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llvm / lld / 0a49d36eee0f66cf76a7ab356ed884123be5fcf5 / . / ELF / Writer.cpp
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//===- Writer.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
//
//===----------------------------------------------------------------------===//
#include "Writer.h"
#include "AArch64ErrataFix.h"
#include "ARMErrataFix.h"
#include "CallGraphSort.h"
#include "Config.h"
#include "LinkerScript.h"
#include "MapFile.h"
#include "OutputSections.h"
#include "Relocations.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "lld/Common/Filesystem.h"
#include "lld/Common/Memory.h"
#include "lld/Common/Strings.h"
#include "lld/Common/Threads.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/Support/RandomNumberGenerator.h"
#include "llvm/Support/SHA1.h"
#include "llvm/Support/xxhash.h"
#include <climits>
using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace llvm::support;
using namespace llvm::support::endian;
using namespace lld;
using namespace lld::elf;
namespace {
// The writer writes a SymbolTable result to a file.
template <class ELFT> class Writer {
public:
Writer() : buffer(errorHandler().outputBuffer) {}
using Elf_Shdr = typename ELFT::Shdr;
using Elf_Ehdr = typename ELFT::Ehdr;
using Elf_Phdr = typename ELFT::Phdr;
void run();
private:
void copyLocalSymbols();
void addSectionSymbols();
void forEachRelSec(llvm::function_ref<void(InputSectionBase &)> fn);
void sortSections();
void resolveShfLinkOrder();
void finalizeAddressDependentContent();
void sortInputSections();
void finalizeSections();
void checkExecuteOnly();
void setReservedSymbolSections();
std::vector<PhdrEntry *> createPhdrs(Partition &part);
void addPhdrForSection(Partition &part, unsigned shType, unsigned pType,
unsigned pFlags);
void assignFileOffsets();
void assignFileOffsetsBinary();
void setPhdrs(Partition &part);
void checkSections();
void fixSectionAlignments();
void openFile();
void writeTrapInstr();
void writeHeader();
void writeSections();
void writeSectionsBinary();
void writeBuildId();
std::unique_ptr<FileOutputBuffer> &buffer;
void addRelIpltSymbols();
void addStartEndSymbols();
void addStartStopSymbols(OutputSection *sec);
uint64_t fileSize;
uint64_t sectionHeaderOff;
};
} // anonymous namespace
static bool isSectionPrefix(StringRef prefix, StringRef name) {
return name.startswith(prefix) || name == prefix.drop_back();
}
StringRef elf::getOutputSectionName(const InputSectionBase *s) {
if (config->relocatable)
return s->name;
// This is for --emit-relocs. If .text.foo is emitted as .text.bar, we want
// to emit .rela.text.foo as .rela.text.bar for consistency (this is not
// technically required, but not doing it is odd). This code guarantees that.
if (auto *isec = dyn_cast<InputSection>(s)) {
if (InputSectionBase *rel = isec->getRelocatedSection()) {
OutputSection *out = rel->getOutputSection();
if (s->type == SHT_RELA)
return saver.save(".rela" + out->name);
return saver.save(".rel" + out->name);
}
}
// This check is for -z keep-text-section-prefix. This option separates text
// sections with prefix ".text.hot", ".text.unlikely", ".text.startup" or
// ".text.exit".
// When enabled, this allows identifying the hot code region (.text.hot) in
// the final binary which can be selectively mapped to huge pages or mlocked,
// for instance.
if (config->zKeepTextSectionPrefix)
for (StringRef v :
{".text.hot.", ".text.unlikely.", ".text.startup.", ".text.exit."})
if (isSectionPrefix(v, s->name))
return v.drop_back();
for (StringRef v :
{".text.", ".rodata.", ".data.rel.ro.", ".data.", ".bss.rel.ro.",
".bss.", ".init_array.", ".fini_array.", ".ctors.", ".dtors.", ".tbss.",
".gcc_except_table.", ".tdata.", ".ARM.exidx.", ".ARM.extab."})
if (isSectionPrefix(v, s->name))
return v.drop_back();
// CommonSection is identified as "COMMON" in linker scripts.
// By default, it should go to .bss section.
if (s->name == "COMMON")
return ".bss";
return s->name;
}
static bool needsInterpSection() {
return !sharedFiles.empty() && !config->dynamicLinker.empty() &&
script->needsInterpSection();
}
template <class ELFT> void elf::writeResult() { Writer<ELFT>().run(); }
static void removeEmptyPTLoad(std::vector<PhdrEntry *> &phdrs) {
llvm::erase_if(phdrs, [&](const PhdrEntry *p) {
if (p->p_type != PT_LOAD)
return false;
if (!p->firstSec)
return true;
uint64_t size = p->lastSec->addr + p->lastSec->size - p->firstSec->addr;
return size == 0;
});
}
void elf::copySectionsIntoPartitions() {
std::vector<InputSectionBase *> newSections;
for (unsigned part = 2; part != partitions.size() + 1; ++part) {
for (InputSectionBase *s : inputSections) {
if (!(s->flags & SHF_ALLOC) || !s->isLive())
continue;
InputSectionBase *copy;
if (s->type == SHT_NOTE)
copy = make<InputSection>(cast<InputSection>(*s));
else if (auto *es = dyn_cast<EhInputSection>(s))
copy = make<EhInputSection>(*es);
else
continue;
copy->partition = part;
newSections.push_back(copy);
}
}
inputSections.insert(inputSections.end(), newSections.begin(),
newSections.end());
}
void elf::combineEhSections() {
for (InputSectionBase *&s : inputSections) {
// Ignore dead sections and the partition end marker (.part.end),
// whose partition number is out of bounds.
if (!s->isLive() || s->partition == 255)
continue;
Partition &part = s->getPartition();
if (auto *es = dyn_cast<EhInputSection>(s)) {
part.ehFrame->addSection(es);
s = nullptr;
} else if (s->kind() == SectionBase::Regular && part.armExidx &&
part.armExidx->addSection(cast<InputSection>(s))) {
s = nullptr;
}
}
std::vector<InputSectionBase *> &v = inputSections;
v.erase(std::remove(v.begin(), v.end(), nullptr), v.end());
}
static Defined *addOptionalRegular(StringRef name, SectionBase *sec,
uint64_t val, uint8_t stOther = STV_HIDDEN,
uint8_t binding = STB_GLOBAL) {
Symbol *s = symtab->find(name);
if (!s || s->isDefined())
return nullptr;
s->resolve(Defined{/*file=*/nullptr, name, binding, stOther, STT_NOTYPE, val,
/*size=*/0, sec});
return cast<Defined>(s);
}
static Defined *addAbsolute(StringRef name) {
Symbol *sym = symtab->addSymbol(Defined{nullptr, name, STB_GLOBAL, STV_HIDDEN,
STT_NOTYPE, 0, 0, nullptr});
return cast<Defined>(sym);
}
// The linker is expected to define some symbols depending on
// the linking result. This function defines such symbols.
void elf::addReservedSymbols() {
if (config->emachine == EM_MIPS) {
// Define _gp for MIPS. st_value of _gp symbol will be updated by Writer
// so that it points to an absolute address which by default is relative
// to GOT. Default offset is 0x7ff0.
// See "Global Data Symbols" in Chapter 6 in the following document:
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
ElfSym::mipsGp = addAbsolute("_gp");
// On MIPS O32 ABI, _gp_disp is a magic symbol designates offset between
// start of function and 'gp' pointer into GOT.
if (symtab->find("_gp_disp"))
ElfSym::mipsGpDisp = addAbsolute("_gp_disp");
// The __gnu_local_gp is a magic symbol equal to the current value of 'gp'
// pointer. This symbol is used in the code generated by .cpload pseudo-op
// in case of using -mno-shared option.
// https://sourceware.org/ml/binutils/2004-12/msg00094.html
if (symtab->find("__gnu_local_gp"))
ElfSym::mipsLocalGp = addAbsolute("__gnu_local_gp");
} else if (config->emachine == EM_PPC) {
// glibc *crt1.o has a undefined reference to _SDA_BASE_. Since we don't
// support Small Data Area, define it arbitrarily as 0.
addOptionalRegular("_SDA_BASE_", nullptr, 0, STV_HIDDEN);
}
// The Power Architecture 64-bit v2 ABI defines a TableOfContents (TOC) which
// combines the typical ELF GOT with the small data sections. It commonly
// includes .got .toc .sdata .sbss. The .TOC. symbol replaces both
// _GLOBAL_OFFSET_TABLE_ and _SDA_BASE_ from the 32-bit ABI. It is used to
// represent the TOC base which is offset by 0x8000 bytes from the start of
// the .got section.
// We do not allow _GLOBAL_OFFSET_TABLE_ to be defined by input objects as the
// correctness of some relocations depends on its value.
StringRef gotSymName =
(config->emachine == EM_PPC64) ? ".TOC." : "_GLOBAL_OFFSET_TABLE_";
if (Symbol *s = symtab->find(gotSymName)) {
if (s->isDefined()) {
error(toString(s->file) + " cannot redefine linker defined symbol '" +
gotSymName + "'");
return;
}
uint64_t gotOff = 0;
if (config->emachine == EM_PPC64)
gotOff = 0x8000;
s->resolve(Defined{/*file=*/nullptr, gotSymName, STB_GLOBAL, STV_HIDDEN,
STT_NOTYPE, gotOff, /*size=*/0, Out::elfHeader});
ElfSym::globalOffsetTable = cast<Defined>(s);
}
// __ehdr_start is the location of ELF file headers. Note that we define
// this symbol unconditionally even when using a linker script, which
// differs from the behavior implemented by GNU linker which only define
// this symbol if ELF headers are in the memory mapped segment.
addOptionalRegular("__ehdr_start", Out::elfHeader, 0, STV_HIDDEN);
// __executable_start is not documented, but the expectation of at
// least the Android libc is that it points to the ELF header.
addOptionalRegular("__executable_start", Out::elfHeader, 0, STV_HIDDEN);
// __dso_handle symbol is passed to cxa_finalize as a marker to identify
// each DSO. The address of the symbol doesn't matter as long as they are
// different in different DSOs, so we chose the start address of the DSO.
addOptionalRegular("__dso_handle", Out::elfHeader, 0, STV_HIDDEN);
// If linker script do layout we do not need to create any standart symbols.
if (script->hasSectionsCommand)
return;
auto add = [](StringRef s, int64_t pos) {
return addOptionalRegular(s, Out::elfHeader, pos, STV_DEFAULT);
};
ElfSym::bss = add("__bss_start", 0);
ElfSym::end1 = add("end", -1);
ElfSym::end2 = add("_end", -1);
ElfSym::etext1 = add("etext", -1);
ElfSym::etext2 = add("_etext", -1);
ElfSym::edata1 = add("edata", -1);
ElfSym::edata2 = add("_edata", -1);
}
static OutputSection *findSection(StringRef name, unsigned partition = 1) {
for (BaseCommand *base : script->sectionCommands)
if (auto *sec = dyn_cast<OutputSection>(base))
if (sec->name == name && sec->partition == partition)
return sec;
return nullptr;
}
template <class ELFT> void elf::createSyntheticSections() {
// Initialize all pointers with NULL. This is needed because
// you can call lld::elf::main more than once as a library.
memset(&Out::first, 0, sizeof(Out));
auto add = [](InputSectionBase *sec) { inputSections.push_back(sec); };
in.shStrTab = make<StringTableSection>(".shstrtab", false);
Out::programHeaders = make<OutputSection>("", 0, SHF_ALLOC);
Out::programHeaders->alignment = config->wordsize;
if (config->strip != StripPolicy::All) {
in.strTab = make<StringTableSection>(".strtab", false);
in.symTab = make<SymbolTableSection<ELFT>>(*in.strTab);
in.symTabShndx = make<SymtabShndxSection>();
}
in.bss = make<BssSection>(".bss", 0, 1);
add(in.bss);
// If there is a SECTIONS command and a .data.rel.ro section name use name
// .data.rel.ro.bss so that we match in the .data.rel.ro output section.
