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llvm / llvm-project / lld / refs/heads/main / . / 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 "InputFiles.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/Arrays.h"
#include "lld/Common/CommonLinkerContext.h"
#include "lld/Common/Filesystem.h"
#include "lld/Common/Strings.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/Support/BLAKE3.h"
#include "llvm/Support/Parallel.h"
#include "llvm/Support/RandomNumberGenerator.h"
#include "llvm/Support/TimeProfiler.h"
#include "llvm/Support/xxhash.h"
#include <climits>
#define DEBUG_TYPE "lld"
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:
LLVM_ELF_IMPORT_TYPES_ELFT(ELFT)
Writer() : buffer(errorHandler().outputBuffer) {}
void run();
private:
void addSectionSymbols();
void sortSections();
void resolveShfLinkOrder();
void finalizeAddressDependentContent();
void optimizeBasicBlockJumps();
void sortInputSections();
void sortOrphanSections();
void finalizeSections();
void checkExecuteOnly();
void setReservedSymbolSections();
SmallVector<PhdrEntry *, 0> 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 &osec);
uint64_t fileSize;
uint64_t sectionHeaderOff;
};
} // anonymous namespace
template <class ELFT> void elf::writeResult() {
Writer<ELFT>().run();
}
static void removeEmptyPTLoad(SmallVector<PhdrEntry *, 0> &phdrs) {
auto it = std::stable_partition(
phdrs.begin(), phdrs.end(), [&](const PhdrEntry *p) {
if (p->p_type != PT_LOAD)
return true;
if (!p->firstSec)
return false;
uint64_t size = p->lastSec->addr + p->lastSec->size - p->firstSec->addr;
return size != 0;
});
// Clear OutputSection::ptLoad for sections contained in removed
// segments.
DenseSet<PhdrEntry *> removed(it, phdrs.end());
for (OutputSection *sec : outputSections)
if (removed.count(sec->ptLoad))
sec->ptLoad = nullptr;
phdrs.erase(it, phdrs.end());
}
void elf::copySectionsIntoPartitions() {
SmallVector<InputSectionBase *, 0> newSections;
const size_t ehSize = ctx.ehInputSections.size();
for (unsigned part = 2; part != partitions.size() + 1; ++part) {
for (InputSectionBase *s : ctx.inputSections) {
if (!(s->flags & SHF_ALLOC) || !s->isLive() || s->type != SHT_NOTE)
continue;
auto *copy = make<InputSection>(cast<InputSection>(*s));
copy->partition = part;
newSections.push_back(copy);
}
for (size_t i = 0; i != ehSize; ++i) {
assert(ctx.ehInputSections[i]->isLive());
auto *copy = make<EhInputSection>(*ctx.ehInputSections[i]);
copy->partition = part;
ctx.ehInputSections.push_back(copy);
}
}
ctx.inputSections.insert(ctx.inputSections.end(), newSections.begin(),
newSections.end());
}
static Defined *addOptionalRegular(StringRef name, SectionBase *sec,
uint64_t val, uint8_t stOther = STV_HIDDEN) {
Symbol *s = symtab.find(name);
if (!s || s->isDefined() || s->isCommon())
return nullptr;
s->resolve(Defined{ctx.internalFile, StringRef(), STB_GLOBAL, stOther,
STT_NOTYPE, val,
/*size=*/0, sec});
s->isUsedInRegularObj = true;
return cast<Defined>(s);
}
// 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) {
auto addAbsolute = [](StringRef name) {
Symbol *sym =
symtab.addSymbol(Defined{ctx.internalFile, name, STB_GLOBAL,
STV_HIDDEN, STT_NOTYPE, 0, 0, nullptr});
sym->isUsedInRegularObj = true;
return cast<Defined>(sym);
};
// 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);
} else if (config->emachine == EM_PPC64) {
addPPC64SaveRestore();
}
// 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{ctx.internalFile, StringRef(), 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 standard 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 void demoteDefined(Defined &sym, DenseMap<SectionBase *, size_t> &map) {
if (map.empty())
for (auto [i, sec] : llvm::enumerate(sym.file->getSections()))
map.try_emplace(sec, i);
// Change WEAK to GLOBAL so that if a scanned relocation references sym,
// maybeReportUndefined will report an error.
uint8_t binding = sym.isWeak() ? uint8_t(STB_GLOBAL) : sym.binding;
Undefined(sym.file, sym.getName(), binding, sym.stOther, sym.type,
/*discardedSecIdx=*/map.lookup(sym.section))
.overwrite(sym);
// Eliminate from the symbol table, otherwise we would leave an undefined
// symbol if the symbol is unreferenced in the absence of GC.
sym.isUsedInRegularObj = false;
}
// If all references to a DSO happen to be weak, the DSO is not added to
// DT_NEEDED. If that happens, replace ShardSymbol with Undefined to avoid
// dangling references to an unneeded DSO. Use a weak binding to avoid
// --no-allow-shlib-undefined diagnostics. Similarly, demote lazy symbols.
//
// In addition, demote symbols defined in discarded sections, so that
// references to /DISCARD/ discarded symbols will lead to errors.
static void demoteSymbolsAndComputeIsPreemptible() {
llvm::TimeTraceScope timeScope("Demote symbols");
DenseMap<InputFile *, DenseMap<SectionBase *, size_t>> sectionIndexMap;
for (Symbol *sym : symtab.getSymbols()) {
if (auto *d = dyn_cast<Defined>(sym)) {
if (d->section && !d->section->isLive())
demoteDefined(*d, sectionIndexMap[d->file]);
} else {
auto *s = dyn_cast<SharedSymbol>(sym);
if (sym->isLazy() || (s && !cast<SharedFile>(s->file)->isNeeded)) {
uint8_t binding = sym->isLazy() ? sym->binding : uint8_t(STB_WEAK);
Undefined(ctx.internalFile, sym->getName(), binding, sym->stOther,
sym->type)
.overwrite(*sym);
sym->versionId = VER_NDX_GLOBAL;
}
}
if (config->hasDynSymTab)
sym->isPreemptible = computeIsPreemptible(*sym);
}
}
static OutputSection *findSection(StringRef name, unsigned partition = 1) {
for (SectionCommand *cmd : script->sectionCommands)
if (auto *osd = dyn_cast<OutputDesc>(cmd))
if (osd->osec.name == name && osd->osec.partition == partition)
return &osd->osec;
return nullptr;
}
// The main function of the writer.
template <class ELFT> void Writer<ELFT>::run() {
// 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 --compressed-debug-sections is specified, 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);
// Handle --print-map(-M)/--Map and --cref. Dump them before checkSections()
// because the files may be useful in case checkSections() or openFile()
// fails, for example, due to an erroneous file size.
writeMapAndCref();
// Handle --print-memory-usage option.
if (config->printMemoryUsage)
script->printMemoryUsage(lld::outs());
if (config->checkSections)
checkSections();
// It does not make sense try to open the file if we have error already.
if (errorCount())
return;
{
llvm::TimeTraceScope timeScope("Write output file");
// Write the result down to a file.
openFile();
if (errorCount())
return;
if (!config->oFormatBinary) {
if (config->zSeparate != SeparateSegmentKind::None)
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;
if (auto e = buffer->commit())
fatal("failed to write output '" + buffer->getPath() +
"': " + toString(std::move(e)));
if (!config->cmseOutputLib.empty())
writeARMCmseImportLib<ELFT>();
}
}
template <class ELFT, class RelTy>
static void markUsedLocalSymbolsImpl(ObjFile<ELFT> *file,
llvm::ArrayRef<RelTy> rels) {
for (const RelTy &rel : rels) {
Symbol &sym = file->getRelocTargetSym(rel);
if (sym.isLocal())
sym.used = true;
}
}
// The function ensures that the "used" field of local symbols reflects the fact
// that the symbol is used in a relocation from a live section.
template <class ELFT> static void markUsedLocalSymbols() {
// With --gc-sections, the field is already filled.
