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Writer.cpp
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Writer.cpp
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//===- Writer.cpp ---------------------------------------------------------===//
//
// The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
#include "Writer.h"
#include "Config.h"
#include "LinkerScript.h"
#include "MapFile.h"
#include "Memory.h"
#include "OutputSections.h"
#include "Relocations.h"
#include "Strings.h"
#include "SymbolTable.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/Support/FileOutputBuffer.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/raw_ostream.h"
#include <climits>
#include <thread>
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:
typedef typename ELFT::uint uintX_t;
typedef typename ELFT::Shdr Elf_Shdr;
typedef typename ELFT::Ehdr Elf_Ehdr;
typedef typename ELFT::Phdr Elf_Phdr;
typedef typename ELFT::Sym Elf_Sym;
typedef typename ELFT::SymRange Elf_Sym_Range;
typedef typename ELFT::Rela Elf_Rela;
void run();
private:
void createSyntheticSections();
void copyLocalSymbols();
void addSectionSymbols();
void addReservedSymbols();
void createSections();
void forEachRelSec(std::function<void(InputSectionBase &)> Fn);
void sortSections();
void finalizeSections();
void addPredefinedSections();
std::vector<PhdrEntry> createPhdrs();
void removeEmptyPTLoad();
void addPtArmExid(std::vector<PhdrEntry> &Phdrs);
void assignAddresses();
void assignFileOffsets();
void assignFileOffsetsBinary();
void setPhdrs();
void fixHeaders();
void fixSectionAlignments();
void fixPredefinedSymbols();
void openFile();
void writeHeader();
void writeSections();
void writeSectionsBinary();
void writeBuildId();
std::unique_ptr<FileOutputBuffer> Buffer;
std::vector<OutputSection *> OutputSections;
OutputSectionFactory Factory{OutputSections};
void addRelIpltSymbols();
void addStartEndSymbols();
void addStartStopSymbols(OutputSection *Sec);
uintX_t getEntryAddr();
OutputSection *findSection(StringRef Name);
std::vector<PhdrEntry> Phdrs;
uintX_t FileSize;
uintX_t SectionHeaderOff;
bool AllocateHeader = true;
};
} // anonymous namespace
StringRef elf::getOutputSectionName(StringRef Name) {
if (Config->Relocatable)
return Name;
// If -emit-relocs is given (which is rare), we need to copy
// relocation sections to the output. If input section .foo is
// output as .bar, we want to rename .rel.foo .rel.bar as well.
if (Config->EmitRelocs) {
for (StringRef V : {".rel.", ".rela."}) {
if (Name.startswith(V)) {
StringRef Inner = getOutputSectionName(Name.substr(V.size() - 1));
return Saver.save(Twine(V.drop_back()) + Inner);
}
}
}
for (StringRef V :
{".text.", ".rodata.", ".data.rel.ro.", ".data.", ".bss.rel.ro.",
".bss.", ".init_array.", ".fini_array.", ".ctors.", ".dtors.", ".tbss.",
".gcc_except_table.", ".tdata.", ".ARM.exidx."}) {
StringRef Prefix = V.drop_back();
if (Name.startswith(V) || Name == Prefix)
return Prefix;
}
// CommonSection is identified as "COMMON" in linker scripts.
// By default, it should go to .bss section.
if (Name == "COMMON")
return ".bss";
// ".zdebug_" is a prefix for ZLIB-compressed sections.
// Because we decompressed input sections, we want to remove 'z'.
if (Name.startswith(".zdebug_"))
return Saver.save(Twine(".") + Name.substr(2));
return Name;
}
template <class ELFT> static bool needsInterpSection() {
return !Symtab<ELFT>::X->getSharedFiles().empty() &&
!Config->DynamicLinker.empty() &&
!Script<ELFT>::X->ignoreInterpSection();
}
template <class ELFT> void elf::writeResult() { Writer<ELFT>().run(); }
template <class ELFT> void Writer<ELFT>::removeEmptyPTLoad() {
auto I = std::remove_if(Phdrs.begin(), Phdrs.end(), [&](const PhdrEntry &P) {
if (P.p_type != PT_LOAD)
return false;
if (!P.First)
return true;
uintX_t Size = P.Last->Addr + P.Last->Size - P.First->Addr;
return Size == 0;
});
Phdrs.erase(I, Phdrs.end());
}
template <class ELFT>
static typename ELFT::uint getOutFlags(InputSectionBase *S) {
return S->Flags & ~(typename ELFT::uint)(SHF_GROUP | SHF_COMPRESSED);
}
// This function scans over the input sections and creates mergeable
// synthetic sections. It removes MergeInputSections from array and
// adds new synthetic ones. Each synthetic section is added to the
// location of the first input section it replaces.