// This makes sure our relro is contiguous.
bool hasDataRelRo =
script->hasSectionsCommand && findSection(".data.rel.ro", 0);
in.bssRelRo =
make<BssSection>(hasDataRelRo ? ".data.rel.ro.bss" : ".bss.rel.ro", 0, 1);
add(in.bssRelRo);
// Add MIPS-specific sections.
if (config->emachine == EM_MIPS) {
if (!config->shared && config->hasDynSymTab) {
in.mipsRldMap = make<MipsRldMapSection>();
add(in.mipsRldMap);
}
if (auto *sec = MipsAbiFlagsSection<ELFT>::create())
add(sec);
if (auto *sec = MipsOptionsSection<ELFT>::create())
add(sec);
if (auto *sec = MipsReginfoSection<ELFT>::create())
add(sec);
}
StringRef relaDynName = config->isRela ? ".rela.dyn" : ".rel.dyn";
for (Partition &part : partitions) {
auto add = [&](InputSectionBase *sec) {
sec->partition = part.getNumber();
inputSections.push_back(sec);
};
if (!part.name.empty()) {
part.elfHeader = make<PartitionElfHeaderSection<ELFT>>();
part.elfHeader->name = part.name;
add(part.elfHeader);
part.programHeaders = make<PartitionProgramHeadersSection<ELFT>>();
add(part.programHeaders);
}
if (config->buildId != BuildIdKind::None) {
part.buildId = make<BuildIdSection>();
add(part.buildId);
}
part.dynStrTab = make<StringTableSection>(".dynstr", true);
part.dynSymTab = make<SymbolTableSection<ELFT>>(*part.dynStrTab);
part.dynamic = make<DynamicSection<ELFT>>();
if (config->androidPackDynRelocs)
part.relaDyn = make<AndroidPackedRelocationSection<ELFT>>(relaDynName);
else
part.relaDyn =
make<RelocationSection<ELFT>>(relaDynName, config->zCombreloc);
if (needsInterpSection())
add(createInterpSection());
if (config->hasDynSymTab) {
part.dynSymTab = make<SymbolTableSection<ELFT>>(*part.dynStrTab);
add(part.dynSymTab);
part.verSym = make<VersionTableSection>();
add(part.verSym);
if (!namedVersionDefs().empty()) {
part.verDef = make<VersionDefinitionSection>();
add(part.verDef);
}
part.verNeed = make<VersionNeedSection<ELFT>>();
add(part.verNeed);
if (config->gnuHash) {
part.gnuHashTab = make<GnuHashTableSection>();
add(part.gnuHashTab);
}
if (config->sysvHash) {
part.hashTab = make<HashTableSection>();
add(part.hashTab);
}
add(part.dynamic);
add(part.dynStrTab);
add(part.relaDyn);
}
if (config->relrPackDynRelocs) {
part.relrDyn = make<RelrSection<ELFT>>();
add(part.relrDyn);
}
if (!config->relocatable) {
if (config->ehFrameHdr) {
part.ehFrameHdr = make<EhFrameHeader>();
add(part.ehFrameHdr);
}
part.ehFrame = make<EhFrameSection>();
add(part.ehFrame);
}
if (config->emachine == EM_ARM && !config->relocatable) {
// The ARMExidxsyntheticsection replaces all the individual .ARM.exidx
// InputSections.
part.armExidx = make<ARMExidxSyntheticSection>();
add(part.armExidx);
}
}
if (partitions.size() != 1) {
// Create the partition end marker. This needs to be in partition number 255
// so that it is sorted after all other partitions. It also has other
// special handling (see createPhdrs() and combineEhSections()).
in.partEnd = make<BssSection>(".part.end", config->maxPageSize, 1);
in.partEnd->partition = 255;
add(in.partEnd);
in.partIndex = make<PartitionIndexSection>();
addOptionalRegular("__part_index_begin", in.partIndex, 0);
addOptionalRegular("__part_index_end", in.partIndex,
in.partIndex->getSize());
add(in.partIndex);
}
// Add .got. MIPS' .got is so different from the other archs,
// it has its own class.
if (config->emachine == EM_MIPS) {
in.mipsGot = make<MipsGotSection>();
add(in.mipsGot);
} else {
in.got = make<GotSection>();
add(in.got);
}
if (config->emachine == EM_PPC) {
in.ppc32Got2 = make<PPC32Got2Section>();
add(in.ppc32Got2);
}
if (config->emachine == EM_PPC64) {
in.ppc64LongBranchTarget = make<PPC64LongBranchTargetSection>();
add(in.ppc64LongBranchTarget);
}
in.gotPlt = make<GotPltSection>();
add(in.gotPlt);
in.igotPlt = make<IgotPltSection>();
add(in.igotPlt);
// _GLOBAL_OFFSET_TABLE_ is defined relative to either .got.plt or .got. Treat
// it as a relocation and ensure the referenced section is created.
if (ElfSym::globalOffsetTable && config->emachine != EM_MIPS) {
if (target->gotBaseSymInGotPlt)
in.gotPlt->hasGotPltOffRel = true;
else
in.got->hasGotOffRel = true;
}
if (config->gdbIndex)
add(GdbIndexSection::create<ELFT>());
// We always need to add rel[a].plt to output if it has entries.
// Even for static linking it can contain R_[*]_IRELATIVE relocations.
in.relaPlt = make<RelocationSection<ELFT>>(
config->isRela ? ".rela.plt" : ".rel.plt", /*sort=*/false);
add(in.relaPlt);
// The relaIplt immediately follows .rel[a].dyn to ensure that the IRelative
// relocations are processed last by the dynamic loader. We cannot place the
// iplt section in .rel.dyn when Android relocation packing is enabled because
// that would cause a section type mismatch. However, because the Android
// dynamic loader reads .rel.plt after .rel.dyn, we can get the desired
// behaviour by placing the iplt section in .rel.plt.
in.relaIplt = make<RelocationSection<ELFT>>(
config->androidPackDynRelocs ? in.relaPlt->name : relaDynName,
/*sort=*/false);
add(in.relaIplt);
in.plt = make<PltSection>(false);
add(in.plt);
in.iplt = make<PltSection>(true);
add(in.iplt);
if (config->andFeatures)
add(make<GnuPropertySection>());
// .note.GNU-stack is always added when we are creating a re-linkable
// object file. Other linkers are using the presence of this marker
// section to control the executable-ness of the stack area, but that
// is irrelevant these days. Stack area should always be non-executable
// by default. So we emit this section unconditionally.
if (config->relocatable)
add(make<GnuStackSection>());
if (in.symTab)
add(in.symTab);
if (in.symTabShndx)
add(in.symTabShndx);
add(in.shStrTab);
if (in.strTab)
add(in.strTab);
}
// The main function of the writer.
template <class ELFT> void Writer<ELFT>::run() {
if (config->discard != DiscardPolicy::All)
copyLocalSymbols();
if (config->copyRelocs)
addSectionSymbols();
// Now that we have a complete set of output sections. This function
// completes section contents. For example, we need to add strings
// to the string table, and add entries to .got and .plt.
// finalizeSections does that.
finalizeSections();
checkExecuteOnly();
if (errorCount())
return;
// If -compressed-debug-sections is specified, we need to compress
// .debug_* sections. Do it right now because it changes the size of
// output sections.
for (OutputSection *sec : outputSections)
sec->maybeCompress<ELFT>();
if (script->hasSectionsCommand)
script->allocateHeaders(mainPart->phdrs);
// Remove empty PT_LOAD to avoid causing the dynamic linker to try to mmap a
// 0 sized region. This has to be done late since only after assignAddresses
// we know the size of the sections.
for (Partition &part : partitions)
removeEmptyPTLoad(part.phdrs);
if (!config->oFormatBinary)
assignFileOffsets();
else
assignFileOffsetsBinary();
for (Partition &part : partitions)
setPhdrs(part);
if (config->relocatable)
for (OutputSection *sec : outputSections)
sec->addr = 0;
if (config->checkSections)
checkSections();
// It does not make sense try to open the file if we have error already.
if (errorCount())
return;
// Write the result down to a file.
openFile();
if (errorCount())
return;
if (!config->oFormatBinary) {
writeTrapInstr();
writeHeader();
writeSections();
} else {
writeSectionsBinary();
}
// Backfill .note.gnu.build-id section content. This is done at last
// because the content is usually a hash value of the entire output file.
writeBuildId();
if (errorCount())
return;
// Handle -Map and -cref options.
writeMapFile();
writeCrossReferenceTable();
if (errorCount())
return;
if (auto e = buffer->commit())
error("failed to write to the output file: " + toString(std::move(e)));
}
static bool shouldKeepInSymtab(const Defined &sym) {
if (sym.isSection())
return false;
if (config->discard == DiscardPolicy::None)
return true;
// If -emit-reloc is given, all symbols including local ones need to be
// copied because they may be referenced by relocations.
if (config->emitRelocs)
return true;
// In ELF assembly .L symbols are normally discarded by the assembler.
// If the assembler fails to do so, the linker discards them if
// * --discard-locals is used.
// * The symbol is in a SHF_MERGE section, which is normally the reason for
// the assembler keeping the .L symbol.
StringRef name = sym.getName();
bool isLocal = name.startswith(".L") || name.empty();
if (!isLocal)
return true;
if (config->discard == DiscardPolicy::Locals)
return false;
SectionBase *sec = sym.section;
return !sec || !(sec->flags & SHF_MERGE);
}
static bool includeInSymtab(const Symbol &b) {
if (!b.isLocal() && !b.isUsedInRegularObj)
return false;
if (auto *d = dyn_cast<Defined>(&b)) {
// Always include absolute symbols.
SectionBase *sec = d->section;
if (!sec)
return true;
sec = sec->repl;
// Exclude symbols pointing to garbage-collected sections.
if (isa<InputSectionBase>(sec) && !sec->isLive())
return false;
if (auto *s = dyn_cast<MergeInputSection>(sec))
if (!s->getSectionPiece(d->value)->live)
return false;
return true;
}
return b.used;
}
// Local symbols are not in the linker's symbol table. This function scans
// each object file's symbol table to copy local symbols to the output.
template <class ELFT> void Writer<ELFT>::copyLocalSymbols() {
if (!in.symTab)
return;
for (InputFile *file : objectFiles) {
ObjFile<ELFT> *f = cast<ObjFile<ELFT>>(file);
for (Symbol *b : f->getLocalSymbols()) {
if (!b->isLocal())
fatal(toString(f) +
": broken object: getLocalSymbols returns a non-local symbol");
auto *dr = dyn_cast<Defined>(b);
// No reason to keep local undefined symbol in symtab.
if (!dr)
continue;
if (!includeInSymtab(*b))
continue;
if (!shouldKeepInSymtab(*dr))
continue;
in.symTab->addSymbol(b);
}
}
}
// Create a section symbol for each output section so that we can represent
// relocations that point to the section. If we know that no relocation is
// referring to a section (that happens if the section is a synthetic one), we
// don't create a section symbol for that section.
template <class ELFT> void Writer<ELFT>::addSectionSymbols() {
for (BaseCommand *base : script->sectionCommands) {
auto *sec = dyn_cast<OutputSection>(base);
if (!sec)
continue;
auto i = llvm::find_if(sec->sectionCommands, [](BaseCommand *base) {
if (auto *isd = dyn_cast<InputSectionDescription>(base))
return !isd->sections.empty();
return false;
});
if (i == sec->sectionCommands.end())
continue;
InputSection *isec = cast<InputSectionDescription>(*i)->sections[0];
// Relocations are not using REL[A] section symbols.
if (isec->type == SHT_REL || isec->type == SHT_RELA)
continue;
// Unlike other synthetic sections, mergeable output sections contain data
// copied from input sections, and there may be a relocation pointing to its
// contents if -r or -emit-reloc are given.
if (isa<SyntheticSection>(isec) && !(isec->flags & SHF_MERGE))
continue;
auto *sym =
make<Defined>(isec->file, "", STB_LOCAL, /*stOther=*/0, STT_SECTION,
/*value=*/0, /*size=*/0, isec);
in.symTab->addSymbol(sym);
}
}
// Today's loaders have a feature to make segments read-only after
// processing dynamic relocations to enhance security. PT_GNU_RELRO
// is defined for that.
//
// This function returns true if a section needs to be put into a
// PT_GNU_RELRO segment.
static bool isRelroSection(const OutputSection *sec) {
if (!config->zRelro)
return false;
uint64_t flags = sec->flags;
// Non-allocatable or non-writable sections don't need RELRO because
// they are not writable or not even mapped to memory in the first place.