// See MarkLive<ELFT>::resolveReloc().
if (config->gcSections)
return;
for (ELFFileBase *file : ctx.objectFiles) {
ObjFile<ELFT> *f = cast<ObjFile<ELFT>>(file);
for (InputSectionBase *s : f->getSections()) {
InputSection *isec = dyn_cast_or_null<InputSection>(s);
if (!isec)
continue;
if (isec->type == SHT_REL)
markUsedLocalSymbolsImpl(f, isec->getDataAs<typename ELFT::Rel>());
else if (isec->type == SHT_RELA)
markUsedLocalSymbolsImpl(f, isec->getDataAs<typename ELFT::Rela>());
}
}
}
static bool shouldKeepInSymtab(const Defined &sym) {
if (sym.isSection())
return false;
// If --emit-reloc or -r is given, preserve symbols referenced by relocations
// from live sections.
if (sym.used && config->copyRelocs)
return true;
// Exclude local symbols pointing to .ARM.exidx sections.
// They are probably mapping symbols "$d", which are optional for these
// sections. After merging the .ARM.exidx sections, some of these symbols
// may become dangling. The easiest way to avoid the issue is not to add
// them to the symbol table from the beginning.
if (config->emachine == EM_ARM && sym.section &&
sym.section->type == SHT_ARM_EXIDX)
return false;
if (config->discard == DiscardPolicy::None)
return true;
if (config->discard == DiscardPolicy::All)
return false;
// 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.
if (sym.getName().starts_with(".L") &&
(config->discard == DiscardPolicy::Locals ||
(sym.section && (sym.section->flags & SHF_MERGE))))
return false;
return true;
}
bool lld::elf::includeInSymtab(const Symbol &b) {
if (auto *d = dyn_cast<Defined>(&b)) {
// Always include absolute symbols.
SectionBase *sec = d->section;
if (!sec)
return true;
assert(sec->isLive());
if (auto *s = dyn_cast<MergeInputSection>(sec))
return s->getSectionPiece(d->value).live;
return true;
}
return b.used || !config->gcSections;
}
// Scan local symbols to:
//
// - demote symbols defined relative to /DISCARD/ discarded input sections so
// that relocations referencing them will lead to errors.
// - copy eligible symbols to .symTab
static void demoteAndCopyLocalSymbols() {
llvm::TimeTraceScope timeScope("Add local symbols");
for (ELFFileBase *file : ctx.objectFiles) {
DenseMap<SectionBase *, size_t> sectionIndexMap;
for (Symbol *b : file->getLocalSymbols()) {
assert(b->isLocal() && "should have been caught in initializeSymbols()");
auto *dr = dyn_cast<Defined>(b);
if (!dr)
continue;
if (dr->section && !dr->section->isLive())
demoteDefined(*dr, sectionIndexMap);
else if (in.symTab && includeInSymtab(*b) && shouldKeepInSymtab(*dr))
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 (SectionCommand *cmd : script->sectionCommands) {
auto *osd = dyn_cast<OutputDesc>(cmd);
if (!osd)
continue;
OutputSection &osec = osd->osec;
InputSectionBase *isec = nullptr;
// Iterate over all input sections and add a STT_SECTION symbol if any input
// section may be a relocation target.
for (SectionCommand *cmd : osec.commands) {
auto *isd = dyn_cast<InputSectionDescription>(cmd);
if (!isd)
continue;
for (InputSectionBase *s : isd->sections) {
// Relocations are not using REL[A] section symbols.
if (isStaticRelSecType(s->type))
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 is given.
if (isa<SyntheticSection>(s) && !(s->flags & SHF_MERGE))
continue;
isec = s;
break;
}
}
if (!isec)
continue;
// Set the symbol to be relative to the output section so that its st_value
// equals the output section address. Note, there may be a gap between the
// start of the output section and isec.
in.symTab->addSymbol(makeDefined(isec->file, "", STB_LOCAL, /*stOther=*/0,
STT_SECTION,
/*value=*/0, /*size=*/0, &osec));
}
}
// 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;
if (sec->relro)
return true;
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;
if (in.relroPadding && sec == in.relroPadding->getParent())
return true;
// .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 == ".fini_array" || s == ".init_array" ||
s == ".openbsd.randomdata" || s == ".preinit_array";
}
// 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_SPECIAL = 1 << 17,
RF_LARGE_ALT = 1 << 15,
RF_WRITE = 1 << 14,
RF_EXEC_WRITE = 1 << 13,
RF_EXEC = 1 << 12,
RF_RODATA = 1 << 11,
RF_LARGE = 1 << 10,
RF_NOT_RELRO = 1 << 9,
RF_NOT_TLS = 1 << 8,
RF_BSS = 1 << 7,
};
static unsigned getSectionRank(OutputSection &osec) {
unsigned rank = osec.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(osec.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 (!(osec.flags & SHF_ALLOC))
return rank | RF_NOT_ALLOC;
if (osec.type == SHT_LLVM_PART_EHDR)
return rank;
if (osec.type == SHT_LLVM_PART_PHDR)
return rank | 1;
// Put .interp first because some loaders want to see that section
// on the first page of the executable file when loaded into memory.
if (osec.name == ".interp")
return rank | 2;
// Put .note sections at the beginning so that they are likely to be included
// in a truncate core file. In particular, .note.gnu.build-id, if available,
// can identify the object file.
if (osec.type == SHT_NOTE)
return rank | 3;
rank |= RF_NOT_SPECIAL;
// Sort sections based on their access permission in the following
// order: R, RX, RXW, RW(RELRO), RW(non-RELRO).
//
// Read-only sections come first such that they go in the PT_LOAD covering the
// program headers at the start of the file.
//
// The layout for writable sections 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.
bool isExec = osec.flags & SHF_EXECINSTR;
bool isWrite = osec.flags & SHF_WRITE;
if (!isWrite && !isExec) {
// Make PROGBITS sections (e.g .rodata .eh_frame) closer to .text to
// alleviate relocation overflow pressure. Large special sections such as
// .dynstr and .dynsym can be away from .text.
if (osec.type == SHT_PROGBITS)
rank |= RF_RODATA;
// Among PROGBITS sections, place .lrodata further from .text.
// For -z lrodata-after-bss, place .lrodata after .lbss like GNU ld. This
// layout has one extra PT_LOAD, but alleviates relocation overflow
// pressure for absolute relocations referencing small data from -fno-pic
// relocatable files.
if (osec.flags & SHF_X86_64_LARGE && config->emachine == EM_X86_64)
rank |= config->zLrodataAfterBss ? RF_LARGE_ALT : 0;
else
rank |= config->zLrodataAfterBss ? 0 : RF_LARGE;
} else if (isExec) {
rank |= isWrite ? RF_EXEC_WRITE : RF_EXEC;
} else {
rank |= RF_WRITE;
// The TLS initialization block needs to be a single contiguous block. Place
// TLS sections directly before the other RELRO sections.
if (!(osec.flags & SHF_TLS))
rank |= RF_NOT_TLS;
if (isRelroSection(&osec))
osec.relro = true;
else
rank |= RF_NOT_RELRO;
// Place .ldata and .lbss after .bss. Making .bss closer to .text
// alleviates relocation overflow pressure.
// For -z lrodata-after-bss, place .lbss/.lrodata/.ldata after .bss.
// .bss/.lbss being adjacent reuses the NOBITS size optimization.
if (osec.flags & SHF_X86_64_LARGE && config->emachine == EM_X86_64) {
rank |= config->zLrodataAfterBss
? (osec.type == SHT_NOBITS ? 1 : RF_LARGE_ALT)
: RF_LARGE;
}
}
// Within TLS sections, or within other RelRo sections, or within non-RelRo
// sections, place non-NOBITS sections first.
if (osec.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.
StringRef name = osec.name;
if (name == ".got")
rank |= 1;
else if (name == ".toc")
rank |= 2;
}
if (config->emachine == EM_MIPS) {
if (osec.name != ".got")
rank |= 1;
// All sections with SHF_MIPS_GPREL flag should be grouped together
// because data in these sections is addressable with a gp relative address.
if (osec.flags & SHF_MIPS_GPREL)
rank |= 2;
}
if (config->emachine == EM_RISCV) {
// .sdata and .sbss are placed closer to make GP relaxation more profitable
// and match GNU ld.
StringRef name = osec.name;
if (name == ".sdata" || (osec.type == SHT_NOBITS && name != ".sbss"))
rank |= 1;
}
return rank;
}
static bool compareSections(const SectionCommand *aCmd,
const SectionCommand *bCmd) {
const OutputSection *a = &cast<OutputDesc>(aCmd)->osec;
const OutputSection *b = &cast<OutputDesc>(bCmd)->osec;
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->addralign);
if (p_type == PT_LOAD)
sec->ptLoad = this;
}
// A statically linked position-dependent executable should only contain
// IRELATIVE relocations and no other dynamic relocations. Encapsulation symbols
// __rel[a]_iplt_{start,end} will be defined for .rel[a].dyn, to be
// processed by the libc runtime. Other executables or DSOs use dynamic tags
// instead.
template <class ELFT> void Writer<ELFT>::addRelIpltSymbols() {
if (config->isPic)
return;
// __rela_iplt_{start,end} are initially defined relative to dummy section 0.
// We'll override Out::elfHeader with relaDyn later when we are sure that
// .rela.dyn will be present in the output.
ElfSym::relaIpltStart = addOptionalRegular(
config->isRela ? "__rela_iplt_start" : "__rel_iplt_start",
Out::elfHeader, 0, STV_HIDDEN);
ElfSym::relaIpltEnd = addOptionalRegular(
config->isRela ? "__rela_iplt_end" : "__rel_iplt_end",
Out::elfHeader, 0, STV_HIDDEN);
}
// 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 *sec = in.gotPlt.get();
if (!target->gotBaseSymInGotPlt)
sec = in.mipsGot ? cast<InputSection>(in.mipsGot.get())
: cast<InputSection>(in.got.get());
ElfSym::globalOffsetTable->section = sec;
}
// .rela_iplt_{start,end} mark the start and the end of .rel[a].dyn.
if (ElfSym::relaIpltStart && mainPart->relaDyn->isNeeded()) {
ElfSym::relaIpltStart->section = mainPart->relaDyn.get();
ElfSym::relaIpltEnd->section = mainPart->relaDyn.get();
ElfSym::relaIpltEnd->value = mainPart->relaDyn->getSize();
}
PhdrEntry *last = nullptr;
OutputSection *lastRO = nullptr;
auto isLarge = [](OutputSection *osec) {
return config->emachine == EM_X86_64 && osec->flags & SHF_X86_64_LARGE;
};
for (Partition &part : partitions) {
for (PhdrEntry *p : part.phdrs) {
if (p->p_type != PT_LOAD)
continue;
last = p;
if (!(p->p_flags & PF_W) && p->lastSec && !isLarge(p->lastSec))
lastRO = p->lastSec;
}
}
if (lastRO) {
// _etext is the first location after the last read-only loadable segment
// that does not contain large sections.
if (ElfSym::etext1)
ElfSym::etext1->section = lastRO;
if (ElfSym::etext2)
ElfSym::etext2->section = lastRO;
}
if (last) {
// _edata points to the end of the last non-large mapped initialized
// section.
OutputSection *edata = nullptr;
for (OutputSection *os : outputSections) {
if (os->type != SHT_NOBITS && !isLarge(os))
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) {
// On RISC-V, set __bss_start to the start of .sbss if present.
OutputSection *sbss =
config->emachine == EM_RISCV ? findSection(".sbss") : nullptr;
ElfSym::bss->section = sbss ? sbss : 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 getRankProximity(OutputSection *a, SectionCommand *b) {
auto *osd = dyn_cast<OutputDesc>(b);
return (osd && osd->osec.hasInputSections)
? llvm::countl_zero(a->sortRank ^ osd->osec.sortRank)
: -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(SectionCommand *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 SmallVectorImpl<SectionCommand *>::iterator
findOrphanPos(SmallVectorImpl<SectionCommand *>::iterator b,
SmallVectorImpl<SectionCommand *>::iterator e) {
OutputSection *sec = &cast<OutputDesc>(*e)->osec;
// As a special case, place .relro_padding before the SymbolAssignment using
// DATA_SEGMENT_RELRO_END, if present.
if (in.relroPadding && sec == in.relroPadding->getParent()) {
auto i = std::find_if(b, e, [=](SectionCommand *a) {
if (auto *assign = dyn_cast<SymbolAssignment>(a))
return assign->dataSegmentRelroEnd;
return false;
});
if (i != e)
return i;
}
// Find the first element that has as close a rank as possible.
auto i = std::max_element(b, e, [=](SectionCommand *a, SectionCommand *b) {
return getRankProximity(sec, a) < getRankProximity(sec, b);
});
if (i == e)
return e;
if (!isa<OutputDesc>(*i))
return e;
auto foundSec = &cast<OutputDesc>(*i)->osec;
// Consider all existing sections with the same proximity.
int proximity = getRankProximity(sec, *i);
unsigned sortRank = sec->sortRank;
if (script->hasPhdrsCommands() || !script->memoryRegions.empty())
// Prevent the orphan section to be placed before the found section. If
// custom program headers are defined, that helps to avoid adding it to a
// previous segment and changing flags of that segment, for example, making
// a read-only segment writable. If memory regions are defined, an orphan
// section should continue the same region as the found section to better
// resemble the behavior of GNU ld.
sortRank = std::max(sortRank, foundSec->sortRank);
for (; i != e; ++i) {
auto *curSecDesc = dyn_cast<OutputDesc>(*i);
if (!curSecDesc || !curSecDesc->osec.hasInputSections)
continue;
if (getRankProximity(sec, curSecDesc) != proximity ||
sortRank < curSecDesc->osec.sortRank)
break;
}
auto isOutputSecWithInputSections = [](SectionCommand *cmd) {
auto *osd = dyn_cast<OutputDesc>(cmd);
return osd && osd->osec.hasInputSections;
};
auto j =
std::find_if(std::make_reverse_iterator(i), std::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;
}
// Adds random priorities to sections not already in the map.
static void maybeShuffle(DenseMap<const InputSectionBase *, int> &order) {
if (config->shuffleSections.empty())
return;
SmallVector<InputSectionBase *, 0> matched, sections = ctx.inputSections;
matched.reserve(sections.size());
for (const auto &patAndSeed : config->shuffleSections) {
matched.clear();
for (InputSectionBase *sec : sections)
if (patAndSeed.first.match(sec->name))
matched.push_back(sec);
const uint32_t seed = patAndSeed.second;
if (seed == UINT32_MAX) {
// If --shuffle-sections <section-glob>=-1, reverse the section order. The
// section order is stable even if the number of sections changes. This is
// useful to catch issues like static initialization order fiasco
// reliably.
std::reverse(matched.begin(), matched.end());
} else {
std::mt19937 g(seed ? seed : std::random_device()());
llvm::shuffle(matched.begin(), matched.end(), g);
}
size_t i = 0;
for (InputSectionBase *&sec : sections)
if (patAndSeed.first.match(sec->name))
sec = matched[i++];
}
// Existing priorities are < 0, so use priorities >= 0 for the missing
// sections.
int prio = 0;
for (InputSectionBase *sec : sections) {
if (order.try_emplace(sec, prio).second)
++prio;
}
}
// 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<CachedHashStringRef, SymbolOrderEntry> symbolOrder;
int priority = -config->symbolOrderingFile.size();
for (StringRef s : config->symbolOrderingFile)
symbolOrder.insert({CachedHashStringRef(s), {priority++, false}});
// Build a map from sections to their priorities.
auto addSym = [&](Symbol &sym) {
auto it = symbolOrder.find(CachedHashStringRef(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)];
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.
for (Symbol *sym : symtab.getSymbols())
addSym(*sym);
for (ELFFileBase *file : ctx.objectFiles)
for (Symbol *sym : file->getLocalSymbols())
addSym(*sym);
if (config->warnSymbolOrdering)
for (auto orderEntry : symbolOrder)
if (!orderEntry.second.present)
warn("symbol ordering file: no such symbol: " + orderEntry.first.val());
return sectionOrder;
}
// Sorts the sections in ISD according to the provided section order.
static void
sortISDBySectionOrder(InputSectionDescription *isd,
const DenseMap<const InputSectionBase *, int> &order,
bool executableOutputSection) {
SmallVector<InputSection *, 0> unorderedSections;
SmallVector<std::pair<InputSection *, int>, 0> orderedSections;
uint64_t unorderedSize = 0;
uint64_t totalSize = 0;
for (InputSection *isec : isd->sections) {
if (executableOutputSection)
totalSize += isec->getSize();
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.
//
// The above is not necessary if total size of input sections in this "isd"
// is small. Note that we assume all input sections are executable if the
// output section is executable (which is not always true but supposed to
// cover most cases).
size_t insPt = 0;
if (executableOutputSection && !orderedSections.empty() &&
target->getThunkSectionSpacing() &&
totalSize >= target->getThunkSectionSpacing()) {
uint64_t unorderedPos = 0;
for (; insPt != unorderedSections.size(); ++insPt) {
unorderedPos += unorderedSections[insPt]->getSize();
if (unorderedPos > unorderedSize / 2)
break;
}
}
isd->sections.clear();
for (InputSection *isec : ArrayRef(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 : ArrayRef(unorderedSections).slice(insPt))
isd->sections.push_back(isec);
}
static void sortSection(OutputSection &osec,
const DenseMap<const InputSectionBase *, int> &order) {
StringRef name = osec.name;
// Never sort these.
if (name == ".init" || name == ".fini")
return;
// Sort input sections by priority using the list provided by
// --symbol-ordering-file or --shuffle-sections=. This is a least significant
// digit radix sort. The sections may be sorted stably again by a more
// significant key.
if (!order.empty())
for (SectionCommand *b : osec.commands)
if (auto *isd = dyn_cast<InputSectionDescription>(b))
sortISDBySectionOrder(isd, order, osec.flags & SHF_EXECINSTR);
if (script->hasSectionsCommand)
return;
if (name == ".init_array" || name == ".fini_array") {
osec.sortInitFini();
} else if (name == ".ctors" || name == ".dtors") {
osec.sortCtorsDtors();
} else if (config->emachine == EM_PPC64 && name == ".toc") {
// .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
assert(osec.commands.size() == 1);
auto *isd = cast<InputSectionDescription>(osec.commands[0]);
llvm::stable_sort(isd->sections,
[](const InputSection *a, const InputSection *b) -> bool {
return a->file->ppc64SmallCodeModelTocRelocs &&
!b->file->ppc64SmallCodeModelTocRelocs;
});
}
}
// 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();
maybeShuffle(order);
for (SectionCommand *cmd : script->sectionCommands)
if (auto *osd = dyn_cast<OutputDesc>(cmd))
sortSection(osd->osec, order);
}
template <class ELFT> void Writer<ELFT>::sortSections() {
llvm::TimeTraceScope timeScope("Sort sections");
// 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) {
script->adjustOutputSections();
return;
}
sortInputSections();
for (SectionCommand *cmd : script->sectionCommands)
if (auto *osd = dyn_cast<OutputDesc>(cmd))
osd->osec.sortRank = getSectionRank(osd->osec);
if (!script->hasSectionsCommand) {
// OutputDescs are mostly contiguous, but may be interleaved with
// SymbolAssignments in the presence of INSERT commands.
auto mid = std::stable_partition(
script->sectionCommands.begin(), script->sectionCommands.end(),
[](SectionCommand *cmd) { return isa<OutputDesc>(cmd); });
std::stable_sort(script->sectionCommands.begin(), mid, compareSections);
}
// Process INSERT commands and update output section attributes. From this
// point onwards the order of script->sectionCommands is fixed.