template <class ELFT> static void combineMergableSections() {
typedef typename ELFT::uint uintX_t;
std::vector<MergeSyntheticSection *> MergeSections;
for (InputSectionBase *&S : InputSections) {
MergeInputSection *MS = dyn_cast<MergeInputSection>(S);
if (!MS)
continue;
// We do not want to handle sections that are not alive, so just remove
// them instead of trying to merge.
if (!MS->Live)
continue;
StringRef OutsecName = getOutputSectionName(MS->Name);
uintX_t Flags = getOutFlags<ELFT>(MS);
uint32_t Alignment = std::max<uintX_t>(MS->Alignment, MS->Entsize);
auto I =
llvm::find_if(MergeSections, [=](MergeSyntheticSection *Sec) {
return Sec->Name == OutsecName && Sec->Flags == Flags &&
Sec->Alignment == Alignment;
});
if (I == MergeSections.end()) {
MergeSyntheticSection *Syn =
make<MergeSyntheticSection>(OutsecName, MS->Type, Flags, Alignment);
MergeSections.push_back(Syn);
I = std::prev(MergeSections.end());
S = Syn;
} else {
S = nullptr;
}
(*I)->addSection(MS);
}
std::vector<InputSectionBase *> &V = InputSections;
V.erase(std::remove(V.begin(), V.end(), nullptr), V.end());
}
template <class ELFT> static void combineEhFrameSections() {
for (InputSectionBase *&S : InputSections) {
EhInputSection *ES = dyn_cast<EhInputSection>(S);
if (!ES || !ES->Live)
continue;
In<ELFT>::EhFrame->addSection(ES);
S = nullptr;
}
std::vector<InputSectionBase *> &V = InputSections;
V.erase(std::remove(V.begin(), V.end(), nullptr), V.end());
}
// The main function of the writer.
template <class ELFT> void Writer<ELFT>::run() {
// Create linker-synthesized sections such as .got or .plt.
// Such sections are of type input section.
createSyntheticSections();
combineMergableSections<ELFT>();
if (!Config->Relocatable)
combineEhFrameSections<ELFT>();
// We need to create some reserved symbols such as _end. Create them.
if (!Config->Relocatable)
addReservedSymbols();
// Create output sections.
Script<ELFT>::X->OutputSections = &OutputSections;
if (ScriptConfig->HasSections) {
// If linker script contains SECTIONS commands, let it create sections.
Script<ELFT>::X->processCommands(Factory);
// Linker scripts may have left some input sections unassigned.
// Assign such sections using the default rule.
Script<ELFT>::X->addOrphanSections(Factory);
} else {
// If linker script does not contain SECTIONS commands, create
// output sections by default rules. We still need to give the
// linker script a chance to run, because it might contain
// non-SECTIONS commands such as ASSERT.
createSections();
Script<ELFT>::X->processCommands(Factory);
}
if (Config->Discard != DiscardPolicy::All)
copyLocalSymbols();
if (Config->copyRelocs())
addSectionSymbols();
// Now that we have a complete set of output sections. This function
// completes section contents. For example, we need to add strings
// to the string table, and add entries to .got and .plt.
// finalizeSections does that.
finalizeSections();
if (ErrorCount)
return;
if (Config->Relocatable) {
assignFileOffsets();
} else {
if (ScriptConfig->HasSections) {
Script<ELFT>::X->assignAddresses(Phdrs);
} else {
fixSectionAlignments();
assignAddresses();
Script<ELFT>::X->processNonSectionCommands();
}
// 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.
removeEmptyPTLoad();
if (!Config->OFormatBinary)
assignFileOffsets();
else
assignFileOffsetsBinary();
setPhdrs();
fixPredefinedSymbols();
}
// It does not make sense try to open the file if we have error already.
if (ErrorCount)
return;
// Write the result down to a file.
openFile();
if (ErrorCount)
return;
if (!Config->OFormatBinary) {
writeHeader();
writeSections();
} else {
writeSectionsBinary();
}
// Backfill .note.gnu.build-id section content. This is done at last
// because the content is usually a hash value of the entire output file.
writeBuildId();
if (ErrorCount)
return;
// Handle -Map option.
writeMapFile<ELFT>(OutputSections);
if (ErrorCount)
return;
if (auto EC = Buffer->commit())
error("failed to write to the output file: " + EC.message());
// Flush the output streams and exit immediately. A full shutdown
// is a good test that we are keeping track of all allocated memory,
// but actually freeing it is a waste of time in a regular linker run.