// RELRO is for sections that are essentially read-only but need to
// be writable only at process startup to allow dynamic linker to
// apply relocations.
if (!(flags & SHF_ALLOC) || !(flags & SHF_WRITE))
return false;
// Once initialized, TLS data segments are used as data templates
// for a thread-local storage. For each new thread, runtime
// allocates memory for a TLS and copy templates there. No thread
// are supposed to use templates directly. Thus, it can be in RELRO.
if (flags & SHF_TLS)
return true;
// .init_array, .preinit_array and .fini_array contain pointers to
// functions that are executed on process startup or exit. These
// pointers are set by the static linker, and they are not expected
// to change at runtime. But if you are an attacker, you could do
// interesting things by manipulating pointers in .fini_array, for
// example. So they are put into RELRO.
uint32_t type = sec->type;
if (type == SHT_INIT_ARRAY || type == SHT_FINI_ARRAY ||
type == SHT_PREINIT_ARRAY)
return true;
// .got contains pointers to external symbols. They are resolved by
// the dynamic linker when a module is loaded into memory, and after
// that they are not expected to change. So, it can be in RELRO.
if (in.got && sec == in.got->getParent())
return true;
// .toc is a GOT-ish section for PowerPC64. Their contents are accessed
// through r2 register, which is reserved for that purpose. Since r2 is used
// for accessing .got as well, .got and .toc need to be close enough in the
// virtual address space. Usually, .toc comes just after .got. Since we place
// .got into RELRO, .toc needs to be placed into RELRO too.
if (sec->name.equals(".toc"))
return true;
// .got.plt contains pointers to external function symbols. They are
// by default resolved lazily, so we usually cannot put it into RELRO.
// However, if "-z now" is given, the lazy symbol resolution is
// disabled, which enables us to put it into RELRO.
if (sec == in.gotPlt->getParent())
return config->zNow;
// .dynamic section contains data for the dynamic linker, and
// there's no need to write to it at runtime, so it's better to put
// it into RELRO.
if (sec->name == ".dynamic")
return true;
// Sections with some special names are put into RELRO. This is a
// bit unfortunate because section names shouldn't be significant in
// ELF in spirit. But in reality many linker features depend on
// magic section names.
StringRef s = sec->name;
return s == ".data.rel.ro" || s == ".bss.rel.ro" || s == ".ctors" ||
s == ".dtors" || s == ".jcr" || s == ".eh_frame" ||
s == ".openbsd.randomdata";
}
// We compute a rank for each section. The rank indicates where the
// section should be placed in the file. Instead of using simple
// numbers (0,1,2...), we use a series of flags. One for each decision
// point when placing the section.
// Using flags has two key properties:
// * It is easy to check if a give branch was taken.
// * It is easy two see how similar two ranks are (see getRankProximity).
enum RankFlags {
RF_NOT_ADDR_SET = 1 << 27,
RF_NOT_ALLOC = 1 << 26,
RF_PARTITION = 1 << 18, // Partition number (8 bits)
RF_NOT_PART_EHDR = 1 << 17,
RF_NOT_PART_PHDR = 1 << 16,
RF_NOT_INTERP = 1 << 15,
RF_NOT_NOTE = 1 << 14,
RF_WRITE = 1 << 13,
RF_EXEC_WRITE = 1 << 12,
RF_EXEC = 1 << 11,
RF_RODATA = 1 << 10,
RF_NOT_RELRO = 1 << 9,
RF_NOT_TLS = 1 << 8,
RF_BSS = 1 << 7,
RF_PPC_NOT_TOCBSS = 1 << 6,
RF_PPC_TOCL = 1 << 5,
RF_PPC_TOC = 1 << 4,
RF_PPC_GOT = 1 << 3,
RF_PPC_BRANCH_LT = 1 << 2,
RF_MIPS_GPREL = 1 << 1,
RF_MIPS_NOT_GOT = 1 << 0
};
static unsigned getSectionRank(const OutputSection *sec) {
unsigned rank = sec->partition * RF_PARTITION;
// We want to put section specified by -T option first, so we
// can start assigning VA starting from them later.
if (config->sectionStartMap.count(sec->name))
return rank;
rank |= RF_NOT_ADDR_SET;
// Allocatable sections go first to reduce the total PT_LOAD size and
// so debug info doesn't change addresses in actual code.
if (!(sec->flags & SHF_ALLOC))
return rank | RF_NOT_ALLOC;
if (sec->type == SHT_LLVM_PART_EHDR)
return rank;
rank |= RF_NOT_PART_EHDR;
if (sec->type == SHT_LLVM_PART_PHDR)
return rank;
rank |= RF_NOT_PART_PHDR;
// Put .interp first because some loaders want to see that section
// on the first page of the executable file when loaded into memory.
if (sec->name == ".interp")
return rank;
rank |= RF_NOT_INTERP;
// Put .note sections (which make up one PT_NOTE) at the beginning so that
// they are likely to be included in a core file even if core file size is
// limited. In particular, we want a .note.gnu.build-id and a .note.tag to be
// included in a core to match core files with executables.
if (sec->type == SHT_NOTE)
return rank;
rank |= RF_NOT_NOTE;
// Sort sections based on their access permission in the following
// order: R, RX, RWX, RW. This order is based on the following
// considerations:
// * Read-only sections come first such that they go in the
// PT_LOAD covering the program headers at the start of the file.
// * Read-only, executable sections come next.
// * Writable, executable sections follow such that .plt on
// architectures where it needs to be writable will be placed
// between .text and .data.
// * Writable sections come last, such that .bss lands at the very
// end of the last PT_LOAD.
bool isExec = sec->flags & SHF_EXECINSTR;
bool isWrite = sec->flags & SHF_WRITE;
if (isExec) {
if (isWrite)
rank |= RF_EXEC_WRITE;
else
rank |= RF_EXEC;
} else if (isWrite) {
rank |= RF_WRITE;
} else if (sec->type == SHT_PROGBITS) {
// Make non-executable and non-writable PROGBITS sections (e.g .rodata
// .eh_frame) closer to .text. They likely contain PC or GOT relative
// relocations and there could be relocation overflow if other huge sections
// (.dynstr .dynsym) were placed in between.
rank |= RF_RODATA;
}
// Place RelRo sections first. After considering SHT_NOBITS below, the
// ordering is PT_LOAD(PT_GNU_RELRO(.data.rel.ro .bss.rel.ro) | .data .bss),
// where | marks where page alignment happens. An alternative ordering is
// PT_LOAD(.data | PT_GNU_RELRO( .data.rel.ro .bss.rel.ro) | .bss), but it may
// waste more bytes due to 2 alignment places.
if (!isRelroSection(sec))
rank |= RF_NOT_RELRO;
// If we got here we know that both A and B are in the same PT_LOAD.
// The TLS initialization block needs to be a single contiguous block in a R/W
// PT_LOAD, so stick TLS sections directly before the other RelRo R/W
// sections. Since p_filesz can be less than p_memsz, place NOBITS sections
// after PROGBITS.
if (!(sec->flags & SHF_TLS))
rank |= RF_NOT_TLS;
// Within TLS sections, or within other RelRo sections, or within non-RelRo
// sections, place non-NOBITS sections first.
if (sec->type == SHT_NOBITS)
rank |= RF_BSS;
// Some architectures have additional ordering restrictions for sections
// within the same PT_LOAD.
if (config->emachine == EM_PPC64) {
// PPC64 has a number of special SHT_PROGBITS+SHF_ALLOC+SHF_WRITE sections
// that we would like to make sure appear is a specific order to maximize
// their coverage by a single signed 16-bit offset from the TOC base
// pointer. Conversely, the special .tocbss section should be first among
// all SHT_NOBITS sections. This will put it next to the loaded special
// PPC64 sections (and, thus, within reach of the TOC base pointer).
StringRef name = sec->name;
if (name != ".tocbss")
rank |= RF_PPC_NOT_TOCBSS;
if (name == ".toc1")
rank |= RF_PPC_TOCL;
if (name == ".toc")
rank |= RF_PPC_TOC;
if (name == ".got")
rank |= RF_PPC_GOT;
if (name == ".branch_lt")
rank |= RF_PPC_BRANCH_LT;
}
if (config->emachine == EM_MIPS) {
// All sections with SHF_MIPS_GPREL flag should be grouped together
// because data in these sections is addressable with a gp relative address.
if (sec->flags & SHF_MIPS_GPREL)
rank |= RF_MIPS_GPREL;
if (sec->name != ".got")
rank |= RF_MIPS_NOT_GOT;
}
return rank;
}
static bool compareSections(const BaseCommand *aCmd, const BaseCommand *bCmd) {
const OutputSection *a = cast<OutputSection>(aCmd);
const OutputSection *b = cast<OutputSection>(bCmd);
if (a->sortRank != b->sortRank)
return a->sortRank < b->sortRank;
if (!(a->sortRank & RF_NOT_ADDR_SET))
return config->sectionStartMap.lookup(a->name) <
config->sectionStartMap.lookup(b->name);
return false;
}
void PhdrEntry::add(OutputSection *sec) {
lastSec = sec;
if (!firstSec)
firstSec = sec;
p_align = std::max(p_align, sec->alignment);
if (p_type == PT_LOAD)
sec->ptLoad = this;
}
// The beginning and the ending of .rel[a].plt section are marked
// with __rel[a]_iplt_{start,end} symbols if it is a statically linked
// executable. The runtime needs these symbols in order to resolve
// all IRELATIVE relocs on startup. For dynamic executables, we don't
// need these symbols, since IRELATIVE relocs are resolved through GOT
// and PLT. For details, see http://www.airs.com/blog/archives/403.
template <class ELFT> void Writer<ELFT>::addRelIpltSymbols() {
if (config->relocatable || needsInterpSection())
return;
// By default, __rela_iplt_{start,end} belong to a dummy section 0
// because .rela.plt might be empty and thus removed from output.
// We'll override Out::elfHeader with In.relaIplt later when we are
// sure that .rela.plt exists in output.
ElfSym::relaIpltStart = addOptionalRegular(
config->isRela ? "__rela_iplt_start" : "__rel_iplt_start",
Out::elfHeader, 0, STV_HIDDEN, STB_WEAK);
ElfSym::relaIpltEnd = addOptionalRegular(
config->isRela ? "__rela_iplt_end" : "__rel_iplt_end",
Out::elfHeader, 0, STV_HIDDEN, STB_WEAK);
}
template <class ELFT>
void Writer<ELFT>::forEachRelSec(
llvm::function_ref<void(InputSectionBase &)> fn) {
// Scan all relocations. Each relocation goes through a series
// of tests to determine if it needs special treatment, such as
// creating GOT, PLT, copy relocations, etc.
// Note that relocations for non-alloc sections are directly
// processed by InputSection::relocateNonAlloc.
for (InputSectionBase *isec : inputSections)
if (isec->isLive() && isa<InputSection>(isec) && (isec->flags & SHF_ALLOC))
fn(*isec);
for (Partition &part : partitions) {
for (EhInputSection *es : part.ehFrame->sections)
fn(*es);
if (part.armExidx && part.armExidx->isLive())
for (InputSection *ex : part.armExidx->exidxSections)
fn(*ex);
}
}
// This function generates assignments for predefined symbols (e.g. _end or
// _etext) and inserts them into the commands sequence to be processed at the
// appropriate time. This ensures that the value is going to be correct by the
// time any references to these symbols are processed and is equivalent to
// defining these symbols explicitly in the linker script.
template <class ELFT> void Writer<ELFT>::setReservedSymbolSections() {
if (ElfSym::globalOffsetTable) {
// The _GLOBAL_OFFSET_TABLE_ symbol is defined by target convention usually
// to the start of the .got or .got.plt section.
InputSection *gotSection = in.gotPlt;
if (!target->gotBaseSymInGotPlt)
gotSection = in.mipsGot ? cast<InputSection>(in.mipsGot)
: cast<InputSection>(in.got);
ElfSym::globalOffsetTable->section = gotSection;
}
// .rela_iplt_{start,end} mark the start and the end of in.relaIplt.
if (ElfSym::relaIpltStart && in.relaIplt->isNeeded()) {
ElfSym::relaIpltStart->section = in.relaIplt;
ElfSym::relaIpltEnd->section = in.relaIplt;
ElfSym::relaIpltEnd->value = in.relaIplt->getSize();
}
PhdrEntry *last = nullptr;
PhdrEntry *lastRO = nullptr;
for (Partition &part : partitions) {
for (PhdrEntry *p : part.phdrs) {
if (p->p_type != PT_LOAD)
continue;
last = p;
if (!(p->p_flags & PF_W))
lastRO = p;
}
}
if (lastRO) {
// _etext is the first location after the last read-only loadable segment.
if (ElfSym::etext1)
ElfSym::etext1->section = lastRO->lastSec;
if (ElfSym::etext2)
ElfSym::etext2->section = lastRO->lastSec;
}
if (last) {
// _edata points to the end of the last mapped initialized section.