script->processInsertCommands();
script->adjustOutputSections();
if (script->hasSectionsCommand)
sortOrphanSections();
script->adjustSectionsAfterSorting();
}
template <class ELFT> void Writer<ELFT>::sortOrphanSections() {
// 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, [](SectionCommand *cmd) {
if (auto *osd = dyn_cast<OutputDesc>(cmd))
return osd->osec.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, [](SectionCommand *cmd) { return !shouldSkip(cmd); });
if (firstSectionOrDotAssignment != e &&
isa<SymbolAssignment>(**firstSectionOrDotAssignment))
++firstSectionOrDotAssignment;
i = firstSectionOrDotAssignment;
while (nonScriptI != e) {
auto pos = findOrphanPos(i, nonScriptI);
OutputSection *orphan = &cast<OutputDesc>(*nonScriptI)->osec;
// 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, [=](SectionCommand *cmd) {
return cast<OutputDesc>(cmd)->osec.sortRank != rank;
});
std::rotate(pos, nonScriptI, end);
nonScriptI = end;
}
}
static bool compareByFilePosition(InputSection *a, InputSection *b) {
InputSection *la = a->flags & SHF_LINK_ORDER ? a->getLinkOrderDep() : nullptr;
InputSection *lb = b->flags & SHF_LINK_ORDER ? b->getLinkOrderDep() : nullptr;
// SHF_LINK_ORDER sections with non-zero sh_link are ordered before
// non-SHF_LINK_ORDER sections and SHF_LINK_ORDER sections with zero sh_link.
if (!la || !lb)
return la && !lb;
OutputSection *aOut = la->getParent();
OutputSection *bOut = lb->getParent();
if (aOut != bOut)
return aOut->addr < bOut->addr;
return la->outSecOff < lb->outSecOff;
}
template <class ELFT> void Writer<ELFT>::resolveShfLinkOrder() {
llvm::TimeTraceScope timeScope("Resolve SHF_LINK_ORDER");
for (OutputSection *sec : outputSections) {
if (!(sec->flags & SHF_LINK_ORDER))
continue;
// 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;
// Link order may be distributed across several InputSectionDescriptions.
// Sorting is performed separately.
SmallVector<InputSection **, 0> scriptSections;
SmallVector<InputSection *, 0> sections;
for (SectionCommand *cmd : sec->commands) {
auto *isd = dyn_cast<InputSectionDescription>(cmd);
if (!isd)
continue;
bool hasLinkOrder = false;
scriptSections.clear();
sections.clear();
for (InputSection *&isec : isd->sections) {
if (isec->flags & SHF_LINK_ORDER) {
InputSection *link = isec->getLinkOrderDep();
if (link && !link->getParent())
error(toString(isec) + ": sh_link points to discarded section " +
toString(link));
hasLinkOrder = true;
}
scriptSections.push_back(&isec);
sections.push_back(isec);
}
if (hasLinkOrder && errorCount() == 0) {
llvm::stable_sort(sections, compareByFilePosition);
for (int i = 0, n = sections.size(); i != n; ++i)
*scriptSections[i] = sections[i];
}
}
}
}
static void finalizeSynthetic(SyntheticSection *sec) {
if (sec && sec->isNeeded() && sec->getParent()) {
llvm::TimeTraceScope timeScope("Finalize synthetic sections", sec->name);
sec->finalizeContents();
}
}
// 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() {
llvm::TimeTraceScope timeScope("Finalize address dependent content");
ThunkCreator tc;
AArch64Err843419Patcher a64p;
ARMErr657417Patcher a32p;
script->assignAddresses();
// .ARM.exidx and SHF_LINK_ORDER do not require precise addresses, but they
// do require the relative addresses of OutputSections because linker scripts
// can assign Virtual Addresses to OutputSections that are not monotonically
// increasing.
for (Partition &part : partitions)
finalizeSynthetic(part.armExidx.get());
resolveShfLinkOrder();
// Converts call x@GDPLT to call __tls_get_addr
if (config->emachine == EM_HEXAGON)
hexagonTLSSymbolUpdate(outputSections);
uint32_t pass = 0, assignPasses = 0;
for (;;) {
bool changed = target->needsThunks ? tc.createThunks(pass, outputSections)
: target->relaxOnce(pass);
++pass;
// With Thunk Size much smaller than branch range we expect to
// converge quickly; if we get to 30 something has gone wrong.
if (changed && pass >= 30) {
error(target->needsThunks ? "thunk creation not converged"
: "relaxation not converged");
break;
}
if (config->fixCortexA53Errata843419) {
if (changed)
script->assignAddresses();
changed |= a64p.createFixes();
}
if (config->fixCortexA8) {
if (changed)
script->assignAddresses();
changed |= a32p.createFixes();
}
finalizeSynthetic(in.got.get());
if (in.mipsGot)
in.mipsGot->updateAllocSize();
for (Partition &part : partitions) {
changed |= part.relaDyn->updateAllocSize();
if (part.relrDyn)
changed |= part.relrDyn->updateAllocSize();
if (part.memtagGlobalDescriptors)
changed |= part.memtagGlobalDescriptors->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;
}
}
}
if (!config->relocatable)
target->finalizeRelax(pass);
if (config->relocatable)
for (OutputSection *sec : outputSections)
sec->addr = 0;
// If addrExpr is set, the address may not be a multiple of the alignment.
// Warn because this is error-prone.
for (SectionCommand *cmd : script->sectionCommands)
if (auto *osd = dyn_cast<OutputDesc>(cmd)) {
OutputSection *osec = &osd->osec;
if (osec->addr % osec->addralign != 0)
warn("address (0x" + Twine::utohexstr(osec->addr) + ") of section " +
osec->name + " is not a multiple of alignment (" +
Twine(osec->addralign) + ")");
}
}
// If Input Sections have been shrunk (basic block sections) then
// update symbol values and sizes associated with these sections. With basic
// block sections, input sections can shrink when the jump instructions at
// the end of the section are relaxed.
static void fixSymbolsAfterShrinking() {
for (InputFile *File : ctx.objectFiles) {
parallelForEach(File->getSymbols(), [&](Symbol *Sym) {
auto *def = dyn_cast<Defined>(Sym);
if (!def)
return;
const SectionBase *sec = def->section;
if (!sec)
return;
const InputSectionBase *inputSec = dyn_cast<InputSectionBase>(sec);
if (!inputSec || !inputSec->bytesDropped)
return;
const size_t OldSize = inputSec->content().size();
const size_t NewSize = OldSize - inputSec->bytesDropped;
if (def->value > NewSize && def->value <= OldSize) {
LLVM_DEBUG(llvm::dbgs()
<< "Moving symbol " << Sym->getName() << " from "
<< def->value << " to "
<< def->value - inputSec->bytesDropped << " bytes\n");
def->value -= inputSec->bytesDropped;
return;
}
if (def->value + def->size > NewSize && def->value <= OldSize &&
def->value + def->size <= OldSize) {
LLVM_DEBUG(llvm::dbgs()
<< "Shrinking symbol " << Sym->getName() << " from "
<< def->size << " to " << def->size - inputSec->bytesDropped
<< " bytes\n");
def->size -= inputSec->bytesDropped;
}
});
}
}
// If basic block sections exist, there are opportunities to delete fall thru
// jumps and shrink jump instructions after basic block reordering. This
// relaxation pass does that. It is only enabled when --optimize-bb-jumps
// option is used.
template <class ELFT> void Writer<ELFT>::optimizeBasicBlockJumps() {
assert(config->optimizeBBJumps);
SmallVector<InputSection *, 0> storage;
script->assignAddresses();
// For every output section that has executable input sections, this
// does the following:
// 1. Deletes all direct jump instructions in input sections that
// jump to the following section as it is not required.
// 2. If there are two consecutive jump instructions, it checks
// if they can be flipped and one can be deleted.
for (OutputSection *osec : outputSections) {
if (!(osec->flags & SHF_EXECINSTR))
continue;
ArrayRef<InputSection *> sections = getInputSections(*osec, storage);
size_t numDeleted = 0;
// Delete all fall through jump instructions. Also, check if two
// consecutive jump instructions can be flipped so that a fall
// through jmp instruction can be deleted.
for (size_t i = 0, e = sections.size(); i != e; ++i) {
InputSection *next = i + 1 < sections.size() ? sections[i + 1] : nullptr;
InputSection &sec = *sections[i];
numDeleted += target->deleteFallThruJmpInsn(sec, sec.file, next);
}
if (numDeleted > 0) {
script->assignAddresses();
LLVM_DEBUG(llvm::dbgs()
<< "Removing " << numDeleted << " fall through jumps\n");
}
}
fixSymbolsAfterShrinking();
for (OutputSection *osec : outputSections)
for (InputSection *is : getInputSections(*osec, storage))
is->trim();
}
// 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. Reverse iterate to find the first synthetic section
// after a non-synthetic one which will be our starting point.
auto start =
llvm::find_if(llvm::reverse(ctx.inputSections), [](InputSectionBase *s) {
return !isa<SyntheticSection>(s);
}).base();
// Remove unused synthetic sections from ctx.inputSections;
DenseSet<InputSectionBase *> unused;
auto end =
std::remove_if(start, ctx.inputSections.end(), [&](InputSectionBase *s) {
auto *sec = cast<SyntheticSection>(s);
if (sec->getParent() && sec->isNeeded())
return false;
unused.insert(sec);
return true;
});
ctx.inputSections.erase(end, ctx.inputSections.end());
// Remove unused synthetic sections from the corresponding input section
// description and orphanSections.
for (auto *sec : unused)
if (OutputSection *osec = cast<SyntheticSection>(sec)->getParent())
for (SectionCommand *cmd : osec->commands)
if (auto *isd = dyn_cast<InputSectionDescription>(cmd))
llvm::erase_if(isd->sections, [&](InputSection *isec) {
return unused.count(isec);
});
llvm::erase_if(script->orphanSections, [&](const InputSectionBase *sec) {
return unused.count(sec);
});
}
// Create output section objects and add them to OutputSections.
template <class ELFT> void Writer<ELFT>::finalizeSections() {
if (!config->relocatable) {
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.
addStartEndSymbols();
for (SectionCommand *cmd : script->sectionCommands)
if (auto *osd = dyn_cast<OutputDesc>(cmd))
addStartStopSymbols(osd->osec);
// 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) {
Symbol *s = symtab.addSymbol(Defined{
ctx.internalFile, "_DYNAMIC", STB_WEAK, STV_HIDDEN, STT_NOTYPE,
/*value=*/0, /*size=*/0, mainPart->dynamic.get()});
s->isUsedInRegularObj = true;
}
// 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) {
ElfSym::riscvGlobalPointer = nullptr;
if (!config->shared) {
OutputSection *sec = findSection(".sdata");
addOptionalRegular(
"__global_pointer$", sec ? sec : Out::elfHeader, 0x800, STV_DEFAULT);
// Set riscvGlobalPointer to be used by the optional global pointer
// relaxation.
if (config->relaxGP) {
Symbol *s = symtab.find("__global_pointer$");
if (s && s->isDefined())
ElfSym::riscvGlobalPointer = cast<Defined>(s);
}
}
}
if (config->emachine == EM_386 || 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{ctx.internalFile, StringRef(), 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.
{
llvm::TimeTraceScope timeScope("Finalize .eh_frame");
for (Partition &part : partitions)
finalizeSynthetic(part.ehFrame.get());
}
}
demoteSymbolsAndComputeIsPreemptible();
if (config->copyRelocs && config->discard != DiscardPolicy::None)
markUsedLocalSymbols<ELFT>();
demoteAndCopyLocalSymbols();
if (config->copyRelocs)
addSectionSymbols();
// Change values of linker-script-defined symbols from placeholders (assigned
// by declareSymbols) to actual definitions.
script->processSymbolAssignments();
if (!config->relocatable) {
llvm::TimeTraceScope timeScope("Scan relocations");
// 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.
ppc64noTocRelax.clear();
scanRelocations<ELFT>();
reportUndefinedSymbols();
postScanRelocations();
if (in.plt && in.plt->isNeeded())
in.plt->addSymbols();
if (in.iplt && in.iplt->isNeeded())
in.iplt->addSymbols();
if (config->unresolvedSymbolsInShlib != UnresolvedPolicy::Ignore) {
auto diagnose =
config->unresolvedSymbolsInShlib == UnresolvedPolicy::ReportError
? errorOrWarn
: warn;
// Error on undefined symbols in a shared object, if all of its DT_NEEDED
// entries 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.
//
// If a DSO reference is resolved by a SharedSymbol, but the SharedSymbol
// is overridden by a hidden visibility Defined (which is later discarded
// due to GC), don't report the diagnostic. However, this may indicate an
// unintended SharedSymbol.
for (SharedFile *file : ctx.sharedFiles) {
bool allNeededIsKnown =
llvm::all_of(file->dtNeeded, [&](StringRef needed) {
return symtab.soNames.count(CachedHashStringRef(needed));
});
if (!allNeededIsKnown)
continue;
for (Symbol *sym : file->requiredSymbols) {
if (sym->dsoDefined)
continue;
if (sym->isUndefined() && !sym->isWeak()) {
diagnose("undefined reference: " + toString(*sym) +
"\n>>> referenced by " + toString(file) +
" (disallowed by --no-allow-shlib-undefined)");
} else if (sym->isDefined() && sym->computeBinding() == STB_LOCAL) {
diagnose("non-exported symbol '" + toString(*sym) + "' in '" +
toString(sym->file) + "' is referenced by DSO '" +
toString(file) + "'");
}
}
}
}
}
{
llvm::TimeTraceScope timeScope("Add symbols to symtabs");
// Now that we have defined all possible global symbols including linker-
// synthesized ones. Visit all symbols to give the finishing touches.
for (Symbol *sym : symtab.getSymbols()) {
if (!sym->isUsedInRegularObj || !includeInSymtab(*sym))
continue;
if (!config->relocatable)
sym->binding = sym->computeBinding();
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.needsDynSymIndex() &&
syms.insert(reloc.sym).second)
part.dynSymTab->addSymbol(reloc.sym);
}
}
if (in.mipsGot)
in.mipsGot->build();
removeUnusedSyntheticSections();
script->diagnoseOrphanHandling();
script->diagnoseMissingSGSectionAddress();
sortSections();
// Create a list of OutputSections, assign sectionIndex, and populate
// in.shStrTab.
for (SectionCommand *cmd : script->sectionCommands)
if (auto *osd = dyn_cast<OutputDesc>(cmd)) {
OutputSection *osec = &osd->osec;
outputSections.push_back(osec);
osec->sectionIndex = outputSections.size();
osec->shName = in.shStrTab->addString(osec->name);
}
// 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; };
}
// With the outputSections available check for GDPLT relocations
// and add __tls_get_addr symbol if needed.
