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llvm-project/lld/ELF/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 "OutputSections.h"
#include "SymbolTable.h"
#include "Target.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/Support/FileOutputBuffer.h"
#include "llvm/Support/StringSaver.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
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;
Writer(SymbolTable<ELFT> &S) : Symtab(S) {}
void run();
private:
// This describes a program header entry.
// Each contains type, access flags and range of output sections that will be
// placed in it.
struct Phdr {
Phdr(unsigned Type, unsigned Flags) {
H.p_type = Type;
H.p_flags = Flags;
}
Elf_Phdr H = {};
OutputSectionBase<ELFT> *First = nullptr;
OutputSectionBase<ELFT> *Last = nullptr;
};
void copyLocalSymbols();
void addReservedSymbols();
bool createSections();
void addPredefinedSections();
bool needsGot();
template <class RelTy>
void scanRelocs(InputSectionBase<ELFT> &C,
iterator_range<const RelTy *> Rels);
void scanRelocs(InputSection<ELFT> &C);
void scanRelocs(InputSectionBase<ELFT> &S, const Elf_Shdr &RelSec);
void createPhdrs();
void assignAddresses();
void assignAddressesRelocatable();
void fixAbsoluteSymbols();
bool openFile();
void writeHeader();
void writeSections();
void writeBuildId();
bool isDiscarded(InputSectionBase<ELFT> *IS) const;
StringRef getOutputSectionName(InputSectionBase<ELFT> *S) const;
bool needsInterpSection() const {
return !Symtab.getSharedFiles().empty() && !Config->DynamicLinker.empty();
}
bool isOutputDynamic() const {
return !Symtab.getSharedFiles().empty() || Config->Pic;
}
void ensureBss();
void addCommonSymbols(std::vector<DefinedCommon *> &Syms);
void addCopyRelSymbols(std::vector<SharedSymbol<ELFT> *> &Syms);
std::unique_ptr<llvm::FileOutputBuffer> Buffer;
BumpPtrAllocator Alloc;
std::vector<OutputSectionBase<ELFT> *> OutputSections;
std::vector<std::unique_ptr<OutputSectionBase<ELFT>>> OwningSections;
// We create a section for the ELF header and one for the program headers.
ArrayRef<OutputSectionBase<ELFT> *> getSections() const {
return makeArrayRef(OutputSections).slice(dummySectionsNum());
}
unsigned getNumSections() const {
return OutputSections.size() + 1 - dummySectionsNum();
}
// Usually there are 2 dummies sections: ELF header and program header.
// Relocatable output does not require program headers to be created.
unsigned dummySectionsNum() const { return Config->Relocatable ? 1 : 2; }
void addRelIpltSymbols();
void addStartEndSymbols();
void addStartStopSymbols(OutputSectionBase<ELFT> *Sec);
SymbolTable<ELFT> &Symtab;
std::vector<Phdr> Phdrs;
uintX_t FileSize;
uintX_t SectionHeaderOff;
// Flag to force GOT to be in output if we have relocations
// that relies on its address.
bool HasGotOffRel = false;
};
} // anonymous namespace
template <class ELFT> void elf::writeResult(SymbolTable<ELFT> *Symtab) {
typedef typename ELFT::uint uintX_t;
// Create singleton output sections.
DynamicSection<ELFT> Dynamic(*Symtab);
EhFrameHeader<ELFT> EhFrameHdr;
GotSection<ELFT> Got;
InterpSection<ELFT> Interp;
PltSection<ELFT> Plt;
RelocationSection<ELFT> RelaDyn(Config->Rela ? ".rela.dyn" : ".rel.dyn");
StringTableSection<ELFT> DynStrTab(".dynstr", true);
StringTableSection<ELFT> ShStrTab(".shstrtab", false);
SymbolTableSection<ELFT> DynSymTab(*Symtab, DynStrTab);
OutputSectionBase<ELFT> ElfHeader("", 0, SHF_ALLOC);
OutputSectionBase<ELFT> ProgramHeaders("", 0, SHF_ALLOC);
ProgramHeaders.updateAlign(sizeof(uintX_t));
// Instantiate optional output sections if they are needed.
std::unique_ptr<BuildIdSection<ELFT>> BuildId;
std::unique_ptr<GnuHashTableSection<ELFT>> GnuHashTab;
std::unique_ptr<GotPltSection<ELFT>> GotPlt;
std::unique_ptr<HashTableSection<ELFT>> HashTab;
std::unique_ptr<RelocationSection<ELFT>> RelaPlt;
std::unique_ptr<StringTableSection<ELFT>> StrTab;
std::unique_ptr<SymbolTableSection<ELFT>> SymTabSec;
std::unique_ptr<OutputSection<ELFT>> MipsRldMap;
if (Config->BuildId)
BuildId.reset(new BuildIdSection<ELFT>);
if (Config->GnuHash)
GnuHashTab.reset(new GnuHashTableSection<ELFT>);
if (Config->SysvHash)
HashTab.reset(new HashTableSection<ELFT>);
if (Target->UseLazyBinding) {
StringRef S = Config->Rela ? ".rela.plt" : ".rel.plt";
GotPlt.reset(new GotPltSection<ELFT>);
RelaPlt.reset(new RelocationSection<ELFT>(S));
}
if (!Config->StripAll) {
StrTab.reset(new StringTableSection<ELFT>(".strtab", false));
SymTabSec.reset(new SymbolTableSection<ELFT>(*Symtab, *StrTab));
}
if (Config->EMachine == EM_MIPS && !Config->Shared) {
// This is a MIPS specific section to hold a space within the data segment
// of executable file which is pointed to by the DT_MIPS_RLD_MAP entry.
// See "Dynamic section" in Chapter 5 in the following document:
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
MipsRldMap.reset(new OutputSection<ELFT>(".rld_map", SHT_PROGBITS,
SHF_ALLOC | SHF_WRITE));
MipsRldMap->setSize(sizeof(uintX_t));
MipsRldMap->updateAlign(sizeof(uintX_t));
}
Out<ELFT>::BuildId = BuildId.get();
Out<ELFT>::DynStrTab = &DynStrTab;
Out<ELFT>::DynSymTab = &DynSymTab;
Out<ELFT>::Dynamic = &Dynamic;
Out<ELFT>::EhFrameHdr = &EhFrameHdr;
Out<ELFT>::GnuHashTab = GnuHashTab.get();
Out<ELFT>::Got = &Got;
Out<ELFT>::GotPlt = GotPlt.get();
Out<ELFT>::HashTab = HashTab.get();
Out<ELFT>::Interp = &Interp;
Out<ELFT>::Plt = &Plt;
Out<ELFT>::RelaDyn = &RelaDyn;
Out<ELFT>::RelaPlt = RelaPlt.get();
Out<ELFT>::ShStrTab = &ShStrTab;
Out<ELFT>::StrTab = StrTab.get();
Out<ELFT>::SymTab = SymTabSec.get();
Out<ELFT>::Bss = nullptr;
Out<ELFT>::MipsRldMap = MipsRldMap.get();
Out<ELFT>::Opd = nullptr;
Out<ELFT>::OpdBuf = nullptr;
Out<ELFT>::TlsPhdr = nullptr;
Out<ELFT>::ElfHeader = &ElfHeader;
Out<ELFT>::ProgramHeaders = &ProgramHeaders;
Writer<ELFT>(*Symtab).run();
}
// The main function of the writer.
template <class ELFT> void Writer<ELFT>::run() {
if (!Config->DiscardAll)
copyLocalSymbols();
addReservedSymbols();
if (!createSections())
return;
if (!Config->Relocatable) {
createPhdrs();
assignAddresses();
} else {
assignAddressesRelocatable();
}
fixAbsoluteSymbols();
if (!openFile())
return;
writeHeader();
writeSections();
writeBuildId();
if (HasError)
return;
check(Buffer->commit());
}
namespace {
template <bool Is64Bits> struct SectionKey {
typedef typename std::conditional<Is64Bits, uint64_t, uint32_t>::type uintX_t;
StringRef Name;
uint32_t Type;
uintX_t Flags;
uintX_t Alignment;
};
}
namespace llvm {
template <bool Is64Bits> struct DenseMapInfo<SectionKey<Is64Bits>> {
static SectionKey<Is64Bits> getEmptyKey() {
return SectionKey<Is64Bits>{DenseMapInfo<StringRef>::getEmptyKey(), 0, 0,
0};
}
static SectionKey<Is64Bits> getTombstoneKey() {
return SectionKey<Is64Bits>{DenseMapInfo<StringRef>::getTombstoneKey(), 0,
0, 0};
}
static unsigned getHashValue(const SectionKey<Is64Bits> &Val) {
return hash_combine(Val.Name, Val.Type, Val.Flags, Val.Alignment);
}
static bool isEqual(const SectionKey<Is64Bits> &LHS,
const SectionKey<Is64Bits> &RHS) {
return DenseMapInfo<StringRef>::isEqual(LHS.Name, RHS.Name) &&
LHS.Type == RHS.Type && LHS.Flags == RHS.Flags &&
LHS.Alignment == RHS.Alignment;
}
};
}
// Returns the number of relocations processed.
template <class ELFT, class RelT>
static unsigned handleTlsRelocation(uint32_t Type, SymbolBody &Body,
InputSectionBase<ELFT> &C, RelT &RI) {
if (Target->pointsToLocalDynamicGotEntry(Type)) {
if (Target->canRelaxTls(Type, nullptr))
return 1;
if (Out<ELFT>::Got->addTlsIndex())
Out<ELFT>::RelaDyn->addReloc({Target->TlsModuleIndexRel,
DynamicReloc<ELFT>::Off_LTlsIndex,
nullptr});
return 1;
}
if (!Body.IsTls)
return 0;
if (Target->isTlsGlobalDynamicRel(Type)) {
if (!Target->canRelaxTls(Type, &Body)) {
if (Out<ELFT>::Got->addDynTlsEntry(Body)) {
Out<ELFT>::RelaDyn->addReloc({Target->TlsModuleIndexRel,
DynamicReloc<ELFT>::Off_GTlsIndex,
&Body});
Out<ELFT>::RelaDyn->addReloc(
{Target->TlsOffsetRel, DynamicReloc<ELFT>::Off_GTlsOