible)
return g.global.lookup(sym) * config->wordsize;
return g.local16.lookup({sym, addend}) * config->wordsize;
}
uint64_t MipsGotSection::getTlsIndexOffset(const InputFile *f) const {
const FileGot &g = gots[*f->mipsGotIndex];
return g.dynTlsSymbols.lookup(nullptr) * config->wordsize;
}
uint64_t MipsGotSection::getGlobalDynOffset(const InputFile *f,
const Symbol &s) const {
const FileGot &g = gots[*f->mipsGotIndex];
Symbol *sym = const_cast<Symbol *>(&s);
return g.dynTlsSymbols.lookup(sym) * config->wordsize;
}
const Symbol *MipsGotSection::getFirstGlobalEntry() const {
if (gots.empty())
return nullptr;
const FileGot &primGot = gots.front();
if (!primGot.global.empty())
return primGot.global.front().first;
if (!primGot.relocs.empty())
return primGot.relocs.front().first;
return nullptr;
}
unsigned MipsGotSection::getLocalEntriesNum() const {
if (gots.empty())
return headerEntriesNum;
return headerEntriesNum + gots.front().getPageEntriesNum() +
gots.front().local16.size();
}
bool MipsGotSection::tryMergeGots(FileGot &dst, FileGot &src, bool isPrimary) {
FileGot tmp = dst;
set_union(tmp.pagesMap, src.pagesMap);
set_union(tmp.local16, src.local16);
set_union(tmp.global, src.global);
set_union(tmp.relocs, src.relocs);
set_union(tmp.tls, src.tls);
set_union(tmp.dynTlsSymbols, src.dynTlsSymbols);
size_t count = isPrimary ? headerEntriesNum : 0;
count += tmp.getIndexedEntriesNum();
if (count * config->wordsize > config->mipsGotSize)
return false;
std::swap(tmp, dst);
return true;
}
void MipsGotSection::finalizeContents() { updateAllocSize(); }
bool MipsGotSection::updateAllocSize() {
size = headerEntriesNum * config->wordsize;
for (const FileGot &g : gots)
size += g.getEntriesNum() * config->wordsize;
return false;
}
void MipsGotSection::build() {
if (gots.empty())
return;
std::vector<FileGot> mergedGots(1);
// For each GOT move non-preemptible symbols from the `Global`
// to `Local16` list. Preemptible symbol might become non-preemptible
// one if, for example, it gets a related copy relocation.
for (FileGot &got : gots) {
for (auto &p: got.global)
if (!p.first->isPreemptible)
got.local16.insert({{p.first, 0}, 0});
got.global.remove_if([&](const std::pair<Symbol *, size_t> &p) {
return !p.first->isPreemptible;
});
}
// For each GOT remove "reloc-only" entry if there is "global"
// entry for the same symbol. And add local entries which indexed
// using 32-bit value at the end of 16-bit entries.
for (FileGot &got : gots) {
got.relocs.remove_if([&](const std::pair<Symbol *, size_t> &p) {
return got.global.count(p.first);
});
set_union(got.local16, got.local32);
got.local32.clear();
}
// Evaluate number of "reloc-only" entries in the resulting GOT.
// To do that put all unique "reloc-only" and "global" entries
// from all GOTs to the future primary GOT.
FileGot *primGot = &mergedGots.front();
for (FileGot &got : gots) {
set_union(primGot->relocs, got.global);
set_union(primGot->relocs, got.relocs);
got.relocs.clear();
}
// Evaluate number of "page" entries in each GOT.
for (FileGot &got : gots) {
for (std::pair<const OutputSection *, FileGot::PageBlock> &p :
got.pagesMap) {
const OutputSection *os = p.first;
uint64_t secSize = 0;
for (BaseCommand *cmd : os->sectionCommands) {
if (auto *isd = dyn_cast<InputSectionDescription>(cmd))
for (InputSection *isec : isd->sections) {
uint64_t off = alignTo(secSize, isec->alignment);
secSize = off + isec->getSize();
}
}
p.second.count = getMipsPageCount(secSize);
}
}
// Merge GOTs. Try to join as much as possible GOTs but do not exceed
// maximum GOT size. At first, try to fill the primary GOT because
// the primary GOT can be accessed in the most effective way. If it
// is not possible, try to fill the last GOT in the list, and finally
// create a new GOT if both attempts failed.
for (FileGot &srcGot : gots) {
InputFile *file = srcGot.file;
if (tryMergeGots(mergedGots.front(), srcGot, true)) {
file->mipsGotIndex = 0;
} else {
// If this is the first time we failed to merge with the primary GOT,
// MergedGots.back() will also be the primary GOT. We must make sure not
// to try to merge again with isPrimary=false, as otherwise, if the
// inputs are just right, we could allow the primary GOT to become 1 or 2
// words bigger due to ignoring the header size.
if (mergedGots.size() == 1 ||
!tryMergeGots(mergedGots.back(), srcGot, false)) {
mergedGots.emplace_back();
std::swap(mergedGots.back(), srcGot);
}
file->mipsGotIndex = mergedGots.size() - 1;
}
}
std::swap(gots, mergedGots);
// Reduce number of "reloc-only" entries in the primary GOT
// by subtracting "global" entries in the primary GOT.
primGot = &gots.front();
primGot->relocs.remove_if([&](const std::pair<Symbol *, size_t> &p) {
return primGot->global.count(p.first);
});
// Calculate indexes for each GOT entry.
size_t index = headerEntriesNum;
for (FileGot &got : gots) {
got.startIndex = &got == primGot ? 0 : index;
for (std::pair<const OutputSection *, FileGot::PageBlock> &p :
got.pagesMap) {
// For each output section referenced by GOT page relocations calculate
// and save into pagesMap an upper bound of MIPS GOT entries required
// to store page addresses of local symbols. We assume the worst case -
// each 64kb page of the output section has at least one GOT relocation
// against it. And take in account the case when the section intersects
// page boundaries.
p.second.firstIndex = index;
index += p.second.count;
}
for (auto &p: got.local16)
p.second = index++;
for (auto &p: got.global)
p.second = index++;
for (auto &p: got.relocs)
p.second = index++;
for (auto &p: got.tls)
p.second = index++;
for (auto &p: got.dynTlsSymbols) {
p.second = index;
index += 2;
}
}
// Update Symbol::gotIndex field to use this
// value later in the `sortMipsSymbols` function.
for (auto &p : primGot->global)
p.first->gotIndex = p.second;
for (auto &p : primGot->relocs)
p.first->gotIndex = p.second;
// Create dynamic relocations.
for (FileGot &got : gots) {
// Create dynamic relocations for TLS entries.
for (std::pair<Symbol *, size_t> &p : got.tls) {
Symbol *s = p.first;
uint64_t offset = p.second * config->wordsize;
// When building a shared library we still need a dynamic relocation
// for the TP-relative offset as we don't know how much other data will
// be allocated before us in the static TLS block.
if (s->isPreemptible || config->shared)
mainPart->relaDyn->addReloc({target->tlsGotRel, this, offset,
DynamicReloc::AgainstSymbolWithTargetVA,
*s, 0, R_ABS});
}
for (std::pair<Symbol *, size_t> &p : got.dynTlsSymbols) {
Symbol *s = p.first;
uint64_t offset = p.second * config->wordsize;
if (s == nullptr) {
if (!config->shared)
continue;
mainPart->relaDyn->addReloc({target->tlsModuleIndexRel, this, offset});
} else {
// When building a shared library we still need a dynamic relocation
// for the module index. Therefore only checking for
// S->isPreemptible is not sufficient (this happens e.g. for
// thread-locals that have been marked as local through a linker script)
if (!s->isPreemptible && !config->shared)
continue;
mainPart->relaDyn->addSymbolReloc(target->tlsModuleIndexRel, this,
offset, *s);
// However, we can skip writing the TLS offset reloc for non-preemptible
// symbols since it is known even in shared libraries
if (!s->isPreemptible)
continue;
offset += config->wordsize;
mainPart->relaDyn->addSymbolReloc(target->tlsOffsetRel, this, offset,
*s);
}
}
// Do not create dynamic relocations for non-TLS
// entries in the primary GOT.
if (&got == primGot)
continue;
// Dynamic relocations for "global" entries.
for (const std::pair<Symbol *, size_t> &p : got.global) {
uint64_t offset = p.second * config->wordsize;
mainPart->relaDyn->addSymbolReloc(target->relativeRel, this, offset,
*p.first);
}
if (!config->isPic)
continue;
// Dynamic relocations for "local" entries in case of PIC.
for (const std::pair<const OutputSection *, FileGot::PageBlock> &l :
got.pagesMap) {
size_t pageCount = l.second.count;
for (size_t pi = 0; pi < pageCount; ++pi) {
uint64_t offset = (l.second.firstIndex + pi) * config->wordsize;
mainPart->relaDyn->addReloc({target->relativeRel, this, offset, l.first,
int64_t(pi * 0x10000)});
}
}
for (const std::pair<GotEntry, size_t> &p : got.local16) {
uint64_t offset = p.second * config->wordsize;
mainPart->relaDyn->addReloc({target->relativeRel, this, offset,
DynamicReloc::AddendOnlyWithTargetVA,
*p.first.first, p.first.second, R_ABS});
}
}
}
bool MipsGotSection::isNeeded() const {
// We add the .got section to the result for dynamic MIPS target because
// its address and properties are mentioned in the .dynamic section.
return !config->relocatable;
}
uint64_t MipsGotSection::getGp(const InputFile *f) const {
// For files without related GOT or files refer a primary GOT
// returns "common" _gp value. For secondary GOTs calculate
// individual _gp values.
if (!f || !f->mipsGotIndex.hasValue() || *f->mipsGotIndex == 0)
return ElfSym::mipsGp->getVA(0);
return getVA() + gots[*f->mipsGotIndex].startIndex * config->wordsize +
0x7ff0;
}
void MipsGotSection::writeTo(uint8_t *buf) {
// Set the MSB of the second GOT slot. This is not required by any
// MIPS ABI documentation, though.
//
// There is a comment in glibc saying that "The MSB of got[1] of a
// gnu object is set to identify gnu objects," and in GNU gold it
// says "the second entry will be used by some runtime loaders".
// But how this field is being used is unclear.
//
// We are not really willing to mimic other linkers behaviors
// without understanding why they do that, but because all files
// generated by GNU tools have this special GOT value, and because
// we've been doing this for years, it is probably a safe bet to
// keep doing this for now. We really need to revisit this to see
// if we had to do this.
writeUint(buf + config->wordsize, (uint64_t)1 << (config->wordsize * 8 - 1));
for (const FileGot &g : gots) {
auto write = [&](size_t i, const Symbol *s, int64_t a) {
uint64_t va = a;
if (s)
va = s->getVA(a);
writeUint(buf + i * config->wordsize, va);
};
// Write 'page address' entries to the local part of the GOT.
for (const std::pair<const OutputSection *, FileGot::PageBlock> &l :
g.pagesMap) {
size_t pageCount = l.second.count;
uint64_t firstPageAddr = getMipsPageAddr(l.first->addr);
for (size_t pi = 0; pi < pageCount; ++pi)
write(l.second.firstIndex + pi, nullptr, firstPageAddr + pi * 0x10000);
}
// Local, global, TLS, reloc-only entries.
// If TLS entry has a corresponding dynamic relocations, leave it
// initialized by zero. Write down adjusted TLS symbol's values otherwise.
// To calculate the adjustments use offsets for thread-local storage.
// http://web.archive.org/web/20190324223224/https://www.linux-mips.org/wiki/NPTL
for (const std::pair<GotEntry, size_t> &p : g.local16)
write(p.second, p.first.first, p.first.second);
// Write VA to the primary GOT only. For secondary GOTs that
// will be done by REL32 dynamic relocations.
if (&g == &gots.front())
for (const std::pair<Symbol *, size_t> &p : g.global)
write(p.second, p.first, 0);
for (const std::pair<Symbol *, size_t> &p : g.relocs)
write(p.second, p.first, 0);
for (const std::pair<Symbol *, size_t> &p : g.tls)
write(p.second, p.first,
p.first->isPreemptible || config->shared ? 0 : -0x7000);
for (const std::pair<Symbol *, size_t> &p : g.dynTlsSymbols) {
if (p.first == nullptr && !config->shared)
write(p.second, nullptr, 1);
else if (p.first && !p.first->isPreemptible) {
// If we are emitting a shared library with relocations we mustn't write
// anything to the GOT here. When using Elf_Rel relocations the value
// one will be treated as an addend and will cause crashes at runtime
if (!config->shared)
write(p.second, nullptr, 1);
write(p.second + 1, p.first, -0x8000);
}
}
}
}
// On PowerPC the .plt section is used to hold the table of function addresses
// instead of the .got.plt, and the type is SHT_NOBITS similar to a .bss
// section. I don't know why we have a BSS style type for the section but it is
// consistent across both 64-bit PowerPC ABIs as well as the 32-bit PowerPC ABI.
GotPltSection::GotPltSection()
: SyntheticSection(SHF_ALLOC | SHF_WRITE, SHT_PROGBITS, config->wordsize,
".got.plt") {
if (config->emachine == EM_PPC) {
name = ".plt";
} else if (config->emachine == EM_PPC64) {
type = SHT_NOBITS;
name = ".plt";
}
}
void GotPltSection::addEntry(Symbol &sym) {
assert(sym.pltIndex == entries.size());
entries.push_back(&sym);
}
size_t GotPltSection::getSize() const {
return (target->gotPltHeaderEntriesNum + entries.size()) *
target->gotEntrySize;
}
void GotPltSection::writeTo(uint8_t *buf) {
target->writeGotPltHeader(buf);
buf += target->gotPltHeaderEntriesNum * target->gotEntrySize;
for (const Symbol *b : entries) {
target->writeGotPlt(buf, *b);
buf += target->gotEntrySize;
}
}
bool GotPltSection::isNeeded() const {
// We need to emit GOTPLT even if it's empty if there's a relocation relative
// to it.
return !entries.empty() || hasGotPltOffRel;
}
static StringRef getIgotPltName() {
// On ARM the IgotPltSection is part of the GotSection.
if (config->emachine == EM_ARM)
return ".got";
// On PowerPC64 the GotPltSection is renamed to '.plt' so the IgotPltSection
// needs to be named the same.
if (config->emachine == EM_PPC64)
return ".plt";
return ".got.plt";
}
// On PowerPC64 the GotPltSection type is SHT_NOBITS so we have to follow suit
// with the IgotPltSection.