if (config->emachine == EM_HEXAGON && hexagonNeedsTLSSymbol(outputSections)) {
Symbol *sym =
symtab.addSymbol(Undefined{ctx.internalFile, "__tls_get_addr",
STB_GLOBAL, STV_DEFAULT, STT_NOTYPE});
sym->isPreemptible = true;
partitions[0].dynSymTab->addSymbol(sym);
}
// 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;
Out::elfHeader->size = sizeof(typename ELFT::Ehdr);
// 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);
}
if (config->emachine == EM_RISCV)
addPhdrForSection(part, SHT_RISCV_ATTRIBUTES, PT_RISCV_ATTRIBUTES,
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();
{
llvm::TimeTraceScope timeScope("Finalize synthetic sections");
finalizeSynthetic(in.bss.get());
finalizeSynthetic(in.bssRelRo.get());
finalizeSynthetic(in.symTabShndx.get());
finalizeSynthetic(in.shStrTab.get());
finalizeSynthetic(in.strTab.get());
finalizeSynthetic(in.got.get());
finalizeSynthetic(in.mipsGot.get());
finalizeSynthetic(in.igotPlt.get());
finalizeSynthetic(in.gotPlt.get());
finalizeSynthetic(in.relaPlt.get());
finalizeSynthetic(in.plt.get());
finalizeSynthetic(in.iplt.get());
finalizeSynthetic(in.ppc32Got2.get());
finalizeSynthetic(in.partIndex.get());
// 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) {
if (part.relaDyn) {
part.relaDyn->mergeRels();
// Compute DT_RELACOUNT to be used by part.dynamic.
part.relaDyn->partitionRels();
finalizeSynthetic(part.relaDyn.get());
}
if (part.relrDyn) {
part.relrDyn->mergeRels();
finalizeSynthetic(part.relrDyn.get());
}
finalizeSynthetic(part.dynSymTab.get());
finalizeSynthetic(part.gnuHashTab.get());
finalizeSynthetic(part.hashTab.get());
finalizeSynthetic(part.verDef.get());
finalizeSynthetic(part.ehFrameHdr.get());
finalizeSynthetic(part.verSym.get());
finalizeSynthetic(part.verNeed.get());
finalizeSynthetic(part.dynamic.get());
}
}
if (!script->hasSectionsCommand && !config->relocatable)
fixSectionAlignments();
// 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();
// All information needed for OutputSection part of Map file is available.
if (errorCount())
return;
{
llvm::TimeTraceScope timeScope("Finalize synthetic sections");
// finalizeAddressDependentContent may have added local symbols to the
// static symbol table.
finalizeSynthetic(in.symTab.get());
finalizeSynthetic(in.debugNames.get());
finalizeSynthetic(in.ppc64LongBranchTarget.get());
finalizeSynthetic(in.armCmseSGSection.get());
}
// Relaxation to delete inter-basic block jumps created by basic block
// sections. Run after in.symTab is finalized as optimizeBasicBlockJumps
// can relax jump instructions based on symbol offset.
if (config->optimizeBBJumps)
optimizeBasicBlockJumps();
// 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();
script->checkFinalScriptConditions();
if (config->emachine == EM_ARM && !config->isLE && config->armBe8) {
addArmInputSectionMappingSymbols();
sortArmMappingSymbols();
}
}
// Ensure data sections are not mixed with executable sections when
// --execute-only is used. --execute-only make pages executable but not
// readable.
template <class ELFT> void Writer<ELFT>::checkExecuteOnly() {
if (!config->executeOnly)
return;
SmallVector<InputSection *, 0> storage;
for (OutputSection *osec : outputSections)
if (osec->flags & SHF_EXECINSTR)
for (InputSection *isec : getInputSections(*osec, storage))
if (!(isec->flags & SHF_EXECINSTR))
error("cannot place " + toString(isec) + " into " +
toString(osec->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 rare situations, the .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 && !script->isDiscarded(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 &osec) {
StringRef s = osec.name;
if (!isValidCIdentifier(s))
return;
addOptionalRegular(saver().save("__start_" + s), &osec, 0,
config->zStartStopVisibility);
addOptionalRegular(saver().save("__stop_" + s), &osec, -1,
config->zStartStopVisibility);
}
static bool needsPtLoad(OutputSection *sec) {
if (!(sec->flags & SHF_ALLOC))
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;
}
// Adjust phdr flags according to certain options.
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;
return flags;
}
// Decide which program headers to create and which sections to include in each
// one.
template <class ELFT>
SmallVector<PhdrEntry *, 0> Writer<ELFT>::createPhdrs(Partition &part) {
SmallVector<PhdrEntry *, 0> 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 processing 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;
}
}
relRo->p_align = 1;
for (OutputSection *sec : outputSections) {
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
// incompatible flags or is loaded at a discontiguous address or memory
// region using AT or AT> linker script command, respectively.
//
// As an exception, we don't create a separate load segment for the ELF
// headers, even if the first "real" output has an AT or AT> attribute.
//
// In addition, NOBITS sections should only be placed at the end of a LOAD
// segment (since it's represented as p_filesz < p_memsz). If we have a
// not-NOBITS section after a NOBITS, we create a new LOAD for the latter
// even if flags match, so as not to require actually writing the
// supposed-to-be-NOBITS section to the output file. (However, we cannot do
// so when hasSectionsCommand, since we cannot introduce the extra alignment
// needed to create a new LOAD)
uint64_t newFlags = computeFlags(sec->getPhdrFlags());
// When --no-rosegment is specified, RO and RX sections are compatible.
uint32_t incompatible = flags ^ newFlags;
if (config->singleRoRx && !(newFlags & PF_W))
incompatible &= ~PF_X;
if (incompatible)
load = nullptr;
bool sameLMARegion =
load && !sec->lmaExpr && sec->lmaRegion == load->firstSec->lmaRegion;
if (load && sec != relroEnd &&
sec->memRegion == load->firstSec->memRegion &&
(sameLMARegion || load->lastSec == Out::programHeaders) &&
(script->hasSectionsCommand || sec->type == SHT_NOBITS ||
load->lastSec->type != SHT_NOBITS)) {
load->p_flags |= newFlags;
} else {
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);
if (config->zGnustack != GnuStackKind::None) {
// 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->zGnustack == GnuStackKind::Exec)
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);
if (OutputSection *cmd = findSection(".note.gnu.property", partNo))
addHdr(PT_GNU_PROPERTY, PF_R)->add(cmd);
// 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->addralign != sec->addralign)
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)
return;
cmd->alignExpr = [align = cmd->addralign]() { return align; };
if (!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.
if (config->zSeparate == SeparateSegmentKind::Loadable ||
(config->zSeparate == SeparateSegmentKind::Code && prev &&
(prev->p_flags & PF_X) != (p->p_flags & PF_X)) ||
cmd->type == SHT_LLVM_PART_EHDR)
cmd->addrExpr = [] {
return alignToPowerOf2(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 alignToPowerOf2(script->getDot(), config->maxPageSize) +
alignToPowerOf2(script->getDot() % config->maxPageSize,
Out::tlsPhdr->p_align);
};
else
cmd->addrExpr = [] {
return alignToPowerOf2(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/PT_TLS. By convention, we keep section offsets monotonically
// increasing rather than setting to zero.