if (Config->ExitEarly)
exitLld(0);
}
// Initialize Out members.
template <class ELFT> void Writer<ELFT>::createSyntheticSections() {
// Initialize all pointers with NULL. This is needed because
// you can call lld::elf::main more than once as a library.
memset(&Out::First, 0, sizeof(Out));
auto Add = [](InputSectionBase *Sec) { InputSections.push_back(Sec); };
In<ELFT>::DynStrTab = make<StringTableSection>(".dynstr", true);
In<ELFT>::Dynamic = make<DynamicSection<ELFT>>();
In<ELFT>::RelaDyn = make<RelocationSection<ELFT>>(
Config->isRela() ? ".rela.dyn" : ".rel.dyn", Config->ZCombreloc);
In<ELFT>::ShStrTab = make<StringTableSection>(".shstrtab", false);
Out::ElfHeader = make<OutputSection>("", 0, SHF_ALLOC);
Out::ElfHeader->Size = sizeof(Elf_Ehdr);
Out::ProgramHeaders = make<OutputSection>("", 0, SHF_ALLOC);
Out::ProgramHeaders->updateAlignment(sizeof(uintX_t));
if (needsInterpSection<ELFT>()) {
In<ELFT>::Interp = createInterpSection();
Add(In<ELFT>::Interp);
} else {
In<ELFT>::Interp = nullptr;
}
if (!Config->Relocatable)
Add(createCommentSection<ELFT>());
if (Config->Strip != StripPolicy::All) {
In<ELFT>::StrTab = make<StringTableSection>(".strtab", false);
In<ELFT>::SymTab = make<SymbolTableSection<ELFT>>(*In<ELFT>::StrTab);
}
if (Config->BuildId != BuildIdKind::None) {
In<ELFT>::BuildId = make<BuildIdSection<ELFT>>();
Add(In<ELFT>::BuildId);
}
InputSection *Common = createCommonSection<ELFT>();
if (!Common->Data.empty()) {
In<ELFT>::Common = Common;
Add(Common);
}
In<ELFT>::Bss = make<BssSection>(".bss");
Add(In<ELFT>::Bss);
In<ELFT>::BssRelRo = make<BssSection>(".bss.rel.ro");
Add(In<ELFT>::BssRelRo);
// Add MIPS-specific sections.
bool HasDynSymTab =
!Symtab<ELFT>::X->getSharedFiles().empty() || Config->pic() ||
Config->ExportDynamic;
if (Config->EMachine == EM_MIPS) {
if (!Config->Shared && HasDynSymTab) {
In<ELFT>::MipsRldMap = make<MipsRldMapSection>();
Add(In<ELFT>::MipsRldMap);
}
if (auto *Sec = MipsAbiFlagsSection<ELFT>::create())
Add(Sec);
if (auto *Sec = MipsOptionsSection<ELFT>::create())
Add(Sec);
if (auto *Sec = MipsReginfoSection<ELFT>::create())
Add(Sec);
}
if (HasDynSymTab) {
In<ELFT>::DynSymTab = make<SymbolTableSection<ELFT>>(*In<ELFT>::DynStrTab);
Add(In<ELFT>::DynSymTab);
In<ELFT>::VerSym = make<VersionTableSection<ELFT>>();
Add(In<ELFT>::VerSym);
if (!Config->VersionDefinitions.empty()) {
In<ELFT>::VerDef = make<VersionDefinitionSection<ELFT>>();
Add(In<ELFT>::VerDef);
}
In<ELFT>::VerNeed = make<VersionNeedSection<ELFT>>();
Add(In<ELFT>::VerNeed);
if (Config->GnuHash) {
In<ELFT>::GnuHashTab = make<GnuHashTableSection<ELFT>>();
Add(In<ELFT>::GnuHashTab);
}
if (Config->SysvHash) {
In<ELFT>::HashTab = make<HashTableSection<ELFT>>();
Add(In<ELFT>::HashTab);
}
Add(In<ELFT>::Dynamic);
Add(In<ELFT>::DynStrTab);
Add(In<ELFT>::RelaDyn);
}
// Add .got. MIPS' .got is so different from the other archs,
// it has its own class.
if (Config->EMachine == EM_MIPS) {
In<ELFT>::MipsGot = make<MipsGotSection<ELFT>>();
Add(In<ELFT>::MipsGot);
} else {
In<ELFT>::Got = make<GotSection<ELFT>>();
Add(In<ELFT>::Got);
}
In<ELFT>::GotPlt = make<GotPltSection>();
Add(In<ELFT>::GotPlt);
In<ELFT>::IgotPlt = make<IgotPltSection>();
Add(In<ELFT>::IgotPlt);
if (Config->GdbIndex) {
In<ELFT>::GdbIndex = make<GdbIndexSection<ELFT>>();
Add(In<ELFT>::GdbIndex);
}
// We always need to add rel[a].plt to output if it has entries.