OutputSection *edata = nullptr;
for (OutputSection *os : outputSections) {
if (os->type != SHT_NOBITS)
edata = os;
if (os == last->lastSec)
break;
}
if (ElfSym::edata1)
ElfSym::edata1->section = edata;
if (ElfSym::edata2)
ElfSym::edata2->section = edata;
// _end is the first location after the uninitialized data region.
if (ElfSym::end1)
ElfSym::end1->section = last->lastSec;
if (ElfSym::end2)
ElfSym::end2->section = last->lastSec;
}
if (ElfSym::bss)
ElfSym::bss->section = findSection(".bss");
// Setup MIPS _gp_disp/__gnu_local_gp symbols which should
// be equal to the _gp symbol's value.
if (ElfSym::mipsGp) {
// Find GP-relative section with the lowest address
// and use this address to calculate default _gp value.
for (OutputSection *os : outputSections) {
if (os->flags & SHF_MIPS_GPREL) {
ElfSym::mipsGp->section = os;
ElfSym::mipsGp->value = 0x7ff0;
break;
}
}
}
}
// We want to find how similar two ranks are.
// The more branches in getSectionRank that match, the more similar they are.
// Since each branch corresponds to a bit flag, we can just use
// countLeadingZeros.
static int getRankProximityAux(OutputSection *a, OutputSection *b) {
return countLeadingZeros(a->sortRank ^ b->sortRank);
}
static int getRankProximity(OutputSection *a, BaseCommand *b) {
auto *sec = dyn_cast<OutputSection>(b);
return (sec && sec->hasInputSections) ? getRankProximityAux(a, sec) : -1;
}
// When placing orphan sections, we want to place them after symbol assignments
// so that an orphan after
// begin_foo = .;
// foo : { *(foo) }
// end_foo = .;
// doesn't break the intended meaning of the begin/end symbols.
// We don't want to go over sections since findOrphanPos is the
// one in charge of deciding the order of the sections.
// We don't want to go over changes to '.', since doing so in
// rx_sec : { *(rx_sec) }
// . = ALIGN(0x1000);
// /* The RW PT_LOAD starts here*/
// rw_sec : { *(rw_sec) }
// would mean that the RW PT_LOAD would become unaligned.
static bool shouldSkip(BaseCommand *cmd) {
if (auto *assign = dyn_cast<SymbolAssignment>(cmd))
return assign->name != ".";
return false;
}
// We want to place orphan sections so that they share as much
// characteristics with their neighbors as possible. For example, if
// both are rw, or both are tls.
static std::vector<BaseCommand *>::iterator
findOrphanPos(std::vector<BaseCommand *>::iterator b,
std::vector<BaseCommand *>::iterator e) {
OutputSection *sec = cast<OutputSection>(*e);
// Find the first element that has as close a rank as possible.
auto i = std::max_element(b, e, [=](BaseCommand *a, BaseCommand *b) {
return getRankProximity(sec, a) < getRankProximity(sec, b);
});
if (i == e)
return e;
// Consider all existing sections with the same proximity.
int proximity = getRankProximity(sec, *i);
for (; i != e; ++i) {
auto *curSec = dyn_cast<OutputSection>(*i);
if (!curSec || !curSec->hasInputSections)
continue;
if (getRankProximity(sec, curSec) != proximity ||
sec->sortRank < curSec->sortRank)
break;
}
auto isOutputSecWithInputSections = [](BaseCommand *cmd) {
auto *os = dyn_cast<OutputSection>(cmd);
return os && os->hasInputSections;
};
auto j = std::find_if(llvm::make_reverse_iterator(i),
llvm::make_reverse_iterator(b),
isOutputSecWithInputSections);
i = j.base();
// As a special case, if the orphan section is the last section, put
// it at the very end, past any other commands.
// This matches bfd's behavior and is convenient when the linker script fully
// specifies the start of the file, but doesn't care about the end (the non
// alloc sections for example).
auto nextSec = std::find_if(i, e, isOutputSecWithInputSections);
if (nextSec == e)
return e;
while (i != e && shouldSkip(*i))
++i;
return i;
}
// Builds section order for handling --symbol-ordering-file.
static DenseMap<const InputSectionBase *, int> buildSectionOrder() {
DenseMap<const InputSectionBase *, int> sectionOrder;
// Use the rarely used option -call-graph-ordering-file to sort sections.
if (!config->callGraphProfile.empty())
return computeCallGraphProfileOrder();
if (config->symbolOrderingFile.empty())
return sectionOrder;
struct SymbolOrderEntry {
int priority;
bool present;
};
// Build a map from symbols to their priorities. Symbols that didn't
// appear in the symbol ordering file have the lowest priority 0.
// All explicitly mentioned symbols have negative (higher) priorities.
DenseMap<StringRef, SymbolOrderEntry> symbolOrder;
int priority = -config->symbolOrderingFile.size();
for (StringRef s : config->symbolOrderingFile)
symbolOrder.insert({s, {priority++, false}});
// Build a map from sections to their priorities.
auto addSym = [&](Symbol &sym) {
auto it = symbolOrder.find(sym.getName());
if (it == symbolOrder.end())
return;
SymbolOrderEntry &ent = it->second;
ent.present = true;
maybeWarnUnorderableSymbol(&sym);
if (auto *d = dyn_cast<Defined>(&sym)) {
if (auto *sec = dyn_cast_or_null<InputSectionBase>(d->section)) {
int &priority = sectionOrder[cast<InputSectionBase>(sec->repl)];
priority = std::min(priority, ent.priority);
}
}
};
// We want both global and local symbols. We get the global ones from the
// symbol table and iterate the object files for the local ones.
symtab->forEachSymbol([&](Symbol *sym) {
if (!sym->isLazy())
addSym(*sym);
});
for (InputFile *file : objectFiles)
for (Symbol *sym : file->getSymbols())
if (sym->isLocal())
addSym(*sym);
if (config->warnSymbolOrdering)
for (auto orderEntry : symbolOrder)
if (!orderEntry.second.present)
warn("symbol ordering file: no such symbol: " + orderEntry.first);
return sectionOrder;
}
// Sorts the sections in ISD according to the provided section order.
static void
sortISDBySectionOrder(InputSectionDescription *isd,
const DenseMap<const InputSectionBase *, int> &order) {
std::vector<InputSection *> unorderedSections;
std::vector<std::pair<InputSection *, int>> orderedSections;
uint64_t unorderedSize = 0;
for (InputSection *isec : isd->sections) {
auto i = order.find(isec);
if (i == order.end()) {
unorderedSections.push_back(isec);
unorderedSize += isec->getSize();
continue;
}
orderedSections.push_back({isec, i->second});
}
llvm::sort(orderedSections, llvm::less_second());
// Find an insertion point for the ordered section list in the unordered
// section list. On targets with limited-range branches, this is the mid-point
// of the unordered section list. This decreases the likelihood that a range
// extension thunk will be needed to enter or exit the ordered region. If the
// ordered section list is a list of hot functions, we can generally expect
// the ordered functions to be called more often than the unordered functions,
// making it more likely that any particular call will be within range, and
// therefore reducing the number of thunks required.
//
// For example, imagine that you have 8MB of hot code and 32MB of cold code.
// If the layout is:
//
// 8MB hot
// 32MB cold
//
// only the first 8-16MB of the cold code (depending on which hot function it
// is actually calling) can call the hot code without a range extension thunk.
// However, if we use this layout:
//
// 16MB cold
// 8MB hot
// 16MB cold
//
// both the last 8-16MB of the first block of cold code and the first 8-16MB
// of the second block of cold code can call the hot code without a thunk. So
// we effectively double the amount of code that could potentially call into
// the hot code without a thunk.
size_t insPt = 0;
if (target->getThunkSectionSpacing() && !orderedSections.empty()) {
uint64_t unorderedPos = 0;
for (; insPt != unorderedSections.size(); ++insPt) {
unorderedPos += unorderedSections[insPt]->getSize();
if (unorderedPos > unorderedSize / 2)
break;
}
}
isd->sections.clear();
for (InputSection *isec : makeArrayRef(unorderedSections).slice(0, insPt))
isd->sections.push_back(isec);
for (std::pair<InputSection *, int> p : orderedSections)
isd->sections.push_back(p.first);
for (InputSection *isec : makeArrayRef(unorderedSections).slice(insPt))
isd->sections.push_back(isec);
}
static void sortSection(OutputSection *sec,
const DenseMap<const InputSectionBase *, int> &order) {
StringRef name = sec->name;
// Sort input sections by section name suffixes for
// __attribute__((init_priority(N))).
if (name == ".init_array" || name == ".fini_array") {
if (!script->hasSectionsCommand)
sec->sortInitFini();
return;
}
// Sort input sections by the special rule for .ctors and .dtors.
if (name == ".ctors" || name == ".dtors") {
if (!script->hasSectionsCommand)
sec->sortCtorsDtors();
return;
}
// Never sort these.
if (name == ".init" || name == ".fini")
return;
// .toc is allocated just after .got and is accessed using GOT-relative
// relocations. Object files compiled with small code model have an
// addressable range of [.got, .got + 0xFFFC] for GOT-relative relocations.
// To reduce the risk of relocation overflow, .toc contents are sorted so that
// sections having smaller relocation offsets are at beginning of .toc
if (config->emachine == EM_PPC64 && name == ".toc") {
if (script->hasSectionsCommand)
return;
assert(sec->sectionCommands.size() == 1);
auto *isd = cast<InputSectionDescription>(sec->sectionCommands[0]);
llvm::stable_sort(isd->sections,
[](const InputSection *a, const InputSection *b) -> bool {
return a->file->ppc64SmallCodeModelTocRelocs &&
!b->file->ppc64SmallCodeModelTocRelocs;
});
return;
}
// Sort input sections by priority using the list provided
// by --symbol-ordering-file.
if (!order.empty())
for (BaseCommand *b : sec->sectionCommands)
if (auto *isd = dyn_cast<InputSectionDescription>(b))
sortISDBySectionOrder(isd, order);
}
// If no layout was provided by linker script, we want to apply default
// sorting for special input sections. This also handles --symbol-ordering-file.
template <class ELFT> void Writer<ELFT>::sortInputSections() {
// Build the order once since it is expensive.
DenseMap<const InputSectionBase *, int> order = buildSectionOrder();
for (BaseCommand *base : script->sectionCommands)
if (auto *sec = dyn_cast<OutputSection>(base))
sortSection(sec, order);
}
template <class ELFT> void Writer<ELFT>::sortSections() {
script->adjustSectionsBeforeSorting();
// Don't sort if using -r. It is not necessary and we want to preserve the
// relative order for SHF_LINK_ORDER sections.
if (config->relocatable)
return;
sortInputSections();
for (BaseCommand *base : script->sectionCommands) {
auto *os = dyn_cast<OutputSection>(base);
if (!os)
continue;
os->sortRank = getSectionRank(os);
// We want to assign rude approximation values to outSecOff fields
// to know the relative order of the input sections. We use it for
// sorting SHF_LINK_ORDER sections. See resolveShfLinkOrder().
uint64_t i = 0;
for (InputSection *sec : getInputSections(os))
sec->outSecOff = i++;
}
if (!script->hasSectionsCommand) {
// We know that all the OutputSections are contiguous in this case.
auto isSection = [](BaseCommand *base) { return isa<OutputSection>(base); };
std::stable_sort(
llvm::find_if(script->sectionCommands, isSection),
llvm::find_if(llvm::reverse(script->sectionCommands), isSection).base(),
compareSections);
return;
}
// Orphan sections are sections present in the input files which are
// not explicitly placed into the output file by the linker script.
//
// The sections in the linker script are already in the correct
// order. We have to figuere out where to insert the orphan
// sections.
//
// The order of the sections in the script is arbitrary and may not agree with
// compareSections. This means that we cannot easily define a strict weak
// ordering. To see why, consider a comparison of a section in the script and
// one not in the script. We have a two simple options:
// * Make them equivalent (a is not less than b, and b is not less than a).
// The problem is then that equivalence has to be transitive and we can
// have sections a, b and c with only b in a script and a less than c
// which breaks this property.
// * Use compareSectionsNonScript. Given that the script order doesn't have
// to match, we can end up with sections a, b, c, d where b and c are in the
// script and c is compareSectionsNonScript less than b. In which case d
// can be equivalent to c, a to b and d < a. As a concrete example:
// .a (rx) # not in script
// .b (rx) # in script
// .c (ro) # in script
// .d (ro) # not in script
//
// The way we define an order then is:
// * Sort only the orphan sections. They are in the end right now.
// * Move each orphan section to its preferred position. We try
// to put each section in the last position where it can share
// a PT_LOAD.
//
// There is some ambiguity as to where exactly a new entry should be
// inserted, because Commands contains not only output section
// commands but also other types of commands such as symbol assignment
// expressions. There's no correct answer here due to the lack of the
// formal specification of the linker script. We use heuristics to
// determine whether a new output command should be added before or
// after another commands. For the details, look at shouldSkip
// function.
auto i = script->sectionCommands.begin();
auto e = script->sectionCommands.end();
auto nonScriptI = std::find_if(i, e, [](BaseCommand *base) {
if (auto *sec = dyn_cast<OutputSection>(base))
return sec->sectionIndex == UINT32_MAX;
return false;
});
// Sort the orphan sections.
std::stable_sort(nonScriptI, e, compareSections);
// As a horrible special case, skip the first . assignment if it is before any
// section. We do this because it is common to set a load address by starting
// the script with ". = 0xabcd" and the expectation is that every section is
// after that.
auto firstSectionOrDotAssignment =
std::find_if(i, e, [](BaseCommand *cmd) { return !shouldSkip(cmd); });
if (firstSectionOrDotAssignment != e &&
isa<SymbolAssignment>(**firstSectionOrDotAssignment))
++firstSectionOrDotAssignment;
i = firstSectionOrDotAssignment;
while (nonScriptI != e) {
auto pos = findOrphanPos(i, nonScriptI);
OutputSection *orphan = cast<OutputSection>(*nonScriptI);
// As an optimization, find all sections with the same sort rank
// and insert them with one rotate.
unsigned rank = orphan->sortRank;
auto end = std::find_if(nonScriptI + 1, e, [=](BaseCommand *cmd) {
return cast<OutputSection>(cmd)->sortRank != rank;
});
std::rotate(pos, nonScriptI, end);
nonScriptI = end;
}
script->adjustSectionsAfterSorting();
}
static bool compareByFilePosition(InputSection *a, InputSection *b) {
InputSection *la = a->getLinkOrderDep();
InputSection *lb = b->getLinkOrderDep();
OutputSection *aOut = la->getParent();
OutputSection *bOut = lb->getParent();
if (aOut != bOut)
return aOut->sectionIndex < bOut->sectionIndex;
return la->outSecOff < lb->outSecOff;
}
template <class ELFT> void Writer<ELFT>::resolveShfLinkOrder() {
for (OutputSection *sec : outputSections) {
if (!(sec->flags & SHF_LINK_ORDER))
continue;
// Link order may be distributed across several InputSectionDescriptions
// but sort must consider them all at once.
std::vector<InputSection **> scriptSections;
std::vector<InputSection *> sections;
for (BaseCommand *base : sec->sectionCommands) {
if (auto *isd = dyn_cast<InputSectionDescription>(base)) {
for (InputSection *&isec : isd->sections) {
scriptSections.push_back(&isec);
sections.push_back(isec);
}
}
}
// The ARM.exidx section use SHF_LINK_ORDER, but we have consolidated
// this processing inside the ARMExidxsyntheticsection::finalizeContents().
if (!config->relocatable && config->emachine == EM_ARM &&
sec->type == SHT_ARM_EXIDX)
continue;
llvm::stable_sort(sections, compareByFilePosition);
for (int i = 0, n = sections.size(); i < n; ++i)
*scriptSections[i] = sections[i];
}
}
// We need to generate and finalize the content that depends on the address of
// InputSections. As the generation of the content may also alter InputSection
// addresses we must converge to a fixed point. We do that here. See the comment
// in Writer<ELFT>::finalizeSections().
template <class ELFT> void Writer<ELFT>::finalizeAddressDependentContent() {
ThunkCreator tc;
AArch64Err843419Patcher a64p;
ARMErr657417Patcher a32p;
script->assignAddresses();
int assignPasses = 0;
for (;;) {
bool changed = target->needsThunks && tc.createThunks(outputSections);
// With Thunk Size much smaller than branch range we expect to
// converge quickly; if we get to 10 something has gone wrong.
if (changed && tc.pass >= 10) {
error("thunk creation not converged");
break;
}
if (config->fixCortexA53Errata843419) {
if (changed)
script->assignAddresses();
changed |= a64p.createFixes();
}
if (config->fixCortexA8) {
if (changed)
script->assignAddresses();
changed |= a32p.createFixes();
}
if (in.mipsGot)
in.mipsGot->updateAllocSize();
for (Partition &part : partitions) {
changed |= part.relaDyn->updateAllocSize();
if (part.relrDyn)
changed |= part.relrDyn->updateAllocSize();
}
const Defined *changedSym = script->assignAddresses();
if (!changed) {
// Some symbols may be dependent on section addresses. When we break the
// loop, the symbol values are finalized because a previous
// assignAddresses() finalized section addresses.
if (!changedSym)
break;
if (++assignPasses == 5) {
errorOrWarn("assignment to symbol " + toString(*changedSym) +
" does not converge");
break;
}
}
}
}
static void finalizeSynthetic(SyntheticSection *sec) {
if (sec && sec->isNeeded() && sec->getParent())
sec->finalizeContents();
}
// In order to allow users to manipulate linker-synthesized sections,
// we had to add synthetic sections to the input section list early,
// even before we make decisions whether they are needed. This allows
// users to write scripts like this: ".mygot : { .got }".
//
// Doing it has an unintended side effects. If it turns out that we
// don't need a .got (for example) at all because there's no
// relocation that needs a .got, we don't want to emit .got.
//
// To deal with the above problem, this function is called after
// scanRelocations is called to remove synthetic sections that turn
// out to be empty.
static void removeUnusedSyntheticSections() {
// All input synthetic sections that can be empty are placed after
// all regular ones. We iterate over them all and exit at first
// non-synthetic.
for (InputSectionBase *s : llvm::reverse(inputSections)) {
SyntheticSection *ss = dyn_cast<SyntheticSection>(s);
if (!ss)
return;
OutputSection *os = ss->getParent();
if (!os || ss->isNeeded())
continue;
// If we reach here, then SS is an unused synthetic section and we want to
// remove it from corresponding input section description of output section.
for (BaseCommand *b : os->sectionCommands)
if (auto *isd = dyn_cast<InputSectionDescription>(b))
llvm::erase_if(isd->sections,
[=](InputSection *isec) { return isec == ss; });
}
}
// Returns true if a symbol can be replaced at load-time by a symbol
// with the same name defined in other ELF executable or DSO.
static bool computeIsPreemptible(const Symbol &b) {
assert(!b.isLocal());
// Only symbols that appear in dynsym can be preempted.
if (!b.includeInDynsym())
return false;
// Only default visibility symbols can be preempted.
if (b.visibility != STV_DEFAULT)
return false;
// At this point copy relocations have not been created yet, so any
// symbol that is not defined locally is preemptible.
if (!b.isDefined())
return true;
if (!config->shared)
return false;
// If the dynamic list is present, it specifies preemptable symbols in a DSO.
if (config->hasDynamicList)
return b.inDynamicList;
// -Bsymbolic means that definitions are not preempted.
if (config->bsymbolic || (config->bsymbolicFunctions && b.isFunc()))
return false;
return true;
}
// Create output section objects and add them to OutputSections.
template <class ELFT> void Writer<ELFT>::finalizeSections() {
Out::preinitArray = findSection(".preinit_array");
Out::initArray = findSection(".init_array");
Out::finiArray = findSection(".fini_array");
// The linker needs to define SECNAME_start, SECNAME_end and SECNAME_stop
// symbols for sections, so that the runtime can get the start and end
// addresses of each section by section name. Add such symbols.
if (!config->relocatable) {
addStartEndSymbols();
for (BaseCommand *base : script->sectionCommands)
if (auto *sec = dyn_cast<OutputSection>(base))
addStartStopSymbols(sec);
}
// Add _DYNAMIC symbol. Unlike GNU gold, our _DYNAMIC symbol has no type.
// It should be okay as no one seems to care about the type.
// Even the author of gold doesn't remember why gold behaves that way.
// https://sourceware.org/ml/binutils/2002-03/msg00360.html
if (mainPart->dynamic->parent)
symtab->addSymbol(Defined{/*file=*/nullptr, "_DYNAMIC", STB_WEAK,
STV_HIDDEN, STT_NOTYPE,
/*value=*/0, /*size=*/0, mainPart->dynamic});
// Define __rel[a]_iplt_{start,end} symbols if needed.
addRelIpltSymbols();
// RISC-V's gp can address +/- 2 KiB, set it to .sdata + 0x800. This symbol
// should only be defined in an executable. If .sdata does not exist, its
// value/section does not matter but it has to be relative, so set its
// st_shndx arbitrarily to 1 (Out::elfHeader).
if (config->emachine == EM_RISCV && !config->shared) {
OutputSection *sec = findSection(".sdata");
ElfSym::riscvGlobalPointer =
addOptionalRegular("__global_pointer$", sec ? sec : Out::elfHeader,
0x800, STV_DEFAULT, STB_GLOBAL);
}
if (config->emachine == EM_X86_64) {
// On targets that support TLSDESC, _TLS_MODULE_BASE_ is defined in such a
// way that:
//
// 1) Without relaxation: it produces a dynamic TLSDESC relocation that
// computes 0.
// 2) With LD->LE relaxation: _TLS_MODULE_BASE_@tpoff = 0 (lowest address in
// the TLS block).
//
// 2) is special cased in @tpoff computation. To satisfy 1), we define it as
// an absolute symbol of zero. This is different from GNU linkers which
// define _TLS_MODULE_BASE_ relative to the first TLS section.
Symbol *s = symtab->find("_TLS_MODULE_BASE_");
if (s && s->isUndefined()) {
s->resolve(Defined{/*file=*/nullptr, s->getName(), STB_GLOBAL, STV_HIDDEN,
STT_TLS, /*value=*/0, 0,
/*section=*/nullptr});
ElfSym::tlsModuleBase = cast<Defined>(s);
}
}
// This responsible for splitting up .eh_frame section into
// pieces. The relocation scan uses those pieces, so this has to be
// earlier.
for (Partition &part : partitions)
finalizeSynthetic(part.ehFrame);
symtab->forEachSymbol(
[](Symbol *s) { s->isPreemptible = computeIsPreemptible(*s); });
// Change values of linker-script-defined symbols from placeholders (assigned
// by declareSymbols) to actual definitions.
script->processSymbolAssignments();
// Scan relocations. This must be done after every symbol is declared so that
// we can correctly decide if a dynamic relocation is needed. This is called
// after processSymbolAssignments() because it needs to know whether a
// linker-script-defined symbol is absolute.
if (!config->relocatable) {
forEachRelSec(scanRelocations<ELFT>);
reportUndefinedSymbols<ELFT>();
}
if (in.plt && in.plt->isNeeded())
in.plt->addSymbols();
if (in.iplt && in.iplt->isNeeded())
in.iplt->addSymbols();
if (!config->allowShlibUndefined) {
// Error on undefined symbols in a shared object, if all of its DT_NEEDED
// entires are seen. These cases would otherwise lead to runtime errors
// reported by the dynamic linker.
//
// ld.bfd traces all DT_NEEDED to emulate the logic of the dynamic linker to
// catch more cases. That is too much for us. Our approach resembles the one
// used in ld.gold, achieves a good balance to be useful but not too smart.
for (SharedFile *file : sharedFiles)
file->allNeededIsKnown =
llvm::all_of(file->dtNeeded, [&](StringRef needed) {
return symtab->soNames.count(needed);
});
symtab->forEachSymbol([](Symbol *sym) {
if (sym->isUndefined() && !sym->isWeak())
if (auto *f = dyn_cast_or_null<SharedFile>(sym->file))
if (f->allNeededIsKnown)
error(toString(f) + ": undefined reference to " + toString(*sym));
});
}
// Now that we have defined all possible global symbols including linker-
// synthesized ones. Visit all symbols to give the finishing touches.
symtab->forEachSymbol([](Symbol *sym) {
if (!includeInSymtab(*sym))
return;
if (in.symTab)
in.symTab->addSymbol(sym);
if (sym->includeInDynsym()) {
partitions[sym->partition - 1].dynSymTab->addSymbol(sym);
if (auto *file = dyn_cast_or_null<SharedFile>(sym->file))
if (file->isNeeded && !sym->isUndefined())
addVerneed(sym);
}
});
// We also need to scan the dynamic relocation tables of the other partitions
// and add any referenced symbols to the partition's dynsym.
for (Partition &part : MutableArrayRef<Partition>(partitions).slice(1)) {
DenseSet<Symbol *> syms;
for (const SymbolTableEntry &e : part.dynSymTab->getSymbols())
syms.insert(e.sym);
for (DynamicReloc &reloc : part.relaDyn->relocs)
if (reloc.sym && !reloc.useSymVA && syms.insert(reloc.sym).second)
part.dynSymTab->addSymbol(reloc.sym);
}
// Do not proceed if there was an undefined symbol.
if (errorCount())
return;
if (in.mipsGot)
in.mipsGot->build();
removeUnusedSyntheticSections();
sortSections();
// Now that we have the final list, create a list of all the
// OutputSections for convenience.
for (BaseCommand *base : script->sectionCommands)
if (auto *sec = dyn_cast<OutputSection>(base))
outputSections.push_back(sec);
// Prefer command line supplied address over other constraints.
for (OutputSection *sec : outputSections) {
auto i = config->sectionStartMap.find(sec->name);
if (i != config->sectionStartMap.end())
sec->addrExpr = [=] { return i->second; };
}
// This is a bit of a hack. A value of 0 means undef, so we set it
// to 1 to make __ehdr_start defined. The section number is not
// particularly relevant.
Out::elfHeader->sectionIndex = 1;
for (size_t i = 0, e = outputSections.size(); i != e; ++i) {
OutputSection *sec = outputSections[i];
sec->sectionIndex = i + 1;
sec->shName = in.shStrTab->addString(sec->name);
}
// Binary and relocatable output does not have PHDRS.
// The headers have to be created before finalize as that can influence the
// image base and the dynamic section on mips includes the image base.
if (!config->relocatable && !config->oFormatBinary) {
for (Partition &part : partitions) {
part.phdrs = script->hasPhdrsCommands() ? script->createPhdrs()
: createPhdrs(part);
if (config->emachine == EM_ARM) {
// PT_ARM_EXIDX is the ARM EHABI equivalent of PT_GNU_EH_FRAME
addPhdrForSection(part, SHT_ARM_EXIDX, PT_ARM_EXIDX, PF_R);
}
if (config->emachine == EM_MIPS) {
// Add separate segments for MIPS-specific sections.
addPhdrForSection(part, SHT_MIPS_REGINFO, PT_MIPS_REGINFO, PF_R);
addPhdrForSection(part, SHT_MIPS_OPTIONS, PT_MIPS_OPTIONS, PF_R);
addPhdrForSection(part, SHT_MIPS_ABIFLAGS, PT_MIPS_ABIFLAGS, PF_R);
}
}
Out::programHeaders->size = sizeof(Elf_Phdr) * mainPart->phdrs.size();
// Find the TLS segment. This happens before the section layout loop so that
// Android relocation packing can look up TLS symbol addresses. We only need
// to care about the main partition here because all TLS symbols were moved
// to the main partition (see MarkLive.cpp).
for (PhdrEntry *p : mainPart->phdrs)
if (p->p_type == PT_TLS)
Out::tlsPhdr = p;
}
// Some symbols are defined in term of program headers. Now that we
// have the headers, we can find out which sections they point to.
setReservedSymbolSections();
finalizeSynthetic(in.bss);
finalizeSynthetic(in.bssRelRo);
finalizeSynthetic(in.symTabShndx);
finalizeSynthetic(in.shStrTab);
finalizeSynthetic(in.strTab);
finalizeSynthetic(in.got);
finalizeSynthetic(in.mipsGot);
finalizeSynthetic(in.igotPlt);
finalizeSynthetic(in.gotPlt);
finalizeSynthetic(in.relaIplt);
finalizeSynthetic(in.relaPlt);
finalizeSynthetic(in.plt);
finalizeSynthetic(in.iplt);
finalizeSynthetic(in.ppc32Got2);
finalizeSynthetic(in.partIndex);
// Dynamic section must be the last one in this list and dynamic
// symbol table section (dynSymTab) must be the first one.
for (Partition &part : partitions) {
finalizeSynthetic(part.armExidx);
finalizeSynthetic(part.dynSymTab);
finalizeSynthetic(part.gnuHashTab);
finalizeSynthetic(part.hashTab);
finalizeSynthetic(part.verDef);
finalizeSynthetic(part.relaDyn);
finalizeSynthetic(part.relrDyn);
finalizeSynthetic(part.ehFrameHdr);
finalizeSynthetic(part.verSym);
finalizeSynthetic(part.verNeed);
finalizeSynthetic(part.dynamic);
}
if (!script->hasSectionsCommand && !config->relocatable)
fixSectionAlignments();
// SHFLinkOrder processing must be processed after relative section placements are
// known but before addresses are allocated.
resolveShfLinkOrder();
// This is used to:
// 1) Create "thunks":
// Jump instructions in many ISAs have small displacements, and therefore
// they cannot jump to arbitrary addresses in memory. For example, RISC-V
// JAL instruction can target only +-1 MiB from PC. It is a linker's
// responsibility to create and insert small pieces of code between
// sections to extend the ranges if jump targets are out of range. Such
// code pieces are called "thunks".
//
// We add thunks at this stage. We couldn't do this before this point
// because this is the earliest point where we know sizes of sections and
// their layouts (that are needed to determine if jump targets are in
// range).
//
// 2) Update the sections. We need to generate content that depends on the
// address of InputSections. For example, MIPS GOT section content or
// android packed relocations sections content.
//
// 3) Assign the final values for the linker script symbols. Linker scripts
// sometimes using forward symbol declarations. We want to set the correct
// values. They also might change after adding the thunks.
finalizeAddressDependentContent();
// finalizeAddressDependentContent may have added local symbols to the static symbol table.
finalizeSynthetic(in.symTab);
finalizeSynthetic(in.ppc64LongBranchTarget);
// Fill other section headers. The dynamic table is finalized
// at the end because some tags like RELSZ depend on result
// of finalizing other sections.
for (OutputSection *sec : outputSections)
sec->finalize();
}
// Ensure data sections are not mixed with executable sections when
// -execute-only is used. -execute-only is a feature to make pages executable
// but not readable, and the feature is currently supported only on AArch64.
template <class ELFT> void Writer<ELFT>::checkExecuteOnly() {
if (!config->executeOnly)
return;
for (OutputSection *os : outputSections)
if (os->flags & SHF_EXECINSTR)
for (InputSection *isec : getInputSections(os))
if (!(isec->flags & SHF_EXECINSTR))
error("cannot place " + toString(isec) + " into " + toString(os->name) +
": -execute-only does not support intermingling data and code");
}
// The linker is expected to define SECNAME_start and SECNAME_end
// symbols for a few sections. This function defines them.
template <class ELFT> void Writer<ELFT>::addStartEndSymbols() {
// If a section does not exist, there's ambiguity as to how we
// define _start and _end symbols for an init/fini section. Since
// the loader assume that the symbols are always defined, we need to
// always define them. But what value? The loader iterates over all
// pointers between _start and _end to run global ctors/dtors, so if
// the section is empty, their symbol values don't actually matter
// as long as _start and _end point to the same location.
//
// That said, we don't want to set the symbols to 0 (which is
// probably the simplest value) because that could cause some
// program to fail to link due to relocation overflow, if their
// program text is above 2 GiB. We use the address of the .text
// section instead to prevent that failure.
//
// In a rare sitaution, .text section may not exist. If that's the
// case, use the image base address as a last resort.
OutputSection *Default = findSection(".text");
if (!Default)
Default = Out::elfHeader;
auto define = [=](StringRef start, StringRef end, OutputSection *os) {
if (os) {
addOptionalRegular(start, os, 0);
addOptionalRegular(end, os, -1);
} else {
addOptionalRegular(start, Default, 0);
addOptionalRegular(end, Default, 0);
}
};
define("__preinit_array_start", "__preinit_array_end", Out::preinitArray);
define("__init_array_start", "__init_array_end", Out::initArray);
define("__fini_array_start", "__fini_array_end", Out::finiArray);
if (OutputSection *sec = findSection(".ARM.exidx"))
define("__exidx_start", "__exidx_end", sec);
}
// If a section name is valid as a C identifier (which is rare because of
// the leading '.'), linkers are expected to define __start_<secname> and
// __stop_<secname> symbols. They are at beginning and end of the section,
// respectively. This is not requested by the ELF standard, but GNU ld and
// gold provide the feature, and used by many programs.
template <class ELFT>
void Writer<ELFT>::addStartStopSymbols(OutputSection *sec) {
StringRef s = sec->name;
if (!isValidCIdentifier(s))
return;
addOptionalRegular(saver.save("__start_" + s), sec, 0, STV_PROTECTED);
addOptionalRegular(saver.save("__stop_" + s), sec, -1, STV_PROTECTED);
}
static bool needsPtLoad(OutputSection *sec) {
if (!(sec->flags & SHF_ALLOC) || sec->noload)
return false;
// Don't allocate VA space for TLS NOBITS sections. The PT_TLS PHDR is
// responsible for allocating space for them, not the PT_LOAD that
// contains the TLS initialization image.
if ((sec->flags & SHF_TLS) && sec->type == SHT_NOBITS)
return false;
return true;
}
// Linker scripts are responsible for aligning addresses. Unfortunately, most
// linker scripts are designed for creating two PT_LOADs only, one RX and one
// RW. This means that there is no alignment in the RO to RX transition and we
// cannot create a PT_LOAD there.
static uint64_t computeFlags(uint64_t flags) {
if (config->omagic)
return PF_R | PF_W | PF_X;
if (config->executeOnly && (flags & PF_X))
return flags & ~PF_R;
if (config->singleRoRx && !(flags & PF_W))
return flags | PF_X;
return flags;
}
// Decide which program headers to create and which sections to include in each
// one.
template <class ELFT>
std::vector<PhdrEntry *> Writer<ELFT>::createPhdrs(Partition &part) {
std::vector<PhdrEntry *> ret;
auto addHdr = [&](unsigned type, unsigned flags) -> PhdrEntry * {
ret.push_back(make<PhdrEntry>(type, flags));
return ret.back();
};
unsigned partNo = part.getNumber();
bool isMain = partNo == 1;
// Add the first PT_LOAD segment for regular output sections.
uint64_t flags = computeFlags(PF_R);
PhdrEntry *load = nullptr;
// nmagic or omagic output does not have PT_PHDR, PT_INTERP, or the readonly
// PT_LOAD.
if (!config->nmagic && !config->omagic) {
// The first phdr entry is PT_PHDR which describes the program header
// itself.
if (isMain)
addHdr(PT_PHDR, PF_R)->add(Out::programHeaders);
else
addHdr(PT_PHDR, PF_R)->add(part.programHeaders->getParent());
// PT_INTERP must be the second entry if exists.
if (OutputSection *cmd = findSection(".interp", partNo))
addHdr(PT_INTERP, cmd->getPhdrFlags())->add(cmd);
// Add the headers. We will remove them if they don't fit.
// In the other partitions the headers are ordinary sections, so they don't
// need to be added here.
if (isMain) {
load = addHdr(PT_LOAD, flags);
load->add(Out::elfHeader);
load->add(Out::programHeaders);
}
}
// PT_GNU_RELRO includes all sections that should be marked as
// read-only by dynamic linker after proccessing relocations.
// Current dynamic loaders only support one PT_GNU_RELRO PHDR, give
// an error message if more than one PT_GNU_RELRO PHDR is required.
PhdrEntry *relRo = make<PhdrEntry>(PT_GNU_RELRO, PF_R);
bool inRelroPhdr = false;
OutputSection *relroEnd = nullptr;
for (OutputSection *sec : outputSections) {
if (sec->partition != partNo || !needsPtLoad(sec))
continue;
if (isRelroSection(sec)) {
inRelroPhdr = true;
if (!relroEnd)
relRo->add(sec);
else
error("section: " + sec->name + " is not contiguous with other relro" +
" sections");
} else if (inRelroPhdr) {
inRelroPhdr = false;
relroEnd = sec;
}
}
for (OutputSection *sec : outputSections) {
if (!(sec->flags & SHF_ALLOC))
break;
if (!needsPtLoad(sec))
continue;
// Normally, sections in partitions other than the current partition are
// ignored. But partition number 255 is a special case: it contains the
// partition end marker (.part.end). It needs to be added to the main
// partition so that a segment is created for it in the main partition,
// which will cause the dynamic loader to reserve space for the other
// partitions.
if (sec->partition != partNo) {
if (isMain && sec->partition == 255)
addHdr(PT_LOAD, computeFlags(sec->getPhdrFlags()))->add(sec);
continue;
}
// Segments are contiguous memory regions that has the same attributes
// (e.g. executable or writable). There is one phdr for each segment.
// Therefore, we need to create a new phdr when the next section has
// different flags or is loaded at a discontiguous address or memory
// region using AT or AT> linker script command, respectively. At the same
// time, we don't want to create a separate load segment for the headers,
// even if the first output section has an AT or AT> attribute.
uint64_t newFlags = computeFlags(sec->getPhdrFlags());
if (!load ||
((sec->lmaExpr ||
(sec->lmaRegion && (sec->lmaRegion != load->firstSec->lmaRegion))) &&
load->lastSec != Out::programHeaders) ||
sec->memRegion != load->firstSec->memRegion || flags != newFlags ||
sec == relroEnd) {
load = addHdr(PT_LOAD, newFlags);
flags = newFlags;
}
load->add(sec);
}
// Add a TLS segment if any.
PhdrEntry *tlsHdr = make<PhdrEntry>(PT_TLS, PF_R);
for (OutputSection *sec : outputSections)
if (sec->partition == partNo && sec->flags & SHF_TLS)
tlsHdr->add(sec);
if (tlsHdr->firstSec)
ret.push_back(tlsHdr);
// Add an entry for .dynamic.
if (OutputSection *sec = part.dynamic->getParent())
addHdr(PT_DYNAMIC, sec->getPhdrFlags())->add(sec);
if (relRo->firstSec)
ret.push_back(relRo);
// PT_GNU_EH_FRAME is a special section pointing on .eh_frame_hdr.
if (part.ehFrame->isNeeded() && part.ehFrameHdr &&
part.ehFrame->getParent() && part.ehFrameHdr->getParent())
addHdr(PT_GNU_EH_FRAME, part.ehFrameHdr->getParent()->getPhdrFlags())
->add(part.ehFrameHdr->getParent());
// PT_OPENBSD_RANDOMIZE is an OpenBSD-specific feature. That makes
// the dynamic linker fill the segment with random data.
if (OutputSection *cmd = findSection(".openbsd.randomdata", partNo))
addHdr(PT_OPENBSD_RANDOMIZE, cmd->getPhdrFlags())->add(cmd);
// PT_GNU_STACK is a special section to tell the loader to make the
// pages for the stack non-executable. If you really want an executable
// stack, you can pass -z execstack, but that's not recommended for
// security reasons.
unsigned perm = PF_R | PF_W;
if (config->zExecstack)
perm |= PF_X;
addHdr(PT_GNU_STACK, perm)->p_memsz = config->zStackSize;
// PT_OPENBSD_WXNEEDED is a OpenBSD-specific header to mark the executable
// is expected to perform W^X violations, such as calling mprotect(2) or
// mmap(2) with PROT_WRITE | PROT_EXEC, which is prohibited by default on
// OpenBSD.
if (config->zWxneeded)
addHdr(PT_OPENBSD_WXNEEDED, PF_X);
// Create one PT_NOTE per a group of contiguous SHT_NOTE sections with the
// same alignment.
PhdrEntry *note = nullptr;
for (OutputSection *sec : outputSections) {
if (sec->partition != partNo)
continue;
if (sec->type == SHT_NOTE && (sec->flags & SHF_ALLOC)) {
if (!note || sec->lmaExpr || note->lastSec->alignment != sec->alignment)
note = addHdr(PT_NOTE, PF_R);
note->add(sec);
} else {
note = nullptr;
}
}
return ret;
}
template <class ELFT>
void Writer<ELFT>::addPhdrForSection(Partition &part, unsigned shType,
unsigned pType, unsigned pFlags) {
unsigned partNo = part.getNumber();
auto i = llvm::find_if(outputSections, [=](OutputSection *cmd) {
return cmd->partition == partNo && cmd->type == shType;
});
if (i == outputSections.end())
return;
PhdrEntry *entry = make<PhdrEntry>(pType, pFlags);
entry->add(*i);
part.phdrs.push_back(entry);
}
// Place the first section of each PT_LOAD to a different page (of maxPageSize).
// This is achieved by assigning an alignment expression to addrExpr of each
// such section.
template <class ELFT> void Writer<ELFT>::fixSectionAlignments() {
const PhdrEntry *prev;
auto pageAlign = [&](const PhdrEntry *p) {
OutputSection *cmd = p->firstSec;
if (cmd && !cmd->addrExpr) {
// Prefer advancing to align(dot, maxPageSize) + dot%maxPageSize to avoid
// padding in the file contents.
//
// When -z separate-code is used we must not have any overlap in pages
// between an executable segment and a non-executable segment. We align to
// the next maximum page size boundary on transitions between executable
// and non-executable segments.
//
// SHT_LLVM_PART_EHDR marks the start of a partition. The partition
// sections will be extracted to a separate file. Align to the next
// maximum page size boundary so that we can find the ELF header at the
// start. We cannot benefit from overlapping p_offset ranges with the
// previous segment anyway.
//
// TODO Enable this technique on all targets.
bool enable = config->emachine != EM_HEXAGON;
if (!enable ||
(config->zSeparateCode && prev &&
(prev->p_flags & PF_X) != (p->p_flags & PF_X)) ||
cmd->type == SHT_LLVM_PART_EHDR)
cmd->addrExpr = [] {
return alignTo(script->getDot(), config->maxPageSize);
};
// PT_TLS is at the start of the first RW PT_LOAD. If `p` includes PT_TLS,
// it must be the RW. Align to p_align(PT_TLS) to make sure
// p_vaddr(PT_LOAD)%p_align(PT_LOAD) = 0. Otherwise, if
// sh_addralign(.tdata) < sh_addralign(.tbss), we will set p_align(PT_TLS)
// to sh_addralign(.tbss), while p_vaddr(PT_TLS)=p_vaddr(PT_LOAD) may not
// be congruent to 0 modulo p_align(PT_TLS).
//
// Technically this is not required, but as of 2019, some dynamic loaders
// don't handle p_vaddr%p_align != 0 correctly, e.g. glibc (i386 and
// x86-64) doesn't make runtime address congruent to p_vaddr modulo
// p_align for dynamic TLS blocks (PR/24606), FreeBSD rtld has the same
// bug, musl (TLS Variant 1 architectures) before 1.1.23 handled TLS
// blocks correctly. We need to keep the workaround for a while.
else if (Out::tlsPhdr && Out::tlsPhdr->firstSec == p->firstSec)
cmd->addrExpr = [] {
return alignTo(script->getDot(), config->maxPageSize) +
alignTo(script->getDot() % config->maxPageSize,
Out::tlsPhdr->p_align);
};
else
cmd->addrExpr = [] {
return alignTo(script->getDot(), config->maxPageSize) +
script->getDot() % config->maxPageSize;
};
}
};
for (Partition &part : partitions) {
prev = nullptr;
for (const PhdrEntry *p : part.phdrs)
if (p->p_type == PT_LOAD && p->firstSec) {
pageAlign(p);
prev = p;
}
}
}
// Compute an in-file position for a given section. The file offset must be the
// same with its virtual address modulo the page size, so that the loader can
// load executables without any address adjustment.
static uint64_t computeFileOffset(OutputSection *os, uint64_t off) {
// The first section in a PT_LOAD has to have congruent offset and address
// modulo the maximum page size.
if (os->ptLoad && os->ptLoad->firstSec == os)
return alignTo(off, os->ptLoad->p_align, os->addr);
// File offsets are not significant for .bss sections other than the first one
// in a PT_LOAD. By convention, we keep section offsets monotonically
// increasing rather than setting to zero.
if (os->type == SHT_NOBITS)
return off;
// If the section is not in a PT_LOAD, we just have to align it.
if (!os->ptLoad)
return alignTo(off, os->alignment);
// If two sections share the same PT_LOAD the file offset is calculated
// using this formula: Off2 = Off1 + (VA2 - VA1).
OutputSection *first = os->ptLoad->firstSec;
return first->offset + os->addr - first->addr;
}
// Set an in-file position to a given section and returns the end position of
// the section.
static uint64_t setFileOffset(OutputSection *os, uint64_t off) {
off = computeFileOffset(os, off);
os->offset = off;
if (os->type == SHT_NOBITS)
return off;
return off + os->size;
}
template <class ELFT> void Writer<ELFT>::assignFileOffsetsBinary() {
uint64_t off = 0;
for (OutputSection *sec : outputSections)
if (sec->flags & SHF_ALLOC)
off = setFileOffset(sec, off);
fileSize = alignTo(off, config->wordsize);
}
static std::string rangeToString(uint64_t addr, uint64_t len) {
return "[0x" + utohexstr(addr) + ", 0x" + utohexstr(addr + len - 1) + "]";
}
// Assign file offsets to output sections.
template <class ELFT> void Writer<ELFT>::assignFileOffsets() {
uint64_t off = 0;
off = setFileOffset(Out::elfHeader, off);
off = setFileOffset(Out::programHeaders, off);
PhdrEntry *lastRX = nullptr;
for (Partition &part : partitions)
for (PhdrEntry *p : part.phdrs)
if (p->p_type == PT_LOAD && (p->p_flags & PF_X))
lastRX = p;
for (OutputSection *sec : outputSections) {
off = setFileOffset(sec, off);
// If this is a last section of the last executable segment and that
// segment is the last loadable segment, align the offset of the
// following section to avoid loading non-segments parts of the file.
if (config->zSeparateCode && lastRX && lastRX->lastSec == sec)
off = alignTo(off, config->commonPageSize);
}
sectionHeaderOff = alignTo(off, config->wordsize);
fileSize = sectionHeaderOff + (outputSections.size() + 1) * sizeof(Elf_Shdr);
// Our logic assumes that sections have rising VA within the same segment.
// With use of linker scripts it is possible to violate this rule and get file
// offset overlaps or overflows. That should never happen with a valid script
// which does not move the location counter backwards and usually scripts do
// not do that. Unfortunately, there are apps in the wild, for example, Linux
// kernel, which control segment distribution explicitly and move the counter
// backwards, so we have to allow doing that to support linking them. We
// perform non-critical checks for overlaps in checkSectionOverlap(), but here
// we want to prevent file size overflows because it would crash the linker.
for (OutputSection *sec : outputSections) {
if (sec->type == SHT_NOBITS)
continue;
if ((sec->offset > fileSize) || (sec->offset + sec->size > fileSize))
error("unable to place section " + sec->name + " at file offset " +
rangeToString(sec->offset, sec->size) +
"; check your linker script for overflows");
}
}
// Finalize the program headers. We call this function after we assign
// file offsets and VAs to all sections.
template <class ELFT> void Writer<ELFT>::setPhdrs(Partition &part) {
for (PhdrEntry *p : part.phdrs) {
OutputSection *first = p->firstSec;
OutputSection *last = p->lastSec;
if (first) {
p->p_filesz = last->offset - first->offset;
if (last->type != SHT_NOBITS)
p->p_filesz += last->size;
p->p_memsz = last->addr + last->size - first->addr;
p->p_offset = first->offset;
p->p_vaddr = first->addr;
// File offsets in partitions other than the main partition are relative
// to the offset of the ELF headers. Perform that adjustment now.
if (part.elfHeader)
p->p_offset -= part.elfHeader->getParent()->offset;
if (!p->hasLMA)
p->p_paddr = first->getLMA();
}
if (p->p_type == PT_GNU_RELRO) {
p->p_align = 1;
// musl/glibc ld.so rounds the size down, so we need to round up
// to protect the last page. This is a no-op on FreeBSD which always
// rounds up.
p->p_memsz = alignTo(p->p_offset + p->p_memsz, config->commonPageSize) -
p->p_offset;
}
}
}
// A helper struct for checkSectionOverlap.
namespace {
struct SectionOffset {
OutputSection *sec;
uint64_t offset;
};
} // namespace
// Check whether sections overlap for a specific address range (file offsets,
// load and virtual adresses).
static void checkOverlap(StringRef name, std::vector<SectionOffset> &sections,
bool isVirtualAddr) {
llvm::sort(sections, [=](const SectionOffset &a, const SectionOffset &b) {
return a.offset < b.offset;
});
// Finding overlap is easy given a vector is sorted by start position.
// If an element starts before the end of the previous element, they overlap.
for (size_t i = 1, end = sections.size(); i < end; ++i) {
SectionOffset a = sections[i - 1];
SectionOffset b = sections[i];
if (b.offset >= a.offset + a.sec->size)
continue;
// If both sections are in OVERLAY we allow the overlapping of virtual
// addresses, because it is what OVERLAY was designed for.
if (isVirtualAddr && a.sec->inOverlay && b.sec->inOverlay)
continue;
errorOrWarn("section " + a.sec->name + " " + name +
" range overlaps with " + b.sec->name + "\n>>> " + a.sec->name +
" range is " + rangeToString(a.offset, a.sec->size) + "\n>>> " +
b.sec->name + " range is " +
rangeToString(b.offset, b.sec->size));
}
}
// Check for overlapping sections and address overflows.
//
// In this function we check that none of the output sections have overlapping
// file offsets. For SHF_ALLOC sections we also check that the load address
// ranges and the virtual address ranges don't overlap
template <class ELFT> void Writer<ELFT>::checkSections() {
// First, check that section's VAs fit in available address space for target.
for (OutputSection *os : outputSections)
if ((os->addr + os->size < os->addr) ||
(!ELFT::Is64Bits && os->addr + os->size > UINT32_MAX))
errorOrWarn("section " + os->name + " at 0x" + utohexstr(os->addr) +
" of size 0x" + utohexstr(os->size) +
" exceeds available address space");
// Check for overlapping file offsets. In this case we need to skip any
// section marked as SHT_NOBITS. These sections don't actually occupy space in
// the file so Sec->Offset + Sec->Size can overlap with others. If --oformat
// binary is specified only add SHF_ALLOC sections are added to the output
// file so we skip any non-allocated sections in that case.
std::vector<SectionOffset> fileOffs;
for (OutputSection *sec : outputSections)
if (sec->size > 0 && sec->type != SHT_NOBITS &&
(!config->oFormatBinary || (sec->flags & SHF_ALLOC)))
fileOffs.push_back({sec, sec->offset});
checkOverlap("file", fileOffs, false);
// When linking with -r there is no need to check for overlapping virtual/load
// addresses since those addresses will only be assigned when the final
// executable/shared object is created.
if (config->relocatable)
return;
// Checking for overlapping virtual and load addresses only needs to take
// into account SHF_ALLOC sections since others will not be loaded.
// Furthermore, we also need to skip SHF_TLS sections since these will be
// mapped to other addresses at runtime and can therefore have overlapping
// ranges in the file.
std::vector<SectionOffset> vmas;
for (OutputSection *sec : outputSections)
if (sec->size > 0 && (sec->flags & SHF_ALLOC) && !(sec->flags & SHF_TLS))
vmas.push_back({sec, sec->addr});
checkOverlap("virtual address", vmas, true);
// Finally, check that the load addresses don't overlap. This will usually be
// the same as the virtual addresses but can be different when using a linker
// script with AT().
std::vector<SectionOffset> lmas;
for (OutputSection *sec : outputSections)
if (sec->size > 0 && (sec->flags & SHF_ALLOC) && !(sec->flags & SHF_TLS))
lmas.push_back({sec, sec->getLMA()});
checkOverlap("load address", lmas, false);
}
// The entry point address is chosen in the following ways.
//
// 1. the '-e' entry command-line option;
// 2. the ENTRY(symbol) command in a linker control script;
// 3. the value of the symbol _start, if present;
// 4. the number represented by the entry symbol, if it is a number;
// 5. the address of the first byte of the .text section, if present;
// 6. the address 0.
static uint64_t getEntryAddr() {
// Case 1, 2 or 3
if (Symbol *b = symtab->find(config->entry))
return b->getVA();
// Case 4
uint64_t addr;
if (to_integer(config->entry, addr))
return addr;
// Case 5
if (OutputSection *sec = findSection(".text")) {
if (config->warnMissingEntry)
warn("cannot find entry symbol " + config->entry + "; defaulting to 0x" +
utohexstr(sec->addr));
return sec->addr;
}
// Case 6
if (config->warnMissingEntry)
warn("cannot find entry symbol " + config->entry +
"; not setting start address");
return 0;
}
static uint16_t getELFType() {
if (config->isPic)
return ET_DYN;
if (config->relocatable)
return ET_REL;
return ET_EXEC;
}
template <class ELFT> void Writer<ELFT>::writeHeader() {
writeEhdr<ELFT>(Out::bufferStart, *mainPart);
writePhdrs<ELFT>(Out::bufferStart + sizeof(Elf_Ehdr), *mainPart);
auto *eHdr = reinterpret_cast<Elf_Ehdr *>(Out::bufferStart);
eHdr->e_type = getELFType();
eHdr->e_entry = getEntryAddr();
eHdr->e_shoff = sectionHeaderOff;
// Write the section header table.
//
// The ELF header can only store numbers up to SHN_LORESERVE in the e_shnum
// and e_shstrndx fields. When the value of one of these fields exceeds
// SHN_LORESERVE ELF requires us to put sentinel values in the ELF header and
// use fields in the section header at index 0 to store
// the value. The sentinel values and fields are:
// e_shnum = 0, SHdrs[0].sh_size = number of sections.
// e_shstrndx = SHN_XINDEX, SHdrs[0].sh_link = .shstrtab section index.
auto *sHdrs = reinterpret_cast<Elf_Shdr *>(Out::bufferStart + eHdr->e_shoff);
size_t num = outputSections.size() + 1;
if (num >= SHN_LORESERVE)
sHdrs->sh_size = num;
else
eHdr->e_shnum = num;
uint32_t strTabIndex = in.shStrTab->getParent()->sectionIndex;
if (strTabIndex >= SHN_LORESERVE) {
sHdrs->sh_link = strTabIndex;
eHdr->e_shstrndx = SHN_XINDEX;
} else {
eHdr->e_shstrndx = strTabIndex;
}
for (OutputSection *sec : outputSections)
sec->writeHeaderTo<ELFT>(++sHdrs);
}
// Open a result file.
template <class ELFT> void Writer<ELFT>::openFile() {
uint64_t maxSize = config->is64 ? INT64_MAX : UINT32_MAX;
if (fileSize != size_t(fileSize) || maxSize < fileSize) {
error("output file too large: " + Twine(fileSize) + " bytes");
return;
}
unlinkAsync(config->outputFile);
unsigned flags = 0;
if (!config->relocatable)
flags = FileOutputBuffer::F_executable;
Expected<std::unique_ptr<FileOutputBuffer>> bufferOrErr =
FileOutputBuffer::create(config->outputFile, fileSize, flags);
if (!bufferOrErr) {
error("failed to open " + config->outputFile + ": " +
llvm::toString(bufferOrErr.takeError()));
return;
}
buffer = std::move(*bufferOrErr);
Out::bufferStart = buffer->getBufferStart();
}
template <class ELFT> void Writer<ELFT>::writeSectionsBinary() {
for (OutputSection *sec : outputSections)
if (sec->flags & SHF_ALLOC)
sec->writeTo<ELFT>(Out::bufferStart + sec->offset);
}
static void fillTrap(uint8_t *i, uint8_t *end) {
for (; i + 4 <= end; i += 4)
memcpy(i, &target->trapInstr, 4);
}
// Fill the last page of executable segments with trap instructions
// instead of leaving them as zero. Even though it is not required by any
// standard, it is in general a good thing to do for security reasons.
//
// We'll leave other pages in segments as-is because the rest will be
// overwritten by output sections.
template <class ELFT> void Writer<ELFT>::writeTrapInstr() {
if (!config->zSeparateCode)
return;
for (Partition &part : partitions) {
// Fill the last page.
for (PhdrEntry *p : part.phdrs)
if (p->p_type == PT_LOAD && (p->p_flags & PF_X))
fillTrap(Out::bufferStart + alignDown(p->firstSec->offset + p->p_filesz,
config->commonPageSize),
Out::bufferStart + alignTo(p->firstSec->offset + p->p_filesz,
config->commonPageSize));
// Round up the file size of the last segment to the page boundary iff it is
// an executable segment to ensure that other tools don't accidentally
// trim the instruction padding (e.g. when stripping the file).
PhdrEntry *last = nullptr;
for (PhdrEntry *p : part.phdrs)
if (p->p_type == PT_LOAD)
last = p;
if (last && (last->p_flags & PF_X))
last->p_memsz = last->p_filesz =
alignTo(last->p_filesz, config->commonPageSize);
}
}
// Write section contents to a mmap'ed file.
template <class ELFT> void Writer<ELFT>::writeSections() {
// In -r or -emit-relocs mode, write the relocation sections first as in
// ELf_Rel targets we might find out that we need to modify the relocated
// section while doing it.
for (OutputSection *sec : outputSections)
if (sec->type == SHT_REL || sec->type == SHT_RELA)
sec->writeTo<ELFT>(Out::bufferStart + sec->offset);
for (OutputSection *sec : outputSections)
if (sec->type != SHT_REL && sec->type != SHT_RELA)
sec->writeTo<ELFT>(Out::bufferStart + sec->offset);
}
// Split one uint8 array into small pieces of uint8 arrays.
static std::vector<ArrayRef<uint8_t>> split(ArrayRef<uint8_t> arr,
size_t chunkSize) {
std::vector<ArrayRef<uint8_t>> ret;
while (arr.size() > chunkSize) {
ret.push_back(arr.take_front(chunkSize));
arr = arr.drop_front(chunkSize);
}
if (!arr.empty())
ret.push_back(arr);
return ret;
}
// Computes a hash value of Data using a given hash function.
// In order to utilize multiple cores, we first split data into 1MB
// chunks, compute a hash for each chunk, and then compute a hash value
// of the hash values.
static void
computeHash(llvm::MutableArrayRef<uint8_t> hashBuf,
llvm::ArrayRef<uint8_t> data,
std::function<void(uint8_t *dest, ArrayRef<uint8_t> arr)> hashFn) {
std::vector<ArrayRef<uint8_t>> chunks = split(data, 1024 * 1024);
std::vector<uint8_t> hashes(chunks.size() * hashBuf.size());
// Compute hash values.
parallelForEachN(0, chunks.size(), [&](size_t i) {
hashFn(hashes.data() + i * hashBuf.size(), chunks[i]);
});
// Write to the final output buffer.
hashFn(hashBuf.data(), hashes);
}
template <class ELFT> void Writer<ELFT>::writeBuildId() {
if (!mainPart->buildId || !mainPart->buildId->getParent())
return;
if (config->buildId == BuildIdKind::Hexstring) {
for (Partition &part : partitions)
part.buildId->writeBuildId(config->buildIdVector);
return;
}
// Compute a hash of all sections of the output file.
size_t hashSize = mainPart->buildId->hashSize;
std::vector<uint8_t> buildId(hashSize);
llvm::ArrayRef<uint8_t> buf{Out::bufferStart, size_t(fileSize)};
switch (config->buildId) {
case BuildIdKind::Fast:
computeHash(buildId, buf, [](uint8_t *dest, ArrayRef<uint8_t> arr) {
write64le(dest, xxHash64(arr));
});
break;
case BuildIdKind::Md5:
computeHash(buildId, buf, [&](uint8_t *dest, ArrayRef<uint8_t> arr) {
memcpy(dest, MD5::hash(arr).data(), hashSize);
});
break;
case BuildIdKind::Sha1:
computeHash(buildId, buf, [&](uint8_t *dest, ArrayRef<uint8_t> arr) {
memcpy(dest, SHA1::hash(arr).data(), hashSize);
});
break;
case BuildIdKind::Uuid:
if (auto ec = llvm::getRandomBytes(buildId.data(), hashSize))
error("entropy source failure: " + ec.message());
break;
default:
llvm_unreachable("unknown BuildIdKind");
}
for (Partition &part : partitions)
part.buildId->writeBuildId(buildId);
}
template void elf::createSyntheticSections<ELF32LE>();
template void elf::createSyntheticSections<ELF32BE>();
template void elf::createSyntheticSections<ELF64LE>();
template void elf::createSyntheticSections<ELF64BE>();
template void elf::writeResult<ELF32LE>();
template void elf::writeResult<ELF32BE>();
template void elf::writeResult<ELF64LE>();
template void elf::writeResult<ELF64BE>();