if (os->type == SHT_NOBITS &&
(!Out::tlsPhdr || Out::tlsPhdr->firstSec != os))
return off;
// If the section is not in a PT_LOAD, we just have to align it.
if (!os->ptLoad)
return alignToPowerOf2(off, os->addralign);
// 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;
}
template <class ELFT> void Writer<ELFT>::assignFileOffsetsBinary() {
// Compute the minimum LMA of all non-empty non-NOBITS sections as minAddr.
auto needsOffset = [](OutputSection &sec) {
return sec.type != SHT_NOBITS && (sec.flags & SHF_ALLOC) && sec.size > 0;
};
uint64_t minAddr = UINT64_MAX;
for (OutputSection *sec : outputSections)
if (needsOffset(*sec)) {
sec->offset = sec->getLMA();
minAddr = std::min(minAddr, sec->offset);
}
// Sections are laid out at LMA minus minAddr.
fileSize = 0;
for (OutputSection *sec : outputSections)
if (needsOffset(*sec)) {
sec->offset -= minAddr;
fileSize = std::max(fileSize, sec->offset + sec->size);
}
}
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() {
Out::programHeaders->offset = Out::elfHeader->size;
uint64_t off = Out::elfHeader->size + Out::programHeaders->size;
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;
// Layout SHF_ALLOC sections before non-SHF_ALLOC sections. A non-SHF_ALLOC
// will not occupy file offsets contained by a PT_LOAD.
for (OutputSection *sec : outputSections) {
if (!(sec->flags & SHF_ALLOC))
continue;
off = computeFileOffset(sec, off);
sec->offset = off;
if (sec->type != SHT_NOBITS)
off += sec->size;
// 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->zSeparate != SeparateSegmentKind::None && lastRX &&
lastRX->lastSec == sec)
off = alignToPowerOf2(off, config->maxPageSize);
}
for (OutputSection *osec : outputSections)
if (!(osec->flags & SHF_ALLOC)) {
osec->offset = alignToPowerOf2(off, osec->addralign);
off = osec->offset + osec->size;
}
sectionHeaderOff = alignToPowerOf2(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;
// .ARM.exidx sections may not be within a single .ARM.exidx
// output section. We always want to describe just the
// SyntheticSection.
if (part.armExidx && p->p_type == PT_ARM_EXIDX) {
p->p_filesz = part.armExidx->getSize();
p->p_memsz = part.armExidx->getSize();
p->p_offset = first->offset + part.armExidx->outSecOff;
p->p_vaddr = first->addr + part.armExidx->outSecOff;
p->p_align = part.armExidx->addralign;
if (part.elfHeader)
p->p_offset -= part.elfHeader->getParent()->offset;
if (!p->hasLMA)
p->p_paddr = first->getLMA() + part.armExidx->outSecOff;
return;
}
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();
}
}
}
// 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 addresses).
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 > uint64_t(UINT32_MAX) + 1))
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 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 (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) {
std::string msg;
raw_string_ostream s(msg);
s << "output file too large: " << Twine(fileSize) << " bytes\n"
<< "section sizes:\n";
for (OutputSection *os : outputSections)
s << os->name << ' ' << os->size << "\n";
error(s.str());
return;
}
unlinkAsync(config->outputFile);
unsigned flags = 0;
if (!config->relocatable)
flags |= FileOutputBuffer::F_executable;
if (!config->mmapOutputFile)
flags |= FileOutputBuffer::F_no_mmap;
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() {
parallel::TaskGroup tg;
for (OutputSection *sec : outputSections)
if (sec->flags & SHF_ALLOC)
sec->writeTo<ELFT>(Out::bufferStart + sec->offset, tg);
}
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() {
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, 4),
Out::bufferStart +
alignToPowerOf2(p->firstSec->offset + p->p_filesz,
config->maxPageSize));
// 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 =
alignToPowerOf2(last->p_filesz, config->maxPageSize);
}
}
// Write section contents to a mmap'ed file.
template <class ELFT> void Writer<ELFT>::writeSections() {
llvm::TimeTraceScope timeScope("Write sections");
{
// 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.
parallel::TaskGroup tg;
for (OutputSection *sec : outputSections)
if (isStaticRelSecType(sec->type))
sec->writeTo<ELFT>(Out::bufferStart + sec->offset, tg);
}
{
parallel::TaskGroup tg;
for (OutputSection *sec : outputSections)
if (!isStaticRelSecType(sec->type))
sec->writeTo<ELFT>(Out::bufferStart + sec->offset, tg);
}
// Finally, check that all dynamic relocation addends were written correctly.
if (config->checkDynamicRelocs && config->writeAddends) {
for (OutputSection *sec : outputSections)
if (isStaticRelSecType(sec->type))
sec->checkDynRelAddends(Out::bufferStart);
}
}
// 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);
const size_t hashesSize = chunks.size() * hashBuf.size();
std::unique_ptr<uint8_t[]> hashes(new uint8_t[hashesSize]);
// Compute hash values.
parallelFor(0, chunks.size(), [&](size_t i) {
hashFn(hashes.get() + i * hashBuf.size(), chunks[i]);
});
// Write to the final output buffer.
hashFn(hashBuf.data(), ArrayRef(hashes.get(), hashesSize));
}
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::unique_ptr<uint8_t[]> buildId(new uint8_t[hashSize]);
MutableArrayRef<uint8_t> output(buildId.get(), hashSize);
llvm::ArrayRef<uint8_t> input{Out::bufferStart, size_t(fileSize)};
// Fedora introduced build ID as "approximation of true uniqueness across all
// binaries that might be used by overlapping sets of people". It does not
// need some security goals that some hash algorithms strive to provide, e.g.
// (second-)preimage and collision resistance. In practice people use 'md5'
// and 'sha1' just for different lengths. Implement them with the more
// efficient BLAKE3.
switch (config->buildId) {
case BuildIdKind::Fast:
computeHash(output, input, [](uint8_t *dest, ArrayRef<uint8_t> arr) {
write64le(dest, xxh3_64bits(arr));
});
break;
case BuildIdKind::Md5:
computeHash(output, input, [&](uint8_t *dest, ArrayRef<uint8_t> arr) {
memcpy(dest, BLAKE3::hash<16>(arr).data(), hashSize);
});
break;
case BuildIdKind::Sha1:
computeHash(output, input, [&](uint8_t *dest, ArrayRef<uint8_t> arr) {
memcpy(dest, BLAKE3::hash<20>(arr).data(), hashSize);
});
break;
case BuildIdKind::Uuid:
if (auto ec = llvm::getRandomBytes(buildId.get(), hashSize))
error("entropy source failure: " + ec.message());
break;
default:
llvm_unreachable("unknown BuildIdKind");
}
for (Partition &part : partitions)
part.buildId->writeBuildId(output);
}
template void elf::writeResult<ELF32LE>();
template void elf::writeResult<ELF32BE>();
template void elf::writeResult<ELF64LE>();
template void elf::writeResult<ELF64BE>();