IgotPltSection::IgotPltSection()
: SyntheticSection(SHF_ALLOC | SHF_WRITE,
config->emachine == EM_PPC64 ? SHT_NOBITS : SHT_PROGBITS,
target->gotEntrySize, getIgotPltName()) {}
void IgotPltSection::addEntry(Symbol &sym) {
assert(sym.pltIndex == entries.size());
entries.push_back(&sym);
}
size_t IgotPltSection::getSize() const {
return entries.size() * target->gotEntrySize;
}
void IgotPltSection::writeTo(uint8_t *buf) {
for (const Symbol *b : entries) {
target->writeIgotPlt(buf, *b);
buf += target->gotEntrySize;
}
}
StringTableSection::StringTableSection(StringRef name, bool dynamic)
: SyntheticSection(dynamic ? (uint64_t)SHF_ALLOC : 0, SHT_STRTAB, 1, name),
dynamic(dynamic) {
// ELF string tables start with a NUL byte.
addString("");
}
// Adds a string to the string table. If `hashIt` is true we hash and check for
// duplicates. It is optional because the name of global symbols are already
// uniqued and hashing them again has a big cost for a small value: uniquing
// them with some other string that happens to be the same.
unsigned StringTableSection::addString(StringRef s, bool hashIt) {
if (hashIt) {
auto r = stringMap.insert(std::make_pair(s, this->size));
if (!r.second)
return r.first->second;
}
unsigned ret = this->size;
this->size = this->size + s.size() + 1;
strings.push_back(s);
return ret;
}
void StringTableSection::writeTo(uint8_t *buf) {
for (StringRef s : strings) {
memcpy(buf, s.data(), s.size());
buf[s.size()] = '\0';
buf += s.size() + 1;
}
}
// Returns the number of entries in .gnu.version_d: the number of
// non-VER_NDX_LOCAL-non-VER_NDX_GLOBAL definitions, plus 1.
// Note that we don't support vd_cnt > 1 yet.
static unsigned getVerDefNum() {
return namedVersionDefs().size() + 1;
}
template <class ELFT>
DynamicSection<ELFT>::DynamicSection()
: SyntheticSection(SHF_ALLOC | SHF_WRITE, SHT_DYNAMIC, config->wordsize,
".dynamic") {
this->entsize = ELFT::Is64Bits ? 16 : 8;
// .dynamic section is not writable on MIPS and on Fuchsia OS
// which passes -z rodynamic.
// See "Special Section" in Chapter 4 in the following document:
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
if (config->emachine == EM_MIPS || config->zRodynamic)
this->flags = SHF_ALLOC;
}
// The output section .rela.dyn may include these synthetic sections:
//
// - part.relaDyn
// - in.relaIplt: this is included if in.relaIplt is named .rela.dyn
// - in.relaPlt: this is included if a linker script places .rela.plt inside
// .rela.dyn
//
// DT_RELASZ is the total size of the included sections.
static uint64_t addRelaSz(RelocationBaseSection *relaDyn) {
size_t size = relaDyn->getSize();
if (in.relaIplt->getParent() == relaDyn->getParent())
size += in.relaIplt->getSize();
if (in.relaPlt->getParent() == relaDyn->getParent())
size += in.relaPlt->getSize();
return size;
}
// A Linker script may assign the RELA relocation sections to the same
// output section. When this occurs we cannot just use the OutputSection
// Size. Moreover the [DT_JMPREL, DT_JMPREL + DT_PLTRELSZ) is permitted to
// overlap with the [DT_RELA, DT_RELA + DT_RELASZ).
static uint64_t addPltRelSz() {
size_t size = in.relaPlt->getSize();
if (in.relaIplt->getParent() == in.relaPlt->getParent() &&
in.relaIplt->name == in.relaPlt->name)
size += in.relaIplt->getSize();
return size;
}
// Add remaining entries to complete .dynamic contents.
template <class ELFT>
std::vector<std::pair<int32_t, uint64_t>>
DynamicSection<ELFT>::computeContents() {
elf::Partition &part = getPartition();
bool isMain = part.name.empty();
std::vector<std::pair<int32_t, uint64_t>> entries;
auto addInt = [&](int32_t tag, uint64_t val) {
entries.emplace_back(tag, val);
};
auto addInSec = [&](int32_t tag, const InputSection *sec) {
entries.emplace_back(tag, sec->getVA());
};
for (StringRef s : config->filterList)
addInt(DT_FILTER, part.dynStrTab->addString(s));
for (StringRef s : config->auxiliaryList)
addInt(DT_AUXILIARY, part.dynStrTab->addString(s));
if (!config->rpath.empty())
addInt(config->enableNewDtags ? DT_RUNPATH : DT_RPATH,
part.dynStrTab->addString(config->rpath));
for (SharedFile *file : sharedFiles)
if (file->isNeeded)
addInt(DT_NEEDED, part.dynStrTab->addString(file->soName));
if (isMain) {
if (!config->soName.empty())
addInt(DT_SONAME, part.dynStrTab->addString(config->soName));
} else {
if (!config->soName.empty())
addInt(DT_NEEDED, part.dynStrTab->addString(config->soName));
addInt(DT_SONAME, part.dynStrTab->addString(part.name));
}
// Set DT_FLAGS and DT_FLAGS_1.
uint32_t dtFlags = 0;
uint32_t dtFlags1 = 0;
if (config->bsymbolic == BsymbolicKind::All)
dtFlags |= DF_SYMBOLIC;
if (config->zGlobal)
dtFlags1 |= DF_1_GLOBAL;
if (config->zInitfirst)
dtFlags1 |= DF_1_INITFIRST;
if (config->zInterpose)
dtFlags1 |= DF_1_INTERPOSE;
if (config->zNodefaultlib)
dtFlags1 |= DF_1_NODEFLIB;
if (config->zNodelete)
dtFlags1 |= DF_1_NODELETE;
if (config->zNodlopen)
dtFlags1 |= DF_1_NOOPEN;
if (config->pie)
dtFlags1 |= DF_1_PIE;
if (config->zNow) {
dtFlags |= DF_BIND_NOW;
dtFlags1 |= DF_1_NOW;
}
if (config->zOrigin) {
dtFlags |= DF_ORIGIN;
dtFlags1 |= DF_1_ORIGIN;
}
if (!config->zText)
dtFlags |= DF_TEXTREL;
if (config->hasTlsIe && config->shared)
dtFlags |= DF_STATIC_TLS;
if (dtFlags)
addInt(DT_FLAGS, dtFlags);
if (dtFlags1)
addInt(DT_FLAGS_1, dtFlags1);
// DT_DEBUG is a pointer to debug information used by debuggers at runtime. We
// need it for each process, so we don't write it for DSOs. The loader writes
// the pointer into this entry.
//
// DT_DEBUG is the only .dynamic entry that needs to be written to. Some
// systems (currently only Fuchsia OS) provide other means to give the
// debugger this information. Such systems may choose make .dynamic read-only.
// If the target is such a system (used -z rodynamic) don't write DT_DEBUG.
if (!config->shared && !config->relocatable && !config->zRodynamic)
addInt(DT_DEBUG, 0);
if (part.relaDyn->isNeeded() ||
(in.relaIplt->isNeeded() &&
part.relaDyn->getParent() == in.relaIplt->getParent())) {
addInSec(part.relaDyn->dynamicTag, part.relaDyn);
entries.emplace_back(part.relaDyn->sizeDynamicTag, addRelaSz(part.relaDyn));
bool isRela = config->isRela;
addInt(isRela ? DT_RELAENT : DT_RELENT,
isRela ? sizeof(Elf_Rela) : sizeof(Elf_Rel));
// MIPS dynamic loader does not support RELCOUNT tag.
// The problem is in the tight relation between dynamic
// relocations and GOT. So do not emit this tag on MIPS.
if (config->emachine != EM_MIPS) {
size_t numRelativeRels = part.relaDyn->getRelativeRelocCount();
if (config->zCombreloc && numRelativeRels)
addInt(isRela ? DT_RELACOUNT : DT_RELCOUNT, numRelativeRels);
}
}
if (part.relrDyn && !part.relrDyn->relocs.empty()) {
addInSec(config->useAndroidRelrTags ? DT_ANDROID_RELR : DT_RELR,
part.relrDyn);
addInt(config->useAndroidRelrTags ? DT_ANDROID_RELRSZ : DT_RELRSZ,
part.relrDyn->getParent()->size);
addInt(config->useAndroidRelrTags ? DT_ANDROID_RELRENT : DT_RELRENT,
sizeof(Elf_Relr));
}
// .rel[a].plt section usually consists of two parts, containing plt and
// iplt relocations. It is possible to have only iplt relocations in the
// output. In that case relaPlt is empty and have zero offset, the same offset
// as relaIplt has. And we still want to emit proper dynamic tags for that
// case, so here we always use relaPlt as marker for the beginning of
// .rel[a].plt section.
if (isMain && (in.relaPlt->isNeeded() || in.relaIplt->isNeeded())) {
addInSec(DT_JMPREL, in.relaPlt);
entries.emplace_back(DT_PLTRELSZ, addPltRelSz());
switch (config->emachine) {
case EM_MIPS:
addInSec(DT_MIPS_PLTGOT, in.gotPlt);
break;
case EM_SPARCV9:
addInSec(DT_PLTGOT, in.plt);
break;
case EM_AARCH64:
if (llvm::find_if(in.relaPlt->relocs, [](const DynamicReloc &r) {
return r.type == target->pltRel &&
r.sym->stOther & STO_AARCH64_VARIANT_PCS;
}) != in.relaPlt->relocs.end())
addInt(DT_AARCH64_VARIANT_PCS, 0);
LLVM_FALLTHROUGH;
default:
addInSec(DT_PLTGOT, in.gotPlt);
break;
}
addInt(DT_PLTREL, config->isRela ? DT_RELA : DT_REL);
}
if (config->emachine == EM_AARCH64) {
if (config->andFeatures & GNU_PROPERTY_AARCH64_FEATURE_1_BTI)
addInt(DT_AARCH64_BTI_PLT, 0);
if (config->zPacPlt)
addInt(DT_AARCH64_PAC_PLT, 0);
}
addInSec(DT_SYMTAB, part.dynSymTab);
addInt(DT_SYMENT, sizeof(Elf_Sym));
addInSec(DT_STRTAB, part.dynStrTab);
addInt(DT_STRSZ, part.dynStrTab->getSize());
if (!config->zText)
addInt(DT_TEXTREL, 0);
if (part.gnuHashTab)
addInSec(DT_GNU_HASH, part.gnuHashTab);
if (part.hashTab)
addInSec(DT_HASH, part.hashTab);
if (isMain) {
if (Out::preinitArray) {
addInt(DT_PREINIT_ARRAY, Out::preinitArray->addr);
addInt(DT_PREINIT_ARRAYSZ, Out::preinitArray->size);
}
if (Out::initArray) {
addInt(DT_INIT_ARRAY, Out::initArray->addr);
addInt(DT_INIT_ARRAYSZ, Out::initArray->size);
}
if (Out::finiArray) {
addInt(DT_FINI_ARRAY, Out::finiArray->addr);
addInt(DT_FINI_ARRAYSZ, Out::finiArray->size);
}
if (Symbol *b = symtab->find(config->init))
if (b->isDefined())
addInt(DT_INIT, b->getVA());
if (Symbol *b = symtab->find(config->fini))
if (b->isDefined())
addInt(DT_FINI, b->getVA());
}
if (part.verSym && part.verSym->isNeeded())
addInSec(DT_VERSYM, part.verSym);
if (part.verDef && part.verDef->isLive()) {
addInSec(DT_VERDEF, part.verDef);
addInt(DT_VERDEFNUM, getVerDefNum());
}
if (part.verNeed && part.verNeed->isNeeded()) {
addInSec(DT_VERNEED, part.verNeed);
unsigned needNum = 0;
for (SharedFile *f : sharedFiles)
if (!f->vernauxs.empty())
++needNum;
addInt(DT_VERNEEDNUM, needNum);
}
if (config->emachine == EM_MIPS) {
addInt(DT_MIPS_RLD_VERSION, 1);
addInt(DT_MIPS_FLAGS, RHF_NOTPOT);
addInt(DT_MIPS_BASE_ADDRESS, target->getImageBase());
addInt(DT_MIPS_SYMTABNO, part.dynSymTab->getNumSymbols());
addInt(DT_MIPS_LOCAL_GOTNO, in.mipsGot->getLocalEntriesNum());
if (const Symbol *b = in.mipsGot->getFirstGlobalEntry())
addInt(DT_MIPS_GOTSYM, b->dynsymIndex);
else
addInt(DT_MIPS_GOTSYM, part.dynSymTab->getNumSymbols());
addInSec(DT_PLTGOT, in.mipsGot);
if (in.mipsRldMap) {
if (!config->pie)
addInSec(DT_MIPS_RLD_MAP, in.mipsRldMap);
// Store the offset to the .rld_map section
// relative to the address of the tag.
addInt(DT_MIPS_RLD_MAP_REL,
in.mipsRldMap->getVA() - (getVA() + entries.size() * entsize));
}
}
// DT_PPC_GOT indicates to glibc Secure PLT is used. If DT_PPC_GOT is absent,
// glibc assumes the old-style BSS PLT layout which we don't support.
if (config->emachine == EM_PPC)
addInSec(DT_PPC_GOT, in.got);
// Glink dynamic tag is required by the V2 abi if the plt section isn't empty.
if (config->emachine == EM_PPC64 && in.plt->isNeeded()) {
// The Glink tag points to 32 bytes before the first lazy symbol resolution
// stub, which starts directly after the header.
addInt(DT_PPC64_GLINK, in.plt->getVA() + target->pltHeaderSize - 32);
}
addInt(DT_NULL, 0);
return entries;
}
template <class ELFT> void DynamicSection<ELFT>::finalizeContents() {
if (OutputSection *sec = getPartition().dynStrTab->getParent())
getParent()->link = sec->sectionIndex;
this->size = computeContents().size() * this->entsize;
}
template <class ELFT> void DynamicSection<ELFT>::writeTo(uint8_t *buf) {
auto *p = reinterpret_cast<Elf_Dyn *>(buf);
for (std::pair<int32_t, uint64_t> kv : computeContents()) {
p->d_tag = kv.first;
p->d_un.d_val = kv.second;
++p;
}
}
uint64_t DynamicReloc::getOffset() const {
return inputSec->getVA(offsetInSec);
}
int64_t DynamicReloc::computeAddend() const {
switch (kind) {
case AddendOnly:
assert(sym == nullptr);
return addend;
case AgainstSymbol:
assert(sym != nullptr);
return addend;
case AddendOnlyWithTargetVA:
case AgainstSymbolWithTargetVA:
return InputSection::getRelocTargetVA(inputSec->file, type, addend,
getOffset(), *sym, expr);
case MipsMultiGotPage:
assert(sym == nullptr);
return getMipsPageAddr(outputSec->addr) + addend;
}
llvm_unreachable("Unknown DynamicReloc::Kind enum");
}
uint32_t DynamicReloc::getSymIndex(SymbolTableBaseSection *symTab) const {
if (needsDynSymIndex())
return symTab->getSymbolIndex(sym);
return 0;
}
RelocationBaseSection::RelocationBaseSection(StringRef name, uint32_t type,
int32_t dynamicTag,
int32_t sizeDynamicTag)
: SyntheticSection(SHF_ALLOC, type, config->wordsize, name),
dynamicTag(dynamicTag), sizeDynamicTag(sizeDynamicTag) {}
void RelocationBaseSection::addSymbolReloc(RelType dynType,
InputSectionBase *isec,
uint64_t offsetInSec, Symbol &sym,
int64_t addend,
Optional<RelType> addendRelType) {
addReloc(DynamicReloc::AgainstSymbol, dynType, isec, offsetInSec, sym, addend,
R_ADDEND, addendRelType ? *addendRelType : target->noneRel);
}
void RelocationBaseSection::addRelativeReloc(
RelType dynType, InputSectionBase *inputSec, uint64_t offsetInSec,
Symbol &sym, int64_t addend, RelType addendRelType, RelExpr expr) {
// This function should only be called for non-preemptible symbols or
// RelExpr values that refer to an address inside the output file (e.g. the
// address of the GOT entry for a potentially preemptible symbol).
assert((!sym.isPreemptible || expr == R_GOT) &&
"cannot add relative relocation against preemptible symbol");
assert(expr != R_ADDEND && "expected non-addend relocation expression");
addReloc(DynamicReloc::AddendOnlyWithTargetVA, dynType, inputSec, offsetInSec,
sym, addend, expr, addendRelType);
}
void RelocationBaseSection::addAddendOnlyRelocIfNonPreemptible(
RelType dynType, InputSectionBase *isec, uint64_t offsetInSec, Symbol &sym,
RelType addendRelType) {
// No need to write an addend to the section for preemptible symbols.
if (sym.isPreemptible)
addReloc({dynType, isec, offsetInSec, DynamicReloc::AgainstSymbol, sym, 0,
R_ABS});
else
addReloc(DynamicReloc::AddendOnlyWithTargetVA, dynType, isec, offsetInSec,
sym, 0, R_ABS, addendRelType);
}
void RelocationBaseSection::addReloc(DynamicReloc::Kind kind, RelType dynType,
InputSectionBase *inputSec,
uint64_t offsetInSec, Symbol &sym,
int64_t addend, RelExpr expr,
RelType addendRelType) {
// Write the addends to the relocated address if required. We skip
// it if the written value would be zero.
if (config->writeAddends && (expr != R_ADDEND || addend != 0))
inputSec->relocations.push_back(
{expr, addendRelType, offsetInSec, addend, &sym});
addReloc({dynType, inputSec, offsetInSec, kind, sym, addend, expr});
}
void RelocationBaseSection::addReloc(const DynamicReloc &reloc) {
if (reloc.type == target->relativeRel)
++numRelativeRelocs;
relocs.push_back(reloc);
}
void RelocationBaseSection::finalizeContents() {
SymbolTableBaseSection *symTab = getPartition().dynSymTab;
// When linking glibc statically, .rel{,a}.plt contains R_*_IRELATIVE
// relocations due to IFUNC (e.g. strcpy). sh_link will be set to 0 in that
// case.
if (symTab && symTab->getParent())
getParent()->link = symTab->getParent()->sectionIndex;
else
getParent()->link = 0;
if (in.relaPlt == this && in.gotPlt->getParent()) {
getParent()->flags |= ELF::SHF_INFO_LINK;
getParent()->info = in.gotPlt->getParent()->sectionIndex;
}
if (in.relaIplt == this && in.igotPlt->getParent()) {
getParent()->flags |= ELF::SHF_INFO_LINK;
getParent()->info = in.igotPlt->getParent()->sectionIndex;
}
}
RelrBaseSection::RelrBaseSection()
: SyntheticSection(SHF_ALLOC,
config->useAndroidRelrTags ? SHT_ANDROID_RELR : SHT_RELR,
config->wordsize, ".relr.dyn") {}
template <class ELFT>
static void encodeDynamicReloc(SymbolTableBaseSection *symTab,
typename ELFT::Rela *p,
const DynamicReloc &rel) {
if (config->isRela)
p->r_addend = rel.computeAddend();
p->r_offset = rel.getOffset();
p->setSymbolAndType(rel.getSymIndex(symTab), rel.type, config->isMips64EL);
}
template <class ELFT>
RelocationSection<ELFT>::RelocationSection(StringRef name, bool sort)
: RelocationBaseSection(name, config->isRela ? SHT_RELA : SHT_REL,
config->isRela ? DT_RELA : DT_REL,
config->isRela ? DT_RELASZ : DT_RELSZ),
sort(sort) {
this->entsize = config->isRela ? sizeof(Elf_Rela) : sizeof(Elf_Rel);
}
template <class ELFT> void RelocationSection<ELFT>::writeTo(uint8_t *buf) {
SymbolTableBaseSection *symTab = getPartition().dynSymTab;
// Sort by (!IsRelative,SymIndex,r_offset). DT_REL[A]COUNT requires us to
// place R_*_RELATIVE first. SymIndex is to improve locality, while r_offset
// is to make results easier to read.
if (sort)
llvm::stable_sort(
relocs, [&](const DynamicReloc &a, const DynamicReloc &b) {
return std::make_tuple(a.type != target->relativeRel,
a.getSymIndex(symTab), a.getOffset()) <
std::make_tuple(b.type != target->relativeRel,
b.getSymIndex(symTab), b.getOffset());
});
for (const DynamicReloc &rel : relocs) {
encodeDynamicReloc<ELFT>(symTab, reinterpret_cast<Elf_Rela *>(buf), rel);
buf += config->isRela ? sizeof(Elf_Rela) : sizeof(Elf_Rel);
}
}
template <class ELFT>
AndroidPackedRelocationSection<ELFT>::AndroidPackedRelocationSection(
StringRef name)
: RelocationBaseSection(
name, config->isRela ? SHT_ANDROID_RELA : SHT_ANDROID_REL,
config->isRela ? DT_ANDROID_RELA : DT_ANDROID_REL,
config->isRela ? DT_ANDROID_RELASZ : DT_ANDROID_RELSZ) {
this->entsize = 1;
}
template <class ELFT>
bool AndroidPackedRelocationSection<ELFT>::updateAllocSize() {
// This function computes the contents of an Android-format packed relocation
// section.
//
// This format compresses relocations by using relocation groups to factor out
// fields that are common between relocations and storing deltas from previous
// relocations in SLEB128 format (which has a short representation for small
// numbers). A good example of a relocation type with common fields is
// R_*_RELATIVE, which is normally used to represent function pointers in
// vtables. In the REL format, each relative relocation has the same r_info
// field, and is only different from other relative relocations in terms of
// the r_offset field. By sorting relocations by offset, grouping them by
// r_info and representing each relocation with only the delta from the
// previous offset, each 8-byte relocation can be compressed to as little as 1
// byte (or less with run-length encoding). This relocation packer was able to
// reduce the size of the relocation section in an Android Chromium DSO from
// 2,911,184 bytes to 174,693 bytes, or 6% of the original size.
//
// A relocation section consists of a header containing the literal bytes
// 'APS2' followed by a sequence of SLEB128-encoded integers. The first two
// elements are the total number of relocations in the section and an initial
// r_offset value. The remaining elements define a sequence of relocation
// groups. Each relocation group starts with a header consisting of the
// following elements:
//
// - the number of relocations in the relocation group
// - flags for the relocation group
// - (if RELOCATION_GROUPED_BY_OFFSET_DELTA_FLAG is set) the r_offset delta
// for each relocation in the group.
// - (if RELOCATION_GROUPED_BY_INFO_FLAG is set) the value of the r_info
// field for each relocation in the group.
// - (if RELOCATION_GROUP_HAS_ADDEND_FLAG and
// RELOCATION_GROUPED_BY_ADDEND_FLAG are set) the r_addend delta for
// each relocation in the group.
//
// Following the relocation group header are descriptions of each of the
// relocations in the group. They consist of the following elements:
//
// - (if RELOCATION_GROUPED_BY_OFFSET_DELTA_FLAG is not set) the r_offset
// delta for this relocation.
// - (if RELOCATION_GROUPED_BY_INFO_FLAG is not set) the value of the r_info
// field for this relocation.
// - (if RELOCATION_GROUP_HAS_ADDEND_FLAG is set and
// RELOCATION_GROUPED_BY_ADDEND_FLAG is not set) the r_addend delta for
// this relocation.
size_t oldSize = relocData.size();
relocData = {'A', 'P', 'S', '2'};
raw_svector_ostream os(relocData);
auto add = [&](int64_t v) { encodeSLEB128(v, os); };
// The format header includes the number of relocations and the initial
// offset (we set this to zero because the first relocation group will
// perform the initial adjustment).
add(relocs.size());
add(0);
std::vector<Elf_Rela> relatives, nonRelatives;
for (const DynamicReloc &rel : relocs) {
Elf_Rela r;
encodeDynamicReloc<ELFT>(getPartition().dynSymTab, &r, rel);
if (r.getType(config->isMips64EL) == target->relativeRel)
relatives.push_back(r);
else
nonRelatives.push_back(r);
}
llvm::sort(relatives, [](const Elf_Rel &a, const Elf_Rel &b) {
return a.r_offset < b.r_offset;
});
// Try to find groups of relative relocations which are spaced one word
// apart from one another. These generally correspond to vtable entries. The
// format allows these groups to be encoded using a sort of run-length
// encoding, but each group will cost 7 bytes in addition to the offset from
// the previous group, so it is only profitable to do this for groups of
// size 8 or larger.
std::vector<Elf_Rela> ungroupedRelatives;
std::vector<std::vector<Elf_Rela>> relativeGroups;
for (auto i = relatives.begin(), e = relatives.end(); i != e;) {
std::vector<Elf_Rela> group;
do {
group.push_back(*i++);
} while (i != e && (i - 1)->r_offset + config->wordsize == i->r_offset);
if (group.size() < 8)
ungroupedRelatives.insert(ungroupedRelatives.end(), group.begin(),
group.end());
else
relativeGroups.emplace_back(std::move(group));
}
// For non-relative relocations, we would like to:
// 1. Have relocations with the same symbol offset to be consecutive, so
// that the runtime linker can speed-up symbol lookup by implementing an
// 1-entry cache.
// 2. Group relocations by r_info to reduce the size of the relocation
// section.
// Since the symbol offset is the high bits in r_info, sorting by r_info
// allows us to do both.
//
// For Rela, we also want to sort by r_addend when r_info is the same. This
// enables us to group by r_addend as well.
llvm::stable_sort(nonRelatives, [](const Elf_Rela &a, const Elf_Rela &b) {
if (a.r_info != b.r_info)
return a.r_info < b.r_info;
if (config->isRela)
return a.r_addend < b.r_addend;
return false;
});
// Group relocations with the same r_info. Note that each group emits a group
// header and that may make the relocation section larger. It is hard to
// estimate the size of a group header as the encoded size of that varies
// based on r_info. However, we can approximate this trade-off by the number
// of values encoded. Each group header contains 3 values, and each relocation
// in a group encodes one less value, as compared to when it is not grouped.
// Therefore, we only group relocations if there are 3 or more of them with
// the same r_info.
//
// For Rela, the addend for most non-relative relocations is zero, and thus we
// can usually get a smaller relocation section if we group relocations with 0
// addend as well.
std::vector<Elf_Rela> ungroupedNonRelatives;
std::vector<std::vector<Elf_Rela>> nonRelativeGroups;
for (auto i = nonRelatives.begin(), e = nonRelatives.end(); i != e;) {
auto j = i + 1;
while (j != e && i->r_info == j->r_info &&
(!config->isRela || i->r_addend == j->r_addend))
++j;
if (j - i < 3 || (config->isRela && i->r_addend != 0))
ungroupedNonRelatives.insert(ungroupedNonRelatives.end(), i, j);
else
nonRelativeGroups.emplace_back(i, j);
i = j;
}
// Sort ungrouped relocations by offset to minimize the encoded length.
llvm::sort(ungroupedNonRelatives, [](const Elf_Rela &a, const Elf_Rela &b) {
return a.r_offset < b.r_offset;
});
unsigned hasAddendIfRela =
config->isRela ? RELOCATION_GROUP_HAS_ADDEND_FLAG : 0;
uint64_t offset = 0;
uint64_t addend = 0;
// Emit the run-length encoding for the groups of adjacent relative
// relocations. Each group is represented using two groups in the packed
// format. The first is used to set the current offset to the start of the
// group (and also encodes the first relocation), and the second encodes the
// remaining relocations.
for (std::vector<Elf_Rela> &g : relativeGroups) {
// The first relocation in the group.
add(1);
add(RELOCATION_GROUPED_BY_OFFSET_DELTA_FLAG |
RELOCATION_GROUPED_BY_INFO_FLAG | hasAddendIfRela);
add(g[0].r_offset - offset);
add(target->relativeRel);
if (config->isRela) {
add(g[0].r_addend - addend);
addend = g[0].r_addend;
}
// The remaining relocations.
add(g.size() - 1);
add(RELOCATION_GROUPED_BY_OFFSET_DELTA_FLAG |
RELOCATION_GROUPED_BY_INFO_FLAG | hasAddendIfRela);
add(config->wordsize);
add(target->relativeRel);
if (config->isRela) {
for (auto i = g.begin() + 1, e = g.end(); i != e; ++i) {
add(i->r_addend - addend);
addend = i->r_addend;
}
}
offset = g.back().r_offset;
}
// Now the ungrouped relatives.
if (!ungroupedRelatives.empty()) {
add(ungroupedRelatives.size());
add(RELOCATION_GROUPED_BY_INFO_FLAG | hasAddendIfRela);
add(target->relativeRel);
for (Elf_Rela &r : ungroupedRelatives) {
add(r.r_offset - offset);
offset = r.r_offset;
if (config->isRela) {
add(r.r_addend - addend);
addend = r.r_addend;
}
}
}
// Grouped non-relatives.
for (ArrayRef<Elf_Rela> g : nonRelativeGroups) {
add(g.size());
add(RELOCATION_GROUPED_BY_INFO_FLAG);
add(g[0].r_info);
for (const Elf_Rela &r : g) {
add(r.r_offset - offset);
offset = r.r_offset;
}
addend = 0;
}
// Finally the ungrouped non-relative relocations.
if (!ungroupedNonRelatives.empty()) {
add(ungroupedNonRelatives.size());
add(hasAddendIfRela);
for (Elf_Rela &r : ungroupedNonRelatives) {
add(r.r_offset - offset);
offset = r.r_offset;
add(r.r_info);
if (config->isRela) {
add(r.r_addend - addend);
addend = r.r_addend;
}
}
}
// Don't allow the section to shrink; otherwise the size of the section can
// oscillate infinitely.
if (relocData.size() < oldSize)
relocData.append(oldSize - relocData.size(), 0);
// Returns whether the section size changed. We need to keep recomputing both
// section layout and the contents of this section until the size converges
// because changing this section's size can affect section layout, which in
// turn can affect the sizes of the LEB-encoded integers stored in this
// section.
return relocData.size() != oldSize;
}
template <class ELFT> RelrSection<ELFT>::RelrSection() {
this->entsize = config->wordsize;
}
template <class ELFT> bool RelrSection<ELFT>::updateAllocSize() {
// This function computes the contents of an SHT_RELR packed relocation
// section.
//
// Proposal for adding SHT_RELR sections to generic-abi is here:
// https://groups.google.com/forum/#!topic/generic-abi/bX460iggiKg
//
// The encoded sequence of Elf64_Relr entries in a SHT_RELR section looks
// like [ AAAAAAAA BBBBBBB1 BBBBBBB1 ... AAAAAAAA BBBBBB1 ... ]
//
// i.e. start with an address, followed by any number of bitmaps. The address
// entry encodes 1 relocation. The subsequent bitmap entries encode up to 63
// relocations each, at subsequent offsets following the last address entry.
//
// The bitmap entries must have 1 in the least significant bit. The assumption
// here is that an address cannot have 1 in lsb. Odd addresses are not
// supported.
//
// Excluding the least significant bit in the bitmap, each non-zero bit in
// the bitmap represents a relocation to be applied to a corresponding machine
// word that follows the base address word. The second least significant bit
// represents the machine word immediately following the initial address, and
// each bit that follows represents the next word, in linear order. As such,
// a single bitmap can encode up to 31 relocations in a 32-bit object, and
// 63 relocations in a 64-bit object.
//
// This encoding has a couple of interesting properties:
// 1. Looking at any entry, it is clear whether it's an address or a bitmap:
// even means address, odd means bitmap.
// 2. Just a simple list of addresses is a valid encoding.
size_t oldSize = relrRelocs.size();
relrRelocs.clear();
// Same as Config->Wordsize but faster because this is a compile-time
// constant.
const size_t wordsize = sizeof(typename ELFT::uint);
// Number of bits to use for the relocation offsets bitmap.
// Must be either 63 or 31.
const size_t nBits = wordsize * 8 - 1;
// Get offsets for all relative relocations and sort them.
std::vector<uint64_t> offsets;
for (const RelativeReloc &rel : relocs)
offsets.push_back(rel.getOffset());
llvm::sort(offsets);
// For each leading relocation, find following ones that can be folded
// as a bitmap and fold them.
for (size_t i = 0, e = offsets.size(); i < e;) {
// Add a leading relocation.
relrRelocs.push_back(Elf_Relr(offsets[i]));
uint64_t base = offsets[i] + wordsize;
++i;
// Find foldable relocations to construct bitmaps.
while (i < e) {
uint64_t bitmap = 0;
while (i < e) {
uint64_t delta = offsets[i] - base;
// If it is too far, it cannot be folded.
if (delta >= nBits * wordsize)
break;
// If it is not a multiple of wordsize away, it cannot be folded.
if (delta % wordsize)
break;
// Fold it.
bitmap |= 1ULL << (delta / wordsize);
++i;
}
if (!bitmap)
break;
relrRelocs.push_back(Elf_Relr((bitmap << 1) | 1));
base += nBits * wordsize;
}
}
// Don't allow the section to shrink; otherwise the size of the section can
// oscillate infinitely. Trailing 1s do not decode to more relocations.
if (relrRelocs.size() < oldSize) {
log(".relr.dyn needs " + Twine(oldSize - relrRelocs.size()) +
" padding word(s)");
relrRelocs.resize(oldSize, Elf_Relr(1));
}
return relrRelocs.size() != oldSize;
}
SymbolTableBaseSection::SymbolTableBaseSection(StringTableSection &strTabSec)
: SyntheticSection(strTabSec.isDynamic() ? (uint64_t)SHF_ALLOC : 0,
strTabSec.isDynamic() ? SHT_DYNSYM : SHT_SYMTAB,
config->wordsize,
strTabSec.isDynamic() ? ".dynsym" : ".symtab"),
strTabSec(strTabSec) {}
// Orders symbols according to their positions in the GOT,
// in compliance with MIPS ABI rules.
// See "Global Offset Table" in Chapter 5 in the following document
// for detailed description:
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
static bool sortMipsSymbols(const SymbolTableEntry &l,
const SymbolTableEntry &r) {
// Sort entries related to non-local preemptible symbols by GOT indexes.
// All other entries go to the beginning of a dynsym in arbitrary order.
if (l.sym->isInGot() && r.sym->isInGot())
return l.sym->gotIndex < r.sym->gotIndex;
if (!l.sym->isInGot() && !r.sym->isInGot())
return false;
return !l.sym->isInGot();
}
void SymbolTableBaseSection::finalizeContents() {
if (OutputSection *sec = strTabSec.getParent())
getParent()->link = sec->sectionIndex;
if (this->type != SHT_DYNSYM) {
sortSymTabSymbols();
return;
}
// If it is a .dynsym, there should be no local symbols, but we need
// to do a few things for the dynamic linker.
// Section's Info field has the index of the first non-local symbol.
// Because the first symbol entry is a null entry, 1 is the first.
getParent()->info = 1;
if (getPartition().gnuHashTab) {
// NB: It also sorts Symbols to meet the GNU hash table requirements.
getPartition().gnuHashTab->addSymbols(symbols);
} else if (config->emachine == EM_MIPS) {
llvm::stable_sort(symbols, sortMipsSymbols);
}
// Only the main partition's dynsym indexes are stored in the symbols
// themselves. All other partitions use a lookup table.
if (this == mainPart->dynSymTab) {
size_t i = 0;
for (const SymbolTableEntry &s : symbols)
s.sym->dynsymIndex = ++i;
}
}
// The ELF spec requires that all local symbols precede global symbols, so we
// sort symbol entries in this function. (For .dynsym, we don't do that because
// symbols for dynamic linking are inherently all globals.)
//
// Aside from above, we put local symbols in groups starting with the STT_FILE
// symbol. That is convenient for purpose of identifying where are local symbols
// coming from.
void SymbolTableBaseSection::sortSymTabSymbols() {
// Move all local symbols before global symbols.
auto e = std::stable_partition(
symbols.begin(), symbols.end(), [](const SymbolTableEntry &s) {
return s.sym->isLocal() || s.sym->computeBinding() == STB_LOCAL;
});
size_t numLocals = e - symbols.begin();
getParent()->info = numLocals + 1;
// We want to group the local symbols by file. For that we rebuild the local
// part of the symbols vector. We do not need to care about the STT_FILE
// symbols, they are already naturally placed first in each group. That
// happens because STT_FILE is always the first symbol in the object and hence
// precede all other local symbols we add for a file.
MapVector<InputFile *, std::vector<SymbolTableEntry>> arr;
for (const SymbolTableEntry &s : llvm::make_range(symbols.begin(), e))
arr[s.sym->file].push_back(s);
auto i = symbols.begin();
for (std::pair<InputFile *, std::vector<SymbolTableEntry>> &p : arr)
for (SymbolTableEntry &entry : p.second)
*i++ = entry;
}
void SymbolTableBaseSection::addSymbol(Symbol *b) {
// Adding a local symbol to a .dynsym is a bug.
assert(this->type != SHT_DYNSYM || !b->isLocal());
bool hashIt = b->isLocal();
symbols.push_back({b, strTabSec.addString(b->getName(), hashIt)});
}
size_t SymbolTableBaseSection::getSymbolIndex(Symbol *sym) {
if (this == mainPart->dynSymTab)
return sym->dynsymIndex;
// Initializes symbol lookup tables lazily. This is used only for -r,
// --emit-relocs and dynsyms in partitions other than the main one.
llvm::call_once(onceFlag, [&] {
symbolIndexMap.reserve(symbols.size());
size_t i = 0;
for (const SymbolTableEntry &e : symbols) {
if (e.sym->type == STT_SECTION)
sectionIndexMap[e.sym->getOutputSection()] = ++i;
else
symbolIndexMap[e.sym] = ++i;
}
});
// Section symbols are mapped based on their output sections
// to maintain their semantics.
if (sym->type == STT_SECTION)
return sectionIndexMap.lookup(sym->getOutputSection());
return symbolIndexMap.lookup(sym);
}
template <class ELFT>
SymbolTableSection<ELFT>::SymbolTableSection(StringTableSection &strTabSec)
: SymbolTableBaseSection(strTabSec) {
this->entsize = sizeof(Elf_Sym);
}
static BssSection *getCommonSec(Symbol *sym) {
if (!config->defineCommon)
if (auto *d = dyn_cast<Defined>(sym))
return dyn_cast_or_null<BssSection>(d->section);
return nullptr;
}
static uint32_t getSymSectionIndex(Symbol *sym) {
if (getCommonSec(sym))
return SHN_COMMON;
if (!isa<Defined>(sym) || sym->needsPltAddr)
return SHN_UNDEF;
if (const OutputSection *os = sym->getOutputSection())
return os->sectionIndex >= SHN_LORESERVE ? (uint32_t)SHN_XINDEX
: os->sectionIndex;
return SHN_ABS;
}
// Write the internal symbol table contents to the output symbol table.
template <class ELFT> void SymbolTableSection<ELFT>::writeTo(uint8_t *buf) {
// The first entry is a null entry as per the ELF spec.
memset(buf, 0, sizeof(Elf_Sym));
buf += sizeof(Elf_Sym);
auto *eSym = reinterpret_cast<Elf_Sym *>(buf);
for (SymbolTableEntry &ent : symbols) {
Symbol *sym = ent.sym;
bool isDefinedHere = type == SHT_SYMTAB || sym->partition == partition;
// Set st_info and st_other.
eSym->st_other = 0;
if (sym->isLocal()) {
eSym->setBindingAndType(STB_LOCAL, sym->type);
} else {
eSym->setBindingAndType(sym->computeBinding(), sym->type);
eSym->setVisibility(sym->visibility);
}
// The 3 most significant bits of st_other are used by OpenPOWER ABI.
// See getPPC64GlobalEntryToLocalEntryOffset() for more details.
if (config->emachine == EM_PPC64)
eSym->st_other |= sym->stOther & 0xe0;
// The most significant bit of st_other is used by AArch64 ABI for the
// variant PCS.
else if (config->emachine == EM_AARCH64)
eSym->st_other |= sym->stOther & STO_AARCH64_VARIANT_PCS;
eSym->st_name = ent.strTabOffset;
if (isDefinedHere)
eSym->st_shndx = getSymSectionIndex(ent.sym);
else
eSym->st_shndx = 0;
// Copy symbol size if it is a defined symbol. st_size is not significant
// for undefined symbols, so whether copying it or not is up to us if that's
// the case. We'll leave it as zero because by not setting a value, we can
// get the exact same outputs for two sets of input files that differ only
// in undefined symbol size in DSOs.
if (eSym->st_shndx == SHN_UNDEF || !isDefinedHere)
eSym->st_size = 0;
else
eSym->st_size = sym->getSize();
// st_value is usually an address of a symbol, but that has a special
// meaning for uninstantiated common symbols (--no-define-common).
if (BssSection *commonSec = getCommonSec(ent.sym))
eSym->st_value = commonSec->alignment;
else if (isDefinedHere)
eSym->st_value = sym->getVA();
else
eSym->st_value = 0;
++eSym;
}
// On MIPS we need to mark symbol which has a PLT entry and requires
// pointer equality by STO_MIPS_PLT flag. That is necessary to help
// dynamic linker distinguish such symbols and MIPS lazy-binding stubs.
// https://sourceware.org/ml/binutils/2008-07/txt00000.txt
if (config->emachine == EM_MIPS) {
auto *eSym = reinterpret_cast<Elf_Sym *>(buf);
for (SymbolTableEntry &ent : symbols) {
Symbol *sym = ent.sym;
if (sym->isInPlt() && sym->needsPltAddr)
eSym->st_other |= STO_MIPS_PLT;
if (isMicroMips()) {
// We already set the less-significant bit for symbols
// marked by the `STO_MIPS_MICROMIPS` flag and for microMIPS PLT
// records. That allows us to distinguish such symbols in
// the `MIPS<ELFT>::relocate()` routine. Now we should
// clear that bit for non-dynamic symbol table, so tools
// like `objdump` will be able to deal with a correct
// symbol position.
if (sym->isDefined() &&
((sym->stOther & STO_MIPS_MICROMIPS) || sym->needsPltAddr)) {
if (!strTabSec.isDynamic())
eSym->st_value &= ~1;
eSym->st_other |= STO_MIPS_MICROMIPS;
}
}
if (config->relocatable)
if (auto *d = dyn_cast<Defined>(sym))
if (isMipsPIC<ELFT>(d))
eSym->st_other |= STO_MIPS_PIC;
++eSym;
}
}
}
SymtabShndxSection::SymtabShndxSection()
: SyntheticSection(0, SHT_SYMTAB_SHNDX, 4, ".symtab_shndx") {
this->entsize = 4;
}
void SymtabShndxSection::writeTo(uint8_t *buf) {
// We write an array of 32 bit values, where each value has 1:1 association
// with an entry in .symtab. If the corresponding entry contains SHN_XINDEX,
// we need to write actual index, otherwise, we must write SHN_UNDEF(0).
buf += 4; // Ignore .symtab[0] entry.
for (const SymbolTableEntry &entry : in.symTab->getSymbols()) {
if (getSymSectionIndex(entry.sym) == SHN_XINDEX)
write32(buf, entry.sym->getOutputSection()->sectionIndex);
buf += 4;
}
}
bool SymtabShndxSection::isNeeded() const {
// SHT_SYMTAB can hold symbols with section indices values up to
// SHN_LORESERVE. If we need more, we want to use extension SHT_SYMTAB_SHNDX
// section. Problem is that we reveal the final section indices a bit too
// late, and we do not know them here. For simplicity, we just always create
// a .symtab_shndx section when the amount of output sections is huge.
size_t size = 0;
for (BaseCommand *base : script->sectionCommands)
if (isa<OutputSection>(base))
++size;
return size >= SHN_LORESERVE;
}
void SymtabShndxSection::finalizeContents() {
getParent()->link = in.symTab->getParent()->sectionIndex;
}
size_t SymtabShndxSection::getSize() const {
return in.symTab->getNumSymbols() * 4;
}
// .hash and .gnu.hash sections contain on-disk hash tables that map
// symbol names to their dynamic symbol table indices. Their purpose
// is to help the dynamic linker resolve symbols quickly. If ELF files
// don't have them, the dynamic linker has to do linear search on all
// dynamic symbols, which makes programs slower. Therefore, a .hash
// section is added to a DSO by default.
//
// The Unix semantics of resolving dynamic symbols is somewhat expensive.
// Each ELF file has a list of DSOs that the ELF file depends on and a
// list of dynamic symbols that need to be resolved from any of the
// DSOs. That means resolving all dynamic symbols takes O(m)*O(n)
// where m is the number of DSOs and n is the number of dynamic
// symbols. For modern large programs, both m and n are large. So
// making each step faster by using hash tables substantially
// improves time to load programs.
//
// (Note that this is not the only way to design the shared library.
// For instance, the Windows DLL takes a different approach. On
// Windows, each dynamic symbol has a name of DLL from which the symbol
// has to be resolved. That makes the cost of symbol resolution O(n).
// This disables some hacky techniques you can use on Unix such as
// LD_PRELOAD, but this is arguably better semantics than the Unix ones.)
//
// Due to historical reasons, we have two different hash tables, .hash
// and .gnu.hash. They are for the same purpose, and .gnu.hash is a new
// and better version of .hash. .hash is just an on-disk hash table, but
// .gnu.hash has a bloom filter in addition to a hash table to skip
// DSOs very quickly. If you are sure that your dynamic linker knows
// about .gnu.hash, you want to specify --hash-style=gnu. Otherwise, a
// safe bet is to specify --hash-style=both for backward compatibility.
GnuHashTableSection::GnuHashTableSection()
: SyntheticSection(SHF_ALLOC, SHT_GNU_HASH, config->wordsize, ".gnu.hash") {
}
void GnuHashTableSection::finalizeContents() {
if (OutputSection *sec = getPartition().dynSymTab->getParent())
getParent()->link = sec->sectionIndex;
// Computes bloom filter size in word size. We want to allocate 12
// bits for each symbol. It must be a power of two.
if (symbols.empty()) {
maskWords = 1;
} else {
uint64_t numBits = symbols.size() * 12;
maskWords = NextPowerOf2(numBits / (config->wordsize * 8));
}
size = 16; // Header
size += config->wordsize * maskWords; // Bloom filter
size += nBuckets * 4; // Hash buckets
size += symbols.size() * 4; // Hash values
}
void GnuHashTableSection::writeTo(uint8_t *buf) {
// The output buffer is not guaranteed to be zero-cleared because we pre-
// fill executable sections with trap instructions. This is a precaution
// for that case, which happens only when --no-rosegment is given.
memset(buf, 0, size);
// Write a header.
write32(buf, nBuckets);
write32(buf + 4, getPartition().dynSymTab->getNumSymbols() - symbols.size());
write32(buf + 8, maskWords);
write32(buf + 12, Shift2);
buf += 16;
// Write the 2-bit bloom filter.
const unsigned c = config->is64 ? 64 : 32;
for (const Entry &sym : symbols) {
// When C = 64, we choose a word with bits [6:...] and set 1 to two bits in
// the word using bits [0:5] and [26:31].
size_t i = (sym.hash / c) & (maskWords - 1);
uint64_t val = readUint(buf + i * config->wordsize);
val |= uint64_t(1) << (sym.hash % c);
val |= uint64_t(1) << ((sym.hash >> Shift2) % c);
writeUint(buf + i * config->wordsize, val);
}
buf += config->wordsize * maskWords;
// Write the hash table.
uint32_t *buckets = reinterpret_cast<uint32_t *>(buf);
uint32_t oldBucket = -1;
uint32_t *values = buckets + nBuckets;
for (auto i = symbols.begin(), e = symbols.end(); i != e; ++i) {
// Write a hash value. It represents a sequence of chains that share the
// same hash modulo value. The last element of each chain is terminated by
// LSB 1.
uint32_t hash = i->hash;
bool isLastInChain = (i + 1) == e || i->bucketIdx != (i + 1)->bucketIdx;
hash = isLastInChain ? hash | 1 : hash & ~1;
write32(values++, hash);
if (i->bucketIdx == oldBucket)
continue;
// Write a hash bucket. Hash buckets contain indices in the following hash
// value table.
write32(buckets + i->bucketIdx,
getPartition().dynSymTab->getSymbolIndex(i->sym));
oldBucket = i->bucketIdx;
}
}
static uint32_t hashGnu(StringRef name) {
uint32_t h = 5381;
for (uint8_t c : name)
h = (h << 5) + h + c;
return h;
}
// Add symbols to this symbol hash table. Note that this function
// destructively sort a given vector -- which is needed because
// GNU-style hash table places some sorting requirements.
void GnuHashTableSection::addSymbols(std::vector<SymbolTableEntry> &v) {
// We cannot use 'auto' for Mid because GCC 6.1 cannot deduce
// its type correctly.
std::vector<SymbolTableEntry>::iterator mid =
std::stable_partition(v.begin(), v.end(), [&](const SymbolTableEntry &s) {
return !s.sym->isDefined() || s.sym->partition != partition;
});
// We chose load factor 4 for the on-disk hash table. For each hash
// collision, the dynamic linker will compare a uint32_t hash value.
// Since the integer comparison is quite fast, we believe we can
// make the load factor even larger. 4 is just a conservative choice.
//
// Note that we don't want to create a zero-sized hash table because
// Android loader as of 2018 doesn't like a .gnu.hash containing such
// table. If that's the case, we create a hash table with one unused
// dummy slot.
nBuckets = std::max<size_t>((v.end() - mid) / 4, 1);
if (mid == v.end())
return;
for (SymbolTableEntry &ent : llvm::make_range(mid, v.end())) {
Symbol *b = ent.sym;
uint32_t hash = hashGnu(b->getName());
uint32_t bucketIdx = hash % nBuckets;
symbols.push_back({b, ent.strTabOffset, hash, bucketIdx});
}
llvm::stable_sort(symbols, [](const Entry &l, const Entry &r) {
return l.bucketIdx < r.bucketIdx;
});
v.erase(mid, v.end());
for (const Entry &ent : symbols)
v.push_back({ent.sym, ent.strTabOffset});
}
HashTableSection::HashTableSection()
: SyntheticSection(SHF_ALLOC, SHT_HASH, 4, ".hash") {
this->entsize = 4;
}
void HashTableSection::finalizeContents() {
SymbolTableBaseSection *symTab = getPartition().dynSymTab;
if (OutputSection *sec = symTab->getParent())
getParent()->link = sec->sectionIndex;
unsigned numEntries = 2; // nbucket and nchain.
numEntries += symTab->getNumSymbols(); // The chain entries.
// Create as many buckets as there are symbols.
numEntries += symTab->getNumSymbols();
this->size = numEntries * 4;
}
void HashTableSection::writeTo(uint8_t *buf) {
SymbolTableBaseSection *symTab = getPartition().dynSymTab;
// See comment in GnuHashTableSection::writeTo.
memset(buf, 0, size);
unsigned numSymbols = symTab->getNumSymbols();
uint32_t *p = reinterpret_cast<uint32_t *>(buf);
write32(p++, numSymbols); // nbucket
write32(p++, numSymbols); // nchain
uint32_t *buckets = p;
uint32_t *chains = p + numSymbols;
for (const SymbolTableEntry &s : symTab->getSymbols()) {
Symbol *sym = s.sym;
StringRef name = sym->getName();
unsigned i = sym->dynsymIndex;
uint32_t hash = hashSysV(name) % numSymbols;
chains[i] = buckets[hash];
write32(buckets + hash, i);
}
}
PltSection::PltSection()
: SyntheticSection(SHF_ALLOC | SHF_EXECINSTR, SHT_PROGBITS, 16, ".plt"),
headerSize(target->pltHeaderSize) {
// On PowerPC, this section contains lazy symbol resolvers.
if (config->emachine == EM_PPC64) {
name = ".glink";
alignment = 4;
}
// On x86 when IBT is enabled, this section contains the second PLT (lazy
// symbol resolvers).
if ((config->emachine == EM_386 || config->emachine == EM_X86_64) &&
(config->andFeatures & GNU_PROPERTY_X86_FEATURE_1_IBT))
name = ".plt.sec";
// The PLT needs to be writable on SPARC as the dynamic linker will
// modify the instructions in the PLT entries.
if (config->emachine == EM_SPARCV9)
this->flags |= SHF_WRITE;
}
void PltSection::writeTo(uint8_t *buf) {
// At beginning of PLT, we have code to call the dynamic
// linker to resolve dynsyms at runtime. Write such code.
target->writePltHeader(buf);
size_t off = headerSize;
for (const Symbol *sym : entries) {
target->writePlt(buf + off, *sym, getVA() + off);
off += target->pltEntrySize;
}
}
void PltSection::addEntry(Symbol &sym) {
sym.pltIndex = entries.size();
entries.push_back(&sym);
}
size_t PltSection::getSize() const {
return headerSize + entries.size() * target->pltEntrySize;
}
bool PltSection::isNeeded() const {
// For -z retpolineplt, .iplt needs the .plt header.
return !entries.empty() || (config->zRetpolineplt && in.iplt->isNeeded());
}
// Used by ARM to add mapping symbols in the PLT section, which aid
// disassembly.
void PltSection::addSymbols() {
target->addPltHeaderSymbols(*this);
size_t off = headerSize;
for (size_t i = 0; i < entries.size(); ++i) {
target->addPltSymbols(*this, off);
off += target->pltEntrySize;
}
}
IpltSection::IpltSection()
: SyntheticSection(SHF_ALLOC | SHF_EXECINSTR, SHT_PROGBITS, 16, ".iplt") {
if (config->emachine == EM_PPC || config->emachine == EM_PPC64) {
name = ".glink";
alignment = 4;
}
}
void IpltSection::writeTo(uint8_t *buf) {
uint32_t off = 0;
for (const Symbol *sym : entries) {
target->writeIplt(buf + off, *sym, getVA() + off);
off += target->ipltEntrySize;
}
}
size_t IpltSection::getSize() const {
return entries.size() * target->ipltEntrySize;
}
void IpltSection::addEntry(Symbol &sym) {
sym.pltIndex = entries.size();
entries.push_back(&sym);
}
// ARM uses mapping symbols to aid disassembly.
void IpltSection::addSymbols() {
size_t off = 0;
for (size_t i = 0, e = entries.size(); i != e; ++i) {
target->addPltSymbols(*this, off);
off += target->pltEntrySize;
}
}
PPC32GlinkSection::PPC32GlinkSection() {
name = ".glink";
alignment = 4;
}
void PPC32GlinkSection::writeTo(uint8_t *buf) {
writePPC32GlinkSection(buf, entries.size());
}
size_t PPC32GlinkSection::getSize() const {
return headerSize + entries.size() * target->pltEntrySize + footerSize;
}
// This is an x86-only extra PLT section and used only when a security
// enhancement feature called CET is enabled. In this comment, I'll explain what
// the feature is and why we have two PLT sections if CET is enabled.
//
// So, what does CET do? CET introduces a new restriction to indirect jump
// instructions. CET works this way. Assume that CET is enabled. Then, if you
// execute an indirect jump instruction, the processor verifies that a special
// "landing pad" instruction (which is actually a repurposed NOP instruction and
// now called "endbr32" or "endbr64") is at the jump target. If the jump target
// does not start with that instruction, the processor raises an exception
// instead of continuing executing code.
//
// If CET is enabled, the compiler emits endbr to all locations where indirect
// jumps may jump to.
//
// This mechanism makes it extremely hard to transfer the control to a middle of
// a function that is not supporsed to be a indirect jump target, preventing
// certain types of attacks such as ROP or JOP.
//
// Note that the processors in the market as of 2019 don't actually support the
// feature. Only the spec is available at the moment.
//
// Now, I'll explain why we have this extra PLT section for CET.
//
// Since you can indirectly jump to a PLT entry, we have to make PLT entries
// start with endbr. The problem is there's no extra space for endbr (which is 4
// bytes long), as the PLT entry is only 16 bytes long and all bytes are already
// used.
//
// In order to deal with the issue, we split a PLT entry into two PLT entries.
// Remember that each PLT entry contains code to jump to an address read from
// .got.plt AND code to resolve a dynamic symbol lazily. With the 2-PLT scheme,
// the former code is written to .plt.sec, and the latter code is written to
// .plt.
//
// Lazy symbol resolution in the 2-PLT scheme works in the usual way, except
// that the regular .plt is now called .plt.sec and .plt is repurposed to
// contain only code for lazy symbol resolution.
//
// In other words, this is how the 2-PLT scheme works. Application code is
// supposed to jump to .plt.sec to call an external function. Each .plt.sec
// entry contains code to read an address from a corresponding .got.plt entry
// and jump to that address. Addresses in .got.plt initially point to .plt, so
// when an application calls an external function for the first time, the
// control is transferred to a function that resolves a symbol name from
// external shared object files. That function then rewrites a .got.plt entry
// with a resolved address, so that the subsequent function calls directly jump
// to a desired location from .plt.sec.
//
// There is an open question as to whether the 2-PLT scheme was desirable or
// not. We could have simply extended the PLT entry size to 32-bytes to
// accommodate endbr, and that scheme would have been much simpler than the
// 2-PLT scheme. One reason to split PLT was, by doing that, we could keep hot
// code (.plt.sec) from cold code (.plt). But as far as I know no one proved
// that the optimization actually makes a difference.
//
// That said, the 2-PLT scheme is a part of the ABI, debuggers and other tools
// depend on it, so we implement the ABI.
IBTPltSection::IBTPltSection()
: SyntheticSection(SHF_ALLOC | SHF_EXECINSTR, SHT_PROGBITS, 16, ".plt") {}
void IBTPltSection::writeTo(uint8_t *buf) {
target->writeIBTPlt(buf, in.plt->getNumEntries());
}
size_t IBTPltSection::getSize() const {
// 16 is the header size of .plt.
return 16 + in.plt->getNumEntries() * target->pltEntrySize;
}
// The string hash function for .gdb_index.
static uint32_t computeGdbHash(StringRef s) {
uint32_t h = 0;
for (uint8_t c : s)
h = h * 67 + toLower(c) - 113;
return h;
}
GdbIndexSection::GdbIndexSection()
: SyntheticSection(0, SHT_PROGBITS, 1, ".gdb_index") {}
// Returns the desired size of an on-disk hash table for a .gdb_index section.
// There's a tradeoff between size and collision rate. We aim 75% utilization.
size_t GdbIndexSection::computeSymtabSize() const {
return std::max<size_t>(NextPowerOf2(symbols.size() * 4 / 3), 1024);
}
// Compute the output section size.
void GdbIndexSection::initOutputSize() {
size = sizeof(GdbIndexHeader) + computeSymtabSize() * 8;
for (GdbChunk &chunk : chunks)
size += chunk.compilationUnits.size() * 16 + chunk.addressAreas.size() * 20;
// Add the constant pool size if exists.
if (!symbols.empty()) {
GdbSymbol &sym = symbols.back();
size += sym.nameOff + sym.name.size() + 1;
}
}
static std::vector<GdbIndexSection::CuEntry> readCuList(DWARFContext &dwarf) {
std::vector<GdbIndexSection::CuEntry> ret;
for (std::unique_ptr<DWARFUnit> &cu : dwarf.compile_units())
ret.push_back({cu->getOffset(), cu->getLength() + 4});
return ret;
}
static std::vector<GdbIndexSection::AddressEntry>
readAddressAreas(DWARFContext &dwarf, InputSection *sec) {
std::vector<GdbIndexSection::AddressEntry> ret;
uint32_t cuIdx = 0;
for (std::unique_ptr<DWARFUnit> &cu : dwarf.compile_units()) {
if (Error e = cu->tryExtractDIEsIfNeeded(false)) {
warn(toString(sec) + ": " + toString(std::move(e)));
return {};
}
Expected<DWARFAddressRangesVector> ranges = cu->collectAddressRanges();
if (!ranges) {
warn(toString(sec) + ": " + toString(ranges.takeError()));
return {};
}
ArrayRef<InputSectionBase *> sections = sec->file->getSections();
for (DWARFAddressRange &r : *ranges) {
if (r.SectionIndex == -1ULL)
continue;
// Range list with zero size has no effect.
InputSectionBase *s = sections[r.SectionIndex];
if (s && s != &InputSection::discarded && s->isLive())
if (r.LowPC != r.HighPC)
ret.push_back({cast<InputSection>(s), r.LowPC, r.HighPC, cuIdx});
}
++cuIdx;
}
return ret;
}
template <class ELFT>
static std::vector<GdbIndexSection::NameAttrEntry>
readPubNamesAndTypes(const LLDDwarfObj<ELFT> &obj,
const std::vector<GdbIndexSection::CuEntry> &cus) {
const LLDDWARFSection &pubNames = obj.getGnuPubnamesSection();
const LLDDWARFSection &pubTypes = obj.getGnuPubtypesSection();
std::vector<GdbIndexSection::NameAttrEntry> ret;
for (const LLDDWARFSection *pub : {&pubNames, &pubTypes}) {
DWARFDataExtractor data(obj, *pub, config->isLE, config->wordsize);
DWARFDebugPubTable table;
table.extract(data, /*GnuStyle=*/true, [&](Error e) {
warn(toString(pub->sec) + ": " + toString(std::move(e)));
});
for (const DWARFDebugPubTable::Set &set : table.getData()) {
// The value written into the constant pool is kind << 24 | cuIndex. As we
// don't know how many compilation units precede this object to compute
// cuIndex, we compute (kind << 24 | cuIndexInThisObject) instead, and add
// the number of preceding compilation units later.
uint32_t i = llvm::partition_point(cus,
[&](GdbIndexSection::CuEntry cu) {
return cu.cuOffset < set.Offset;
}) -
cus.begin();
for (const DWARFDebugPubTable::Entry &ent : set.Entries)
ret.push_back({{ent.Name, computeGdbHash(ent.Name)},
(ent.Descriptor.toBits() << 24) | i});
}
}
return ret;
}
// Create a list of symbols from a given list of symbol names and types
// by uniquifying them by name.
static std::vector<GdbIndexSection::GdbSymbol>
createSymbols(ArrayRef<std::vector<GdbIndexSection::NameAttrEntry>> nameAttrs,
const std::vector<GdbIndexSection::GdbChunk> &chunks) {
using GdbSymbol = GdbIndexSection::GdbSymbol;
using NameAttrEntry = GdbIndexSection::NameAttrEntry;
// For each chunk, compute the number of compilation units preceding it.
uint32_t cuIdx = 0;
std::vector<uint32_t> cuIdxs(chunks.size());
for (uint32_t i = 0, e = chunks.size(); i != e; ++i) {
cuIdxs[i] = cuIdx;
cuIdx += chunks[i].compilationUnits.size();
}
// The number of symbols we will handle in this function is of the order
// of millions for very large executables, so we use multi-threading to
// speed it up.
constexpr size_t numShards = 32;
size_t concurrency = PowerOf2Floor(
std::min<size_t>(hardware_concurrency(parallel::strategy.ThreadsRequested)
.compute_thread_count(),
numShards));
// A sharded map to uniquify symbols by name.
std::vector<DenseMap<CachedHashStringRef, size_t>> map(numShards);
size_t shift = 32 - countTrailingZeros(numShards);
// Instantiate GdbSymbols while uniqufying them by name.
std::vector<std::vector<GdbSymbol>> symbols(numShards);
parallelForEachN(0, concurrency, [&](size_t threadId) {
uint32_t i = 0;
for (ArrayRef<NameAttrEntry> entries : nameAttrs) {
for (const NameAttrEntry &ent : entries) {
size_t shardId = ent.name.hash() >> shift;
if ((shardId & (concurrency - 1)) != threadId)
continue;
uint32_t v = ent.cuIndexAndAttrs + cuIdxs[i];
size_t &idx = map[shardId][ent.name];
if (idx) {
symbols[shardId][idx - 1].cuVector.push_back(v);
continue;
}
idx = symbols[shardId].size() + 1;
symbols[shardId].push_back({ent.name, {v}, 0, 0});
}
++i;
}
});
size_t numSymbols = 0;
for (ArrayRef<GdbSymbol> v : symbols)
numSymbols += v.size();
// The return type is a flattened vector, so we'll copy each vector
// contents to Ret.
std::vector<GdbSymbol> ret;
ret.reserve(numSymbols);
for (std::vector<GdbSymbol> &vec : symbols)
for (GdbSymbol &sym : vec)
ret.push_back(std::move(sym));
// CU vectors and symbol names are adjacent in the output file.
// We can compute their offsets in the output file now.
size_t off = 0;
for (GdbSymbol &sym : ret) {
sym.cuVectorOff = off;
off += (sym.cuVector.size() + 1) * 4;
}
for (GdbSymbol &sym : ret) {
sym.nameOff = off;
off += sym.name.size() + 1;
}
return ret;
}
// Returns a newly-created .gdb_index section.
template <class ELFT> GdbIndexSection *GdbIndexSection::create() {
// Collect InputFiles with .debug_info. See the comment in
// LLDDwarfObj<ELFT>::LLDDwarfObj. If we do lightweight parsing in the future,
// note that isec->data() may uncompress the full content, which should be
// parallelized.
SetVector<InputFile *> files;
for (InputSectionBase *s : inputSections) {
InputSection *isec = dyn_cast<InputSection>(s);
if (!isec)
continue;
// .debug_gnu_pub{names,types} are useless in executables.
// They are present in input object files solely for creating
// a .gdb_index. So we can remove them from the output.
if (s->name == ".debug_gnu_pubnames" || s->name == ".debug_gnu_pubtypes")
s->markDead();
else if (isec->name == ".debug_info")
files.insert(isec->file);
}
// Drop .rel[a].debug_gnu_pub{names,types} for --emit-relocs.
llvm::erase_if(inputSections, [](InputSectionBase *s) {
if (auto *isec = dyn_cast<InputSection>(s))
if (InputSectionBase *rel = isec->getRelocatedSection())
return !rel->isLive();
return !s->isLive();
});
std::vector<GdbChunk> chunks(files.size());
std::vector<std::vector<NameAttrEntry>> nameAttrs(files.size());
parallelForEachN(0, files.size(), [&](size_t i) {
// To keep memory usage low, we don't want to keep cached DWARFContext, so
// avoid getDwarf() here.
ObjFile<ELFT> *file = cast<ObjFile<ELFT>>(files[i]);
DWARFContext dwarf(std::make_unique<LLDDwarfObj<ELFT>>(file));
auto &dobj = static_cast<const LLDDwarfObj<ELFT> &>(dwarf.getDWARFObj());
// If the are multiple compile units .debug_info (very rare ld -r --unique),
// this only picks the last one. Other address ranges are lost.
chunks[i].sec = dobj.getInfoSection();
chunks[i].compilationUnits = readCuList(dwarf);
chunks[i].addressAreas = readAddressAreas(dwarf, chunks[i].sec);
nameAttrs[i] = readPubNamesAndTypes<ELFT>(dobj, chunks[i].compilationUnits);
});
auto *ret = make<GdbIndexSection>();
ret->chunks = std::move(chunks);
ret->symbols = createSymbols(nameAttrs, ret->chunks);
ret->initOutputSize();
return ret;
}
void GdbIndexSection::writeTo(uint8_t *buf) {
// Write the header.
auto *hdr = reinterpret_cast<GdbIndexHeader *>(buf);
uint8_t *start = buf;
hdr->version = 7;
buf += sizeof(*hdr);
// Write the CU list.
hdr->cuListOff = buf - start;
for (GdbChunk &chunk : chunks) {
for (CuEntry &cu : chunk.compilationUnits) {
write64le(buf, chunk.sec->outSecOff + cu.cuOffset);
write64le(buf + 8, cu.cuLength);
buf += 16;
}
}
// Write the address area.
hdr->cuTypesOff = buf - start;
hdr->addressAreaOff = buf - start;
uint32_t cuOff = 0;
for (GdbChunk &chunk : chunks) {
for (AddressEntry &e : chunk.addressAreas) {
// In the case of ICF there may be duplicate address range entries.
const uint64_t baseAddr = e.section->repl->getVA(0);
write64le(buf, baseAddr + e.lowAddress);
write64le(buf + 8, baseAddr + e.highAddress);
write32le(buf + 16, e.cuIndex + cuOff);
buf += 20;
}
cuOff += chunk.compilationUnits.size();
}
// Write the on-disk open-addressing hash table containing symbols.
hdr->symtabOff = buf - start;
size_t symtabSize = computeSymtabSize();
uint32_t mask = symtabSize - 1;
for (GdbSymbol &sym : symbols) {
uint32_t h = sym.name.hash();
uint32_t i = h & mask;
uint32_t step = ((h * 17) & mask) | 1;
while (read32le(buf + i * 8))
i = (i + step) & mask;
write32le(buf + i * 8, sym.nameOff);
write32le(buf + i * 8 + 4, sym.cuVectorOff);
}
buf += symtabSize * 8;
// Write the string pool.
hdr->constantPoolOff = buf - start;
parallelForEach(symbols, [&](GdbSymbol &sym) {
memcpy(buf + sym.nameOff, sym.name.data(), sym.name.size());
});
// Write the CU vectors.
for (GdbSymbol &sym : symbols) {
write32le(buf, sym.cuVector.size());
buf += 4;
for (uint32_t val : sym.cuVector) {
write32le(buf, val);
buf += 4;
}
}
}
bool GdbIndexSection::isNeeded() const { return !chunks.empty(); }
EhFrameHeader::EhFrameHeader()
: SyntheticSection(SHF_ALLOC, SHT_PROGBITS, 4, ".eh_frame_hdr") {}
void EhFrameHeader::writeTo(uint8_t *buf) {
// Unlike most sections, the EhFrameHeader section is written while writing
// another section, namely EhFrameSection, which calls the write() function
// below from its writeTo() function. This is necessary because the contents
// of EhFrameHeader depend on the relocated contents of EhFrameSection and we
// don't know which order the sections will be written in.
}
// .eh_frame_hdr contains a binary search table of pointers to FDEs.
// Each entry of the search table consists of two values,
// the starting PC from where FDEs covers, and the FDE's address.
// It is sorted by PC.
void EhFrameHeader::write() {
uint8_t *buf = Out::bufferStart + getParent()->offset + outSecOff;
using FdeData = EhFrameSection::FdeData;
std::vector<FdeData> fdes = getPartition().ehFrame->getFdeData();
buf[0] = 1;
buf[1] = DW_EH_PE_pcrel | DW_EH_PE_sdata4;
buf[2] = DW_EH_PE_udata4;
buf[3] = DW_EH_PE_datarel | DW_EH_PE_sdata4;
write32(buf + 4,
getPartition().ehFrame->getParent()->addr - this->getVA() - 4);
write32(buf + 8, fdes.size());
buf += 12;
for (FdeData &fde : fdes) {
write32(buf, fde.pcRel);
write32(buf + 4, fde.fdeVARel);
buf += 8;
}
}
size_t EhFrameHeader::getSize() const {
// .eh_frame_hdr has a 12 bytes header followed by an array of FDEs.
return 12 + getPartition().ehFrame->numFdes * 8;
}
bool EhFrameHeader::isNeeded() const {
return isLive() && getPartition().ehFrame->isNeeded();
}
VersionDefinitionSection::VersionDefinitionSection()
: SyntheticSection(SHF_ALLOC, SHT_GNU_verdef, sizeof(uint32_t),
".gnu.version_d") {}
StringRef VersionDefinitionSection::getFileDefName() {
if (!getPartition().name.empty())
return getPartition().name;
if (!config->soName.empty())
return config->soName;
return config->outputFile;
}
void VersionDefinitionSection::finalizeContents() {
fileDefNameOff = getPartition().dynStrTab->addString(getFileDefName());
for (const VersionDefinition &v : namedVersionDefs())
verDefNameOffs.push_back(getPartition().dynStrTab->addString(v.name));
if (OutputSection *sec = getPartition().dynStrTab->getParent())
getParent()->link = sec->sectionIndex;
// sh_info should be set to the number of definitions. This fact is missed in
// documentation, but confirmed by binutils community:
// https://sourceware.org/ml/binutils/2014-11/msg00355.html
getParent()->info = getVerDefNum();
}
void VersionDefinitionSection::writeOne(uint8_t *buf, uint32_t index,
StringRef name, size_t nameOff) {
uint16_t flags = index == 1 ? VER_FLG_BASE : 0;
// Write a verdef.
write16(buf, 1); // vd_version
write16(buf + 2, flags); // vd_flags
write16(buf + 4, index); // vd_ndx
write16(buf + 6, 1); // vd_cnt
write32(buf + 8, hashSysV(name)); // vd_hash
write32(buf + 12, 20); // vd_aux
write32(buf + 16, 28); // vd_next
// Write a veraux.
write32(buf + 20, nameOff); // vda_name
write32(buf + 24, 0); // vda_next
}
void VersionDefinitionSection::writeTo(uint8_t *buf) {
writeOne(buf, 1, getFileDefName(), fileDefNameOff);
auto nameOffIt = verDefNameOffs.begin();
for (const VersionDefinition &v : namedVersionDefs()) {
buf += EntrySize;
writeOne(buf, v.id, v.name, *nameOffIt++);
}
// Need to terminate the last version definition.
write32(buf + 16, 0); // vd_next
}
size_t VersionDefinitionSection::getSize() const {
return EntrySize * getVerDefNum();
}
// .gnu.version is a table where each entry is 2 byte long.
VersionTableSection::VersionTableSection()
: SyntheticSection(SHF_ALLOC, SHT_GNU_versym, sizeof(uint16_t),
".gnu.version") {
this->entsize = 2;
}
void VersionTableSection::finalizeContents() {
// At the moment of june 2016 GNU docs does not mention that sh_link field
// should be set, but Sun docs do. Also readelf relies on this field.
getParent()->link = getPartition().dynSymTab->getParent()->sectionIndex;
}
size_t VersionTableSection::getSize() const {
return (getPartition().dynSymTab->getSymbols().size() + 1) * 2;
}
void VersionTableSection::writeTo(uint8_t *buf) {
buf += 2;
for (const SymbolTableEntry &s : getPartition().dynSymTab->getSymbols()) {
// For an unfetched lazy symbol (undefined weak), it must have been
// converted to Undefined and have VER_NDX_GLOBAL version here.
assert(!s.sym->isLazy());
write16(buf, s.sym->versionId);
buf += 2;
}
}
bool VersionTableSection::isNeeded() const {
return isLive() &&
(getPartition().verDef || getPartition().verNeed->isNeeded());
}
void elf::addVerneed(Symbol *ss) {
auto &file = cast<SharedFile>(*ss->file);
if (ss->verdefIndex == VER_NDX_GLOBAL) {
ss->versionId = VER_NDX_GLOBAL;
return;
}
if (file.vernauxs.empty())
file.vernauxs.resize(file.verdefs.size());
// Select a version identifier for the vernaux data structure, if we haven't
// already allocated one. The verdef identifiers cover the range
// [1..getVerDefNum()]; this causes the vernaux identifiers to start from
// getVerDefNum()+1.
if (file.vernauxs[ss->verdefIndex] == 0)
file.vernauxs[ss->verdefIndex] = ++SharedFile::vernauxNum + getVerDefNum();
ss->versionId = file.vernauxs[ss->verdefIndex];
}
template <class ELFT>
VersionNeedSection<ELFT>::VersionNeedSection()
: SyntheticSection(SHF_ALLOC, SHT_GNU_verneed, sizeof(uint32_t),
".gnu.version_r") {}
template <class ELFT> void VersionNeedSection<ELFT>::finalizeContents() {
for (SharedFile *f : sharedFiles) {
if (f->vernauxs.empty())
continue;
verneeds.emplace_back();
Verneed &vn = verneeds.back();
vn.nameStrTab = getPartition().dynStrTab->addString(f->soName);
for (unsigned i = 0; i != f->vernauxs.size(); ++i) {
if (f->vernauxs[i] == 0)
continue;
auto *verdef =
reinterpret_cast<const typename ELFT::Verdef *>(f->verdefs[i]);
vn.vernauxs.push_back(
{verdef->vd_hash, f->vernauxs[i],
getPartition().dynStrTab->addString(f->getStringTable().data() +
verdef->getAux()->vda_name)});
}
}
if (OutputSection *sec = getPartition().dynStrTab->getParent())
getParent()->link = sec->sectionIndex;
getParent()->info = verneeds.size();
}
template <class ELFT> void VersionNeedSection<ELFT>::writeTo(uint8_t *buf) {
// The Elf_Verneeds need to appear first, followed by the Elf_Vernauxs.
auto *verneed = reinterpret_cast<Elf_Verneed *>(buf);
auto *vernaux = reinterpret_cast<Elf_Vernaux *>(verneed + verneeds.size());
for (auto &vn : verneeds) {
// Create an Elf_Verneed for this DSO.
verneed->vn_version = 1;
verneed->vn_cnt = vn.vernauxs.size();
verneed->vn_file = vn.nameStrTab;
verneed->vn_aux =
reinterpret_cast<char *>(vernaux) - reinterpret_cast<char *>(verneed);
verneed->vn_next = sizeof(Elf_Verneed);
++verneed;
// Create the Elf_Vernauxs for this Elf_Verneed.
for (auto &vna : vn.vernauxs) {
vernaux->vna_hash = vna.hash;
vernaux->vna_flags = 0;
vernaux->vna_other = vna.verneedIndex;
vernaux->vna_name = vna.nameStrTab;
vernaux->vna_next = sizeof(Elf_Vernaux);
++vernaux;
}
vernaux[-1].vna_next = 0;
}
verneed[-1].vn_next = 0;
}
template <class ELFT> size_t VersionNeedSection<ELFT>::getSize() const {
return verneeds.size() * sizeof(Elf_Verneed) +
SharedFile::vernauxNum * sizeof(Elf_Vernaux);
}
template <class ELFT> bool VersionNeedSection<ELFT>::isNeeded() const {
return isLive() && SharedFile::vernauxNum != 0;
}
void MergeSyntheticSection::addSection(MergeInputSection *ms) {
ms->parent = this;
sections.push_back(ms);
assert(alignment == ms->alignment || !(ms->flags & SHF_STRINGS));
alignment = std::max(alignment, ms->alignment);
}
MergeTailSection::MergeTailSection(StringRef name, uint32_t type,
uint64_t flags, uint32_t alignment)
: MergeSyntheticSection(name, type, flags, alignment),
builder(StringTableBuilder::RAW, alignment) {}
size_t MergeTailSection::getSize() const { return builder.getSize(); }
void MergeTailSection::writeTo(uint8_t *buf) { builder.write(buf); }
void MergeTailSection::finalizeContents() {
// Add all string pieces to the string table builder to create section
// contents.
for (MergeInputSection *sec : sections)
for (size_t i = 0, e = sec->pieces.size(); i != e; ++i)
if (sec->pieces[i].live)
builder.add(sec->getData(i));
// Fix the string table content. After this, the contents will never change.
builder.finalize();
// finalize() fixed tail-optimized strings, so we can now get
// offsets of strings. Get an offset for each string and save it
// to a corresponding SectionPiece for easy access.
for (MergeInputSection *sec : sections)
for (size_t i = 0, e = sec->pieces.size(); i != e; ++i)
if (sec->pieces[i].live)
sec->pieces[i].outputOff = builder.getOffset(sec->getData(i));
}
void MergeNoTailSection::writeTo(uint8_t *buf) {
for (size_t i = 0; i < numShards; ++i)
shards[i].write(buf + shardOffsets[i]);
}
// This function is very hot (i.e. it can take several seconds to finish)
// because sometimes the number of inputs is in an order of magnitude of
// millions. So, we use multi-threading.
//
// For any strings S and T, we know S is not mergeable with T if S's hash
// value is different from T's. If that's the case, we can safely put S and
// T into different string builders without worrying about merge misses.
// We do it in parallel.
void MergeNoTailSection::finalizeContents() {
// Initializes string table builders.
for (size_t i = 0; i < numShards; ++i)
shards.emplace_back(StringTableBuilder::RAW, alignment);
// Concurrency level. Must be a power of 2 to avoid expensive modulo
// operations in the following tight loop.
size_t concurrency = PowerOf2Floor(
std::min<size_t>(hardware_concurrency(parallel::strategy.ThreadsRequested)
.compute_thread_count(),
numShards));
// Add section pieces to the builders.
parallelForEachN(0, concurrency, [&](size_t threadId) {
for (MergeInputSection *sec : sections) {
for (size_t i = 0, e = sec->pieces.size(); i != e; ++i) {
if (!sec->pieces[i].live)
continue;
size_t shardId = getShardId(sec->pieces[i].hash);
if ((shardId & (concurrency - 1)) == threadId)
sec->pieces[i].outputOff = shards[shardId].add(sec->getData(i));
}
}
});
// Compute an in-section offset for each shard.
size_t off = 0;
for (size_t i = 0; i < numShards; ++i) {
shards[i].finalizeInOrder();
if (shards[i].getSize() > 0)
off = alignTo(off, alignment);
shardOffsets[i] = off;
off += shards[i].getSize();
}
size = off;
// So far, section pieces have offsets from beginning of shards, but
// we want offsets from beginning of the whole section. Fix them.
parallelForEach(sections, [&](MergeInputSection *sec) {
for (size_t i = 0, e = sec->pieces.size(); i != e; ++i)
if (sec->pieces[i].live)
sec->pieces[i].outputOff +=
shardOffsets[getShardId(sec->pieces[i].hash)];
});
}
MergeSyntheticSection *elf::createMergeSynthetic(StringRef name, uint32_t type,
uint64_t flags,
uint32_t alignment) {
bool shouldTailMerge = (flags & SHF_STRINGS) && config->optimize >= 2;
if (shouldTailMerge)
return make<MergeTailSection>(name, type, flags, alignment);
return make<MergeNoTailSection>(name, type, flags, alignment);
}
template <class ELFT> void elf::splitSections() {
llvm::TimeTraceScope timeScope("Split sections");
// splitIntoPieces needs to be called on each MergeInputSection
// before calling finalizeContents().
parallelForEach(inputSections, [](InputSectionBase *sec) {
if (auto *s = dyn_cast<MergeInputSection>(sec))
s->splitIntoPieces();
else if (auto *eh = dyn_cast<EhInputSection>(sec))
eh->split<ELFT>();
});
}
MipsRldMapSection::MipsRldMapSection()
: SyntheticSection(SHF_ALLOC | SHF_WRITE, SHT_PROGBITS, config->wordsize,
".rld_map") {}
ARMExidxSyntheticSection::ARMExidxSyntheticSection()
: SyntheticSection(SHF_ALLOC | SHF_LINK_ORDER, SHT_ARM_EXIDX,
config->wordsize, ".ARM.exidx") {}
static InputSection *findExidxSection(InputSection *isec) {
for (InputSection *d : isec->dependentSections)
if (d->type == SHT_ARM_EXIDX && d->isLive())
return d;
return nullptr;
}
static bool isValidExidxSectionDep(InputSection *isec) {
return (isec->flags & SHF_ALLOC) && (isec->flags & SHF_EXECINSTR) &&
isec->getSize() > 0;
}
bool ARMExidxSyntheticSection::addSection(InputSection *isec) {
if (isec->type == SHT_ARM_EXIDX) {
if (InputSection *dep = isec->getLinkOrderDep())
if (isValidExidxSectionDep(dep)) {
exidxSections.push_back(isec);
// Every exidxSection is 8 bytes, we need an estimate of
// size before assignAddresses can be called. Final size
// will only be known after finalize is called.
size += 8;
}
return true;
}
if (isValidExidxSectionDep(isec)) {
executableSections.push_back(isec);
return false;
}
// FIXME: we do not output a relocation section when --emit-relocs is used
// as we do not have relocation sections for linker generated table entries
// and we would have to erase at a late stage relocations from merged entries.
// Given that exception tables are already position independent and a binary
// analyzer could derive the relocations we choose to erase the relocations.
if (config->emitRelocs && isec->type == SHT_REL)
if (InputSectionBase *ex = isec->getRelocatedSection())
if (isa<InputSection>(ex) && ex->type == SHT_ARM_EXIDX)
return true;
return false;
}
// References to .ARM.Extab Sections have bit 31 clear and are not the
// special EXIDX_CANTUNWIND bit-pattern.
static bool isExtabRef(uint32_t unwind) {
return (unwind & 0x80000000) == 0 && unwind != 0x1;
}
// Return true if the .ARM.exidx section Cur can be merged into the .ARM.exidx
// section Prev, where Cur follows Prev in the table. This can be done if the
// unwinding instructions in Cur are identical to Prev. Linker generated
// EXIDX_CANTUNWIND entries are represented by nullptr as they do not have an
// InputSection.
static bool isDuplicateArmExidxSec(InputSection *prev, InputSection *cur) {
struct ExidxEntry {
ulittle32_t fn;
ulittle32_t unwind;
};
// Get the last table Entry from the previous .ARM.exidx section. If Prev is
// nullptr then it will be a synthesized EXIDX_CANTUNWIND entry.
ExidxEntry prevEntry = {ulittle32_t(0), ulittle32_t(1)};
if (prev)
prevEntry = prev->getDataAs<ExidxEntry>().back();
if (isExtabRef(prevEntry.unwind))
return false;
// We consider the unwind instructions of an .ARM.exidx table entry
// a duplicate if the previous unwind instructions if:
// - Both are the special EXIDX_CANTUNWIND.
// - Both are the same inline unwind instructions.
// We do not attempt to follow and check links into .ARM.extab tables as
// consecutive identical entries are rare and the effort to check that they
// are identical is high.
// If Cur is nullptr then this is synthesized EXIDX_CANTUNWIND entry.
if (cur == nullptr)
return prevEntry.unwind == 1;
for (const ExidxEntry entry : cur->getDataAs<ExidxEntry>())
if (isExtabRef(entry.unwind) || entry.unwind != prevEntry.unwind)
return false;
// All table entries in this .ARM.exidx Section can be merged into the
// previous Section.
return true;
}
// The .ARM.exidx table must be sorted in ascending order of the address of the
// functions the table describes. Optionally duplicate adjacent table entries
// can be removed. At the end of the function the executableSections must be
// sorted in ascending order of address, Sentinel is set to the InputSection
// with the highest address and any InputSections that have mergeable
// .ARM.exidx table entries are removed from it.
void ARMExidxSyntheticSection::finalizeContents() {
// The executableSections and exidxSections that we use to derive the final
// contents of this SyntheticSection are populated before
// processSectionCommands() and ICF. A /DISCARD/ entry in SECTIONS command or
// ICF may remove executable InputSections and their dependent .ARM.exidx
// section that we recorded earlier.
auto isDiscarded = [](const InputSection *isec) { return !isec->isLive(); };
llvm::erase_if(exidxSections, isDiscarded);
// We need to remove discarded InputSections and InputSections without
// .ARM.exidx sections that if we generated the .ARM.exidx it would be out
// of range.
auto isDiscardedOrOutOfRange = [this](InputSection *isec) {
if (!isec->isLive())
return true;
if (findExidxSection(isec))
return false;
int64_t off = static_cast<int64_t>(isec->getVA() - getVA());
return off != llvm::SignExtend64(off, 31);
};
llvm::erase_if(executableSections, isDiscardedOrOutOfRange);
// Sort the executable sections that may or may not have associated
// .ARM.exidx sections by order of ascending address. This requires the
// relative positions of InputSections and OutputSections to be known.
auto compareByFilePosition = [](const InputSection *a,
const InputSection *b) {
OutputSection *aOut = a->getParent();
OutputSection *bOut = b->getParent();
if (aOut != bOut)
return aOut->addr < bOut->addr;
return a->outSecOff < b->outSecOff;
};
llvm::stable_sort(executableSections, compareByFilePosition);
sentinel = executableSections.back();
// Optionally merge adjacent duplicate entries.
if (config->mergeArmExidx) {
std::vector<InputSection *> selectedSections;
selectedSections.reserve(executableSections.size());
selectedSections.push_back(executableSections[0]);
size_t prev = 0;
for (size_t i = 1; i < executableSections.size(); ++i) {
InputSection *ex1 = findExidxSection(executableSections[prev]);
InputSection *ex2 = findExidxSection(executableSections[i]);
if (!isDuplicateArmExidxSec(ex1, ex2)) {
selectedSections.push_back(executableSections[i]);
prev = i;
}
}
executableSections = std::move(selectedSections);
}
size_t offset = 0;
size = 0;
for (InputSection *isec : executableSections) {
if (InputSection *d = findExidxSection(isec)) {
d->outSecOff = offset;
d->parent = getParent();
offset += d->getSize();
} else {
offset += 8;
}
}
// Size includes Sentinel.
size = offset + 8;
}
InputSection *ARMExidxSyntheticSection::getLinkOrderDep() const {
return executableSections.front();
}
// To write the .ARM.exidx table from the ExecutableSections we have three cases
// 1.) The InputSection has a .ARM.exidx InputSection in its dependent sections.
// We write the .ARM.exidx section contents and apply its relocations.
// 2.) The InputSection does not have a dependent .ARM.exidx InputSection. We
// must write the contents of an EXIDX_CANTUNWIND directly. We use the
// start of the InputSection as the purpose of the linker generated
// section is to terminate the address range of the previous entry.
// 3.) A trailing EXIDX_CANTUNWIND sentinel section is required at the end of
// the table to terminate the address range of the final entry.
void ARMExidxSyntheticSection::writeTo(uint8_t *buf) {
const uint8_t cantUnwindData[8] = {0, 0, 0, 0, // PREL31 to target
1, 0, 0, 0}; // EXIDX_CANTUNWIND
uint64_t offset = 0;
for (InputSection *isec : executableSections) {
assert(isec->getParent() != nullptr);
if (InputSection *d = findExidxSection(isec)) {
memcpy(buf + offset, d->data().data(), d->data().size());
d->relocateAlloc(buf + d->outSecOff, buf + d->outSecOff + d->getSize());
offset += d->getSize();
} else {
// A Linker generated CANTUNWIND section.
memcpy(buf + offset, cantUnwindData, sizeof(cantUnwindData));
uint64_t s = isec->getVA();
uint64_t p = getVA() + offset;
target->relocateNoSym(buf + offset, R_ARM_PREL31, s - p);
offset += 8;
}
}
// Write Sentinel.
memcpy(buf + offset, cantUnwindData, sizeof(cantUnwindData));
uint64_t s = sentinel->getVA(sentinel->getSize());
uint64_t p = getVA() + offset;
target->relocateNoSym(buf + offset, R_ARM_PREL31, s - p);
assert(size == offset + 8);
}
bool ARMExidxSyntheticSection::isNeeded() const {
return llvm::any_of(exidxSections,
[](InputSection *isec) { return isec->isLive(); });
}
bool ARMExidxSyntheticSection::classof(const SectionBase *d) {
return d->kind() == InputSectionBase::Synthetic && d->type == SHT_ARM_EXIDX;
}
ThunkSection::ThunkSection(OutputSection *os, uint64_t off)
: SyntheticSection(SHF_ALLOC | SHF_EXECINSTR, SHT_PROGBITS,
config->emachine == EM_PPC64 ? 16 : 4, ".text.thunk") {
this->parent = os;
this->outSecOff = off;
}
size_t ThunkSection::getSize() const {
if (roundUpSizeForErrata)
return alignTo(size, 4096);
return size;
}
void ThunkSection::addThunk(Thunk *t) {
thunks.push_back(t);
t->addSymbols(*this);
}
void ThunkSection::writeTo(uint8_t *buf) {
for (Thunk *t : thunks)
t->writeTo(buf + t->offset);
}
InputSection *ThunkSection::getTargetInputSection() const {
if (thunks.empty())
return nullptr;
const Thunk *t = thunks.front();
return t->getTargetInputSection();
}
bool ThunkSection::assignOffsets() {
uint64_t off = 0;
for (Thunk *t : thunks) {
off = alignTo(off, t->alignment);
t->setOffset(off);
uint32_t size = t->size();
t->getThunkTargetSym()->size = size;
off += size;
}
bool changed = off != size;
size = off;
return changed;
}
PPC32Got2Section::PPC32Got2Section()
: SyntheticSection(SHF_ALLOC | SHF_WRITE, SHT_PROGBITS, 4, ".got2") {}
bool PPC32Got2Section::isNeeded() const {
// See the comment below. This is not needed if there is no other
// InputSection.
for (BaseCommand *base : getParent()->sectionCommands)
if (auto *isd = dyn_cast<InputSectionDescription>(base))
for (InputSection *isec : isd->sections)
if (isec != this)
return true;
return false;
}
void PPC32Got2Section::finalizeContents() {
// PPC32 may create multiple GOT sections for -fPIC/-fPIE, one per file in
// .got2 . This function computes outSecOff of each .got2 to be used in
// PPC32PltCallStub::writeTo(). The purpose of this empty synthetic section is
// to collect input sections named ".got2".
uint32_t offset = 0;
for (BaseCommand *base : getParent()->sectionCommands)
if (auto *isd = dyn_cast<InputSectionDescription>(base)) {
for (InputSection *isec : isd->sections) {
if (isec == this)
continue;
isec->file->ppc32Got2OutSecOff = offset;
offset += (uint32_t)isec->getSize();
}
}
}
// If linking position-dependent code then the table will store the addresses
// directly in the binary so the section has type SHT_PROGBITS. If linking
// position-independent code the section has type SHT_NOBITS since it will be
// allocated and filled in by the dynamic linker.
PPC64LongBranchTargetSection::PPC64LongBranchTargetSection()
: SyntheticSection(SHF_ALLOC | SHF_WRITE,
config->isPic ? SHT_NOBITS : SHT_PROGBITS, 8,
".branch_lt") {}
uint64_t PPC64LongBranchTargetSection::getEntryVA(const Symbol *sym,
int64_t addend) {
return getVA() + entry_index.find({sym, addend})->second * 8;
}
Optional<uint32_t> PPC64LongBranchTargetSection::addEntry(const Symbol *sym,
int64_t addend) {
auto res =
entry_index.try_emplace(std::make_pair(sym, addend), entries.size());
if (!res.second)
return None;
entries.emplace_back(sym, addend);
return res.first->second;
}
size_t PPC64LongBranchTargetSection::getSize() const {
return entries.size() * 8;
}
void PPC64LongBranchTargetSection::writeTo(uint8_t *buf) {
// If linking non-pic we have the final addresses of the targets and they get
// written to the table directly. For pic the dynamic linker will allocate
// the section and fill it it.
if (config->isPic)
return;
for (auto entry : entries) {
const Symbol *sym = entry.first;
int64_t addend = entry.second;
assert(sym->getVA());
// Need calls to branch to the local entry-point since a long-branch
// must be a local-call.
write64(buf, sym->getVA(addend) +
getPPC64GlobalEntryToLocalEntryOffset(sym->stOther));
buf += 8;
}
}
bool PPC64LongBranchTargetSection::isNeeded() const {
// `removeUnusedSyntheticSections()` is called before thunk allocation which
// is too early to determine if this section will be empty or not. We need
// Finalized to keep the section alive until after thunk creation. Finalized
// only gets set to true once `finalizeSections()` is called after thunk
// creation. Because of this, if we don't create any long-branch thunks we end
// up with an empty .branch_lt section in the binary.
return !finalized || !entries.empty();
}
static uint8_t getAbiVersion() {
// MIPS non-PIC executable gets ABI version 1.
if (config->emachine == EM_MIPS) {
if (!config->isPic && !config->relocatable &&
(config->eflags & (EF_MIPS_PIC | EF_MIPS_CPIC)) == EF_MIPS_CPIC)
return 1;
return 0;
}
if (config->emachine == EM_AMDGPU) {
uint8_t ver = objectFiles[0]->abiVersion;
for (InputFile *file : makeArrayRef(objectFiles).slice(1))
if (file->abiVersion != ver)
error("incompatible ABI version: " + toString(file));
return ver;
}
return 0;
}
template <typename ELFT> void elf::writeEhdr(uint8_t *buf, Partition &part) {
// For executable segments, the trap instructions are written before writing
// the header. Setting Elf header bytes to zero ensures that any unused bytes
// in header are zero-cleared, instead of having trap instructions.
memset(buf, 0, sizeof(typename ELFT::Ehdr));
memcpy(buf, "\177ELF", 4);
auto *eHdr = reinterpret_cast<typename ELFT::Ehdr *>(buf);
eHdr->e_ident[EI_CLASS] = config->is64 ? ELFCLASS64 : ELFCLASS32;
eHdr->e_ident[EI_DATA] = config->isLE ? ELFDATA2LSB : ELFDATA2MSB;
eHdr->e_ident[EI_VERSION] = EV_CURRENT;
eHdr->e_ident[EI_OSABI] = config->osabi;
eHdr->e_ident[EI_ABIVERSION] = getAbiVersion();
eHdr->e_machine = config->emachine;
eHdr->e_version = EV_CURRENT;
eHdr->e_flags = config->eflags;
eHdr->e_ehsize = sizeof(typename ELFT::Ehdr);
eHdr->e_phnum = part.phdrs.size();
eHdr->e_shentsize = sizeof(typename ELFT::Shdr);
if (!config->relocatable) {
eHdr->e_phoff = sizeof(typename ELFT::Ehdr);
eHdr->e_phentsize = sizeof(typename ELFT::Phdr);
}
}
template <typename ELFT> void elf::writePhdrs(uint8_t *buf, Partition &part) {
// Write the program header table.
auto *hBuf = reinterpret_cast<typename ELFT::Phdr *>(buf);
for (PhdrEntry *p : part.phdrs) {
hBuf->p_type = p->p_type;
hBuf->p_flags = p->p_flags;
hBuf->p_offset = p->p_offset;
hBuf->p_vaddr = p->p_vaddr;
hBuf->p_paddr = p->p_paddr;
hBuf->p_filesz = p->p_filesz;
hBuf->p_memsz = p->p_memsz;
hBuf->p_align = p->p_align;
++hBuf;
}
}
template <typename ELFT>
PartitionElfHeaderSection<ELFT>::PartitionElfHeaderSection()
: SyntheticSection(SHF_ALLOC, SHT_LLVM_PART_EHDR, 1, "") {}
template <typename ELFT>
size_t PartitionElfHeaderSection<ELFT>::getSize() const {
return sizeof(typename ELFT::Ehdr);
}
template <typename ELFT>
void PartitionElfHeaderSection<ELFT>::writeTo(uint8_t *buf) {
writeEhdr<ELFT>(buf, getPartition());
// Loadable partitions are always ET_DYN.
auto *eHdr = reinterpret_cast<typename ELFT::Ehdr *>(buf);
eHdr->e_type = ET_DYN;
}
template <typename ELFT>
PartitionProgramHeadersSection<ELFT>::PartitionProgramHeadersSection()
: SyntheticSection(SHF_ALLOC, SHT_LLVM_PART_PHDR, 1, ".phdrs") {}
template <typename ELFT>
size_t PartitionProgramHeadersSection<ELFT>::getSize() const {
return sizeof(typename ELFT::Phdr) * getPartition().phdrs.size();
}
template <typename ELFT>
void PartitionProgramHeadersSection<ELFT>::writeTo(uint8_t *buf) {
writePhdrs<ELFT>(buf, getPartition());
}
PartitionIndexSection::PartitionIndexSection()
: SyntheticSection(SHF_ALLOC, SHT_PROGBITS, 4, ".rodata") {}
size_t PartitionIndexSection::getSize() const {
return 12 * (partitions.size() - 1);
}
void PartitionIndexSection::finalizeContents() {
for (size_t i = 1; i != partitions.size(); ++i)
partitions[i].nameStrTab = mainPart->dynStrTab->addString(partitions[i].name);
}
void PartitionIndexSection::writeTo(uint8_t *buf) {
uint64_t va = getVA();
for (size_t i = 1; i != partitions.size(); ++i) {
write32(buf, mainPart->dynStrTab->getVA() + partitions[i].nameStrTab - va);
write32(buf + 4, partitions[i].elfHeader->getVA() - (va + 4));
SyntheticSection *next =
i == partitions.size() - 1 ? in.partEnd : partitions[i + 1].elfHeader;
write32(buf + 8, next->getVA() - partitions[i].elfHeader->getVA());
va += 12;
buf += 12;
}
}
InStruct elf::in;
std::vector<Partition> elf::partitions;
Partition *elf::mainPart;
template GdbIndexSection *GdbIndexSection::create<ELF32LE>();
template GdbIndexSection *GdbIndexSection::create<ELF32BE>();
template GdbIndexSection *GdbIndexSection::create<ELF64LE>();
template GdbIndexSection *GdbIndexSection::create<ELF64BE>();
template void elf::splitSections<ELF32LE>();
template void elf::splitSections<ELF32BE>();
template void elf::splitSections<ELF64LE>();
template void elf::splitSections<ELF64BE>();
template class elf::MipsAbiFlagsSection<ELF32LE>;
template class elf::MipsAbiFlagsSection<ELF32BE>;
template class elf::MipsAbiFlagsSection<ELF64LE>;
template class elf::MipsAbiFlagsSection<ELF64BE>;
template class elf::MipsOptionsSection<ELF32LE>;
template class elf::MipsOptionsSection<ELF32BE>;
template class elf::MipsOptionsSection<ELF64LE>;
template class elf::MipsOptionsSection<ELF64BE>;
template void EhFrameSection::iterateFDEWithLSDA<ELF32LE>(
function_ref<void(InputSection &)>);
template void EhFrameSection::iterateFDEWithLSDA<ELF32BE>(
function_ref<void(InputSection &)>);
template void EhFrameSection::iterateFDEWithLSDA<ELF64LE>(
function_ref<void(InputSection &)>);
template void EhFrameSection::iterateFDEWithLSDA<ELF64BE>(
function_ref<void(InputSection &)>);
template class elf::MipsReginfoSection<ELF32LE>;
template class elf::MipsReginfoSection<ELF32BE>;
template class elf::MipsReginfoSection<ELF64LE>;
template class elf::MipsReginfoSection<ELF64BE>;
template class elf::DynamicSection<ELF32LE>;
template class elf::DynamicSection<ELF32BE>;
template class elf::DynamicSection<ELF64LE>;
template class elf::DynamicSection<ELF64BE>;
template class elf::RelocationSection<ELF32LE>;
template class elf::RelocationSection<ELF32BE>;
template class elf::RelocationSection<ELF64LE>;
template class elf::RelocationSection<ELF64BE>;
template class elf::AndroidPackedRelocationSection<ELF32LE>;
template class elf::AndroidPackedRelocationSection<ELF32BE>;
template class elf::AndroidPackedRelocationSection<ELF64LE>;
template class elf::AndroidPackedRelocationSection<ELF64BE>;
template class elf::RelrSection<ELF32LE>;
template class elf::RelrSection<ELF32BE>;
template class elf::RelrSection<ELF64LE>;
template class elf::RelrSection<ELF64BE>;
template class elf::SymbolTableSection<ELF32LE>;
template class elf::SymbolTableSection<ELF32BE>;
template class elf::SymbolTableSection<ELF64LE>;
template class elf::SymbolTableSection<ELF64BE>;
template class elf::VersionNeedSection<ELF32LE>;
template class elf::VersionNeedSection<ELF32BE>;
template class elf::VersionNeedSection<ELF64LE>;
template class elf::VersionNeedSection<ELF64BE>;
template void elf::writeEhdr<ELF32LE>(uint8_t *Buf, Partition &Part);
template void elf::writeEhdr<ELF32BE>(uint8_t *Buf, Partition &Part);
template void elf::writeEhdr<ELF64LE>(uint8_t *Buf, Partition &Part);
template void elf::writeEhdr<ELF64BE>(uint8_t *Buf, Partition &Part);
template void elf::writePhdrs<ELF32LE>(uint8_t *Buf, Partition &Part);
template void elf::writePhdrs<ELF32BE>(uint8_t *Buf, Partition &Part);
template void elf::writePhdrs<ELF64LE>(uint8_t *Buf, Partition &Part);
template void elf::writePhdrs<ELF64BE>(uint8_t *Buf, Partition &Part);
template class elf::PartitionElfHeaderSection<ELF32LE>;
template class elf::PartitionElfHeaderSection<ELF32BE>;
template class elf::PartitionElfHeaderSection<ELF64LE>;
template class elf::PartitionElfHeaderSection<ELF64BE>;
template class elf::PartitionProgramHeadersSection<ELF32LE>;
template class elf::PartitionProgramHeadersSection<ELF32BE>;
template class elf::PartitionProgramHeadersSection<ELF64LE>;
template class elf::PartitionProgramHeadersSection<ELF64BE>;