// Even for static linking it can contain R_[*]_IRELATIVE relocations.
In<ELFT>::RelaPlt = make<RelocationSection<ELFT>>(
Config->isRela() ? ".rela.plt" : ".rel.plt", false /*Sort*/);
Add(In<ELFT>::RelaPlt);
// The RelaIplt immediately follows .rel.plt (.rel.dyn for ARM) to ensure
// that the IRelative relocations are processed last by the dynamic loader
In<ELFT>::RelaIplt = make<RelocationSection<ELFT>>(
(Config->EMachine == EM_ARM) ? ".rel.dyn" : In<ELFT>::RelaPlt->Name,
false /*Sort*/);
Add(In<ELFT>::RelaIplt);
In<ELFT>::Plt = make<PltSection<ELFT>>(Target->PltHeaderSize);
Add(In<ELFT>::Plt);
In<ELFT>::Iplt = make<PltSection<ELFT>>(0);
Add(In<ELFT>::Iplt);
if (!Config->Relocatable) {
if (Config->EhFrameHdr) {
In<ELFT>::EhFrameHdr = make<EhFrameHeader<ELFT>>();
Add(In<ELFT>::EhFrameHdr);
}
In<ELFT>::EhFrame = make<EhFrameSection<ELFT>>();
Add(In<ELFT>::EhFrame);
}
if (In<ELFT>::SymTab)
Add(In<ELFT>::SymTab);
Add(In<ELFT>::ShStrTab);
if (In<ELFT>::StrTab)
Add(In<ELFT>::StrTab);
}
static bool shouldKeepInSymtab(SectionBase *Sec, StringRef SymName,
const SymbolBody &B) {
if (B.isFile() || B.isSection())
return false;
// If sym references a section in a discarded group, don't keep it.
if (Sec == &InputSection::Discarded)
return false;
if (Config->Discard == DiscardPolicy::None)
return true;
// In ELF assembly .L symbols are normally discarded by the assembler.
// If the assembler fails to do so, the linker discards them if
// * --discard-locals is used.
// * The symbol is in a SHF_MERGE section, which is normally the reason for
// the assembler keeping the .L symbol.
if (!SymName.startswith(".L") && !SymName.empty())
return true;
if (Config->Discard == DiscardPolicy::Locals)
return false;
return !Sec || !(Sec->Flags & SHF_MERGE);
}
static bool includeInSymtab(const SymbolBody &B) {
if (!B.isLocal() && !B.symbol()->IsUsedInRegularObj)
return false;
if (auto *D = dyn_cast<DefinedRegular>(&B)) {
// Always include absolute symbols.
SectionBase *Sec = D->Section;
if (!Sec)
return true;
if (auto *IS = dyn_cast<InputSectionBase>(Sec)) {
Sec = IS->Repl;
IS = cast<InputSectionBase>(Sec);
// Exclude symbols pointing to garbage-collected sections.
if (!IS->Live)
return false;
}
if (auto *S = dyn_cast<MergeInputSection>(Sec))
if (!S->getSectionPiece(D->Value)->Live)
return false;
}
return true;
}
// Local symbols are not in the linker's symbol table. This function scans
// each object file's symbol table to copy local symbols to the output.
template <class ELFT> void Writer<ELFT>::copyLocalSymbols() {
if (!In<ELFT>::SymTab)
return;
for (elf::ObjectFile<ELFT> *F : Symtab<ELFT>::X->getObjectFiles()) {
for (SymbolBody *B : F->getLocalSymbols()) {
if (!B->IsLocal)
fatal(toString(F) +
": broken object: getLocalSymbols returns a non-local symbol");
auto *DR = dyn_cast<DefinedRegular>(B);
// No reason to keep local undefined symbol in symtab.
if (!DR)
continue;
if (!includeInSymtab(*B))
continue;
SectionBase *Sec = DR->Section;
if (!shouldKeepInSymtab(Sec, B->getName(), *B))
continue;
In<ELFT>::SymTab->addSymbol(B);
}
}
}
template <class ELFT> void Writer<ELFT>::addSectionSymbols() {
// Create one STT_SECTION symbol for each output section we might
// have a relocation with.
for (OutputSection *Sec : OutputSections) {
if (Sec->Sections.empty())
continue;
InputSection *IS = Sec->Sections[0];
if (isa<SyntheticSection>(IS) || IS->Type == SHT_REL ||
IS->Type == SHT_RELA)
continue;
auto *Sym =
make<DefinedRegular>("", /*IsLocal=*/true, /*StOther=*/0, STT_SECTION,
/*Value=*/0, /*Size=*/0, IS, nullptr);
In<ELFT>::SymTab->addSymbol(Sym);
}
}
// PPC64 has a number of special SHT_PROGBITS+SHF_ALLOC+SHF_WRITE sections that
// we would like to make sure appear is a specific order to maximize their
// coverage by a single signed 16-bit offset from the TOC base pointer.
// Conversely, the special .tocbss section should be first among all SHT_NOBITS
// sections. This will put it next to the loaded special PPC64 sections (and,
// thus, within reach of the TOC base pointer).
static int getPPC64SectionRank(StringRef SectionName) {
return StringSwitch<int>(SectionName)
.Case(".tocbss", 0)
.Case(".branch_lt", 2)
.Case(".toc", 3)
.Case(".toc1", 4)
.Case(".opd", 5)
.Default(1);
}
// All sections with SHF_MIPS_GPREL flag should be grouped together
// because data in these sections is addressable with a gp relative address.
static int getMipsSectionRank(const OutputSection *S) {
if ((S->Flags & SHF_MIPS_GPREL) == 0)
return 0;
if (S->Name == ".got")
return 1;
return 2;
}
// 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.
template <class ELFT> bool elf::isRelroSection(const OutputSection *Sec) {
if (!Config->ZRelro)
return false;
uint64_t Flags = Sec->Flags;
if (!(Flags & SHF_ALLOC) || !(Flags & SHF_WRITE))
return false;
if (Flags & SHF_TLS)
return true;
uint32_t Type = Sec->Type;
if (Type == SHT_INIT_ARRAY || Type == SHT_FINI_ARRAY ||
Type == SHT_PREINIT_ARRAY)
return true;
if (Sec == In<ELFT>::GotPlt->OutSec)
return Config->ZNow;
if (Sec == In<ELFT>::Dynamic->OutSec)
return true;
if (In<ELFT>::Got && Sec == In<ELFT>::Got->OutSec)
return true;
if (Sec == In<ELFT>::BssRelRo->OutSec)
return true;
StringRef S = Sec->Name;
return S == ".data.rel.ro" || S == ".ctors" || S == ".dtors" || S == ".jcr" ||
S == ".eh_frame" || S == ".openbsd.randomdata";
}
template <class ELFT>
static bool compareSectionsNonScript(const OutputSection *A,
const OutputSection *B) {
// Put .interp first because some loaders want to see that section
// on the first page of the executable file when loaded into memory.
bool AIsInterp = A->Name == ".interp";
bool BIsInterp = B->Name == ".interp";
if (AIsInterp != BIsInterp)
return AIsInterp;
// Allocatable sections go first to reduce the total PT_LOAD size and
// so debug info doesn't change addresses in actual code.
bool AIsAlloc = A->Flags & SHF_ALLOC;
bool BIsAlloc = B->Flags & SHF_ALLOC;
if (AIsAlloc != BIsAlloc)
return AIsAlloc;
// We don't have any special requirements for the relative order of two non
// allocatable sections.
if (!AIsAlloc)
return false;
// We want to put section specified by -T option first, so we
// can start assigning VA starting from them later.
auto AAddrSetI = Config->SectionStartMap.find(A->Name);
auto BAddrSetI = Config->SectionStartMap.find(B->Name);
bool AHasAddrSet = AAddrSetI != Config->SectionStartMap.end();
bool BHasAddrSet = BAddrSetI != Config->SectionStartMap.end();
if (AHasAddrSet != BHasAddrSet)
return AHasAddrSet;
if (AHasAddrSet)
return AAddrSetI->second < BAddrSetI->second;
// We want the read only sections first so that they go in the PT_LOAD
// covering the program headers at the start of the file.
bool AIsWritable = A->Flags & SHF_WRITE;
bool BIsWritable = B->Flags & SHF_WRITE;
if (AIsWritable != BIsWritable)
return BIsWritable;
if (!Config->SingleRoRx) {
// For a corresponding reason, put non exec sections first (the program
// header PT_LOAD is not executable).
// We only do that if we are not using linker scripts, since with linker
// scripts ro and rx sections are in the same PT_LOAD, so their relative
// order is not important. The same applies for -no-rosegment.
bool AIsExec = A->Flags & SHF_EXECINSTR;
bool BIsExec = B->Flags & SHF_EXECINSTR;
if (AIsExec != BIsExec)
return BIsExec;
}
// If we got here we know that both A and B are in the same PT_LOAD.
bool AIsTls = A->Flags & SHF_TLS;
bool BIsTls = B->Flags & SHF_TLS;
bool AIsNoBits = A->Type == SHT_NOBITS;
bool BIsNoBits = B->Type == SHT_NOBITS;
// The first requirement we have is to put (non-TLS) nobits sections last. The
// reason is that the only thing the dynamic linker will see about them is a
// p_memsz that is larger than p_filesz. Seeing that it zeros the end of the
// PT_LOAD, so that has to correspond to the nobits sections.
bool AIsNonTlsNoBits = AIsNoBits && !AIsTls;
bool BIsNonTlsNoBits = BIsNoBits && !BIsTls;
if (AIsNonTlsNoBits != BIsNonTlsNoBits)
return BIsNonTlsNoBits;
// We place nobits RelRo sections before plain r/w ones, and non-nobits RelRo
// sections after r/w ones, so that the RelRo sections are contiguous.
bool AIsRelRo = isRelroSection<ELFT>(A);
bool BIsRelRo = isRelroSection<ELFT>(B);
if (AIsRelRo != BIsRelRo)
return AIsNonTlsNoBits ? AIsRelRo : BIsRelRo;
// The TLS initialization block needs to be a single contiguous block in a R/W
// PT_LOAD, so stick TLS sections directly before the other RelRo R/W
// sections. The TLS NOBITS sections are placed here as they don't take up
// virtual address space in the PT_LOAD.
if (AIsTls != BIsTls)
return AIsTls;
// Within the TLS initialization block, the non-nobits sections need to appear
// first.
if (AIsNoBits != BIsNoBits)
return BIsNoBits;
// Some architectures have additional ordering restrictions for sections
// within the same PT_LOAD.
if (Config->EMachine == EM_PPC64)
return getPPC64SectionRank(A->Name) < getPPC64SectionRank(B->Name);
if (Config->EMachine == EM_MIPS)
return getMipsSectionRank(A) < getMipsSectionRank(B);
return false;
}
// Output section ordering is determined by this function.
template <class ELFT>
static bool compareSections(const OutputSection *A, const OutputSection *B) {
// For now, put sections mentioned in a linker script first.
int AIndex = Script<ELFT>::X->getSectionIndex(A->Name);
int BIndex = Script<ELFT>::X->getSectionIndex(B->Name);
bool AInScript = AIndex != INT_MAX;
bool BInScript = BIndex != INT_MAX;
if (AInScript != BInScript)
return AInScript;
// If both are in the script, use that order.
if (AInScript)
return AIndex < BIndex;
return compareSectionsNonScript<ELFT>(A, B);
}
// Program header entry
PhdrEntry::PhdrEntry(unsigned Type, unsigned Flags) {
p_type = Type;
p_flags = Flags;
}
void PhdrEntry::add(OutputSection *Sec) {
Last = Sec;
if (!First)
First = Sec;
p_align = std::max(p_align, Sec->Alignment);
if (p_type == PT_LOAD)
Sec->FirstInPtLoad = First;
}
template <class ELFT>
static Symbol *addRegular(StringRef Name, SectionBase *Sec, uint64_t Value,
uint8_t StOther = STV_HIDDEN,
uint8_t Binding = STB_WEAK) {
// The linker generated symbols are added as STB_WEAK to allow user defined
// ones to override them.
return Symtab<ELFT>::X->addRegular(Name, StOther, STT_NOTYPE, Value,
/*Size=*/0, Binding, Sec,
/*File=*/nullptr);
}
template <class ELFT>
static DefinedRegular *
addOptionalRegular(StringRef Name, SectionBase *Sec, uint64_t Val,
uint8_t StOther = STV_HIDDEN, uint8_t Binding = STB_GLOBAL) {
SymbolBody *S = Symtab<ELFT>::X->find(Name);
if (!S)
return nullptr;
if (S->isInCurrentDSO())
return nullptr;
return cast<DefinedRegular>(
addRegular<ELFT>(Name, Sec, Val, StOther, Binding)->body());
}
// The beginning and the ending of .rel[a].plt section are marked
// with __rel[a]_iplt_{start,end} symbols if it is a statically linked
// executable. The runtime needs these symbols in order to resolve
// all IRELATIVE relocs on startup. For dynamic executables, we don't
// need these symbols, since IRELATIVE relocs are resolved through GOT
// and PLT. For details, see http://www.airs.com/blog/archives/403.
template <class ELFT> void Writer<ELFT>::addRelIpltSymbols() {
if (In<ELFT>::DynSymTab)
return;
StringRef S = Config->isRela() ? "__rela_iplt_start" : "__rel_iplt_start";
addOptionalRegular<ELFT>(S, In<ELFT>::RelaIplt, 0, STV_HIDDEN, STB_WEAK);
S = Config->isRela() ? "__rela_iplt_end" : "__rel_iplt_end";
addOptionalRegular<ELFT>(S, In<ELFT>::RelaIplt, -1, STV_HIDDEN, STB_WEAK);
}
// The linker is expected to define some symbols depending on
// the linking result. This function defines such symbols.
template <class ELFT> void Writer<ELFT>::addReservedSymbols() {
if (Config->EMachine == EM_MIPS) {
// Define _gp for MIPS. st_value of _gp symbol will be updated by Writer
// so that it points to an absolute address which by default is relative
// to GOT. Default offset is 0x7ff0.
// See "Global Data Symbols" in Chapter 6 in the following document:
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
ElfSym::MipsGp = Symtab<ELFT>::X->addAbsolute("_gp", STV_HIDDEN, STB_LOCAL);
// On MIPS O32 ABI, _gp_disp is a magic symbol designates offset between
// start of function and 'gp' pointer into GOT. To simplify relocation
// calculation we assign _gp value to it and calculate corresponding
// relocations as relative to this value.
if (Symtab<ELFT>::X->find("_gp_disp"))
ElfSym::MipsGpDisp =
Symtab<ELFT>::X->addAbsolute("_gp_disp", STV_HIDDEN, STB_LOCAL);
// 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<ELFT>::X->find("__gnu_local_gp"))
ElfSym::MipsLocalGp =
Symtab<ELFT>::X->addAbsolute("__gnu_local_gp", STV_HIDDEN, STB_LOCAL);
}
// In the assembly for 32 bit x86 the _GLOBAL_OFFSET_TABLE_ symbol
// is magical and is used to produce a R_386_GOTPC relocation.
// The R_386_GOTPC relocation value doesn't actually depend on the
// symbol value, so it could use an index of STN_UNDEF which, according
// to the spec, means the symbol value is 0.
// Unfortunately both gas and MC keep the _GLOBAL_OFFSET_TABLE_ symbol in
// the object file.
// The situation is even stranger on x86_64 where the assembly doesn't
// need the magical symbol, but gas still puts _GLOBAL_OFFSET_TABLE_ as
// an undefined symbol in the .o files.
// Given that the symbol is effectively unused, we just create a dummy
// hidden one to avoid the undefined symbol error.
Symtab<ELFT>::X->addIgnored("_GLOBAL_OFFSET_TABLE_");
// __tls_get_addr is defined by the dynamic linker for dynamic ELFs. For
// static linking the linker is required to optimize away any references to
// __tls_get_addr, so it's not defined anywhere. Create a hidden definition
// to avoid the undefined symbol error. As usual special cases are ARM and
// MIPS - the libc for these targets defines __tls_get_addr itself because
// there are no TLS optimizations for these targets.
if (!In<ELFT>::DynSymTab &&
(Config->EMachine != EM_MIPS && Config->EMachine != EM_ARM))
Symtab<ELFT>::X->addIgnored("__tls_get_addr");
// If linker script do layout we do not need to create any standart symbols.
if (ScriptConfig->HasSections)
return;
// __ehdr_start is the location of ELF file headers.
addOptionalRegular<ELFT>("__ehdr_start", Out::ElfHeader, 0, STV_HIDDEN);
auto Define = [](StringRef S, DefinedRegular *&Sym1, DefinedRegular *&Sym2) {
Sym1 = addOptionalRegular<ELFT>(S, Out::ElfHeader, 0, STV_DEFAULT);
assert(S.startswith("_"));
S = S.substr(1);
Sym2 = addOptionalRegular<ELFT>(S, Out::ElfHeader, 0, STV_DEFAULT);
};
Define("_end", ElfSym::End, ElfSym::End2);
Define("_etext", ElfSym::Etext, ElfSym::Etext2);
Define("_edata", ElfSym::Edata, ElfSym::Edata2);
}
// Sort input sections by section name suffixes for
// __attribute__((init_priority(N))).
template <class ELFT> static void sortInitFini(OutputSection *S) {
if (S)
reinterpret_cast<OutputSection *>(S)->sortInitFini();
}
// Sort input sections by the special rule for .ctors and .dtors.
template <class ELFT> static void sortCtorsDtors(OutputSection *S) {
if (S)
reinterpret_cast<OutputSection *>(S)->sortCtorsDtors();
}
// Sort input sections using the list provided by --symbol-ordering-file.
template <class ELFT>
static void sortBySymbolsOrder(ArrayRef<OutputSection *> OutputSections) {
if (Config->SymbolOrderingFile.empty())
return;
// Build a map from symbols to their priorities. Symbols that didn't
// appear in the symbol ordering file have the lowest priority 0.
// All explicitly mentioned symbols have negative (higher) priorities.
DenseMap<StringRef, int> SymbolOrder;
int Priority = -Config->SymbolOrderingFile.size();
for (StringRef S : Config->SymbolOrderingFile)
SymbolOrder.insert({S, Priority++});
// Build a map from sections to their priorities.
DenseMap<SectionBase *, int> SectionOrder;
for (elf::ObjectFile<ELFT> *File : Symtab<ELFT>::X->getObjectFiles()) {
for (SymbolBody *Body : File->getSymbols()) {
auto *D = dyn_cast<DefinedRegular>(Body);
if (!D || !D->Section)
continue;
int &Priority = SectionOrder[D->Section];
Priority = std::min(Priority, SymbolOrder.lookup(D->getName()));
}
}
// Sort sections by priority.
for (OutputSection *Base : OutputSections)
if (auto *Sec = dyn_cast<OutputSection>(Base))
Sec->sort([&](InputSectionBase *S) { return SectionOrder.lookup(S); });
}
template <class ELFT>
void Writer<ELFT>::forEachRelSec(std::function<void(InputSectionBase &)> Fn) {
for (InputSectionBase *IS : InputSections) {
if (!IS->Live)
continue;
// Scan all relocations. Each relocation goes through a series
// of tests to determine if it needs special treatment, such as
// creating GOT, PLT, copy relocations, etc.
// Note that relocations for non-alloc sections are directly
// processed by InputSection::relocateNonAlloc.
if (!(IS->Flags & SHF_ALLOC))
continue;
if (isa<InputSection>(IS) || isa<EhInputSection>(IS))
Fn(*IS);
}
if (!Config->Relocatable) {
for (EhInputSection *ES : In<ELFT>::EhFrame->Sections)
Fn(*ES);
}
}
template <class ELFT> void Writer<ELFT>::createSections() {
for (InputSectionBase *IS : InputSections)
if (IS)
Factory.addInputSec(IS, getOutputSectionName(IS->Name));
sortBySymbolsOrder<ELFT>(OutputSections);
sortInitFini<ELFT>(findSection(".init_array"));
sortInitFini<ELFT>(findSection(".fini_array"));
sortCtorsDtors<ELFT>(findSection(".ctors"));
sortCtorsDtors<ELFT>(findSection(".dtors"));
for (OutputSection *Sec : OutputSections)
Sec->assignOffsets();
}
static bool canSharePtLoad(const OutputSection &S1, const OutputSection &S2) {
if (!(S1.Flags & SHF_ALLOC) || !(S2.Flags & SHF_ALLOC))
return false;
bool S1IsWrite = S1.Flags & SHF_WRITE;
bool S2IsWrite = S2.Flags & SHF_WRITE;
if (S1IsWrite != S2IsWrite)
return false;
if (!S1IsWrite)
return true; // RO and RX share a PT_LOAD with linker scripts.
return (S1.Flags & SHF_EXECINSTR) == (S2.Flags & SHF_EXECINSTR);
}
template <class ELFT> void Writer<ELFT>::sortSections() {
// Don't sort if using -r. It is not necessary and we want to preserve the
// relative order for SHF_LINK_ORDER sections.
if (Config->Relocatable)
return;
if (!ScriptConfig->HasSections) {
std::stable_sort(OutputSections.begin(), OutputSections.end(),
compareSectionsNonScript<ELFT>);
return;
}
Script<ELFT>::X->adjustSectionsBeforeSorting();
// The order of the sections in the script is arbitrary and may not agree with
// compareSectionsNonScript. 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:
// * First put script sections at the start and sort the script and
// non-script sections independently.
// * Move each non-script section to its preferred position. We try
// to put each section in the last position where it it can share
// a PT_LOAD.
std::stable_sort(OutputSections.begin(), OutputSections.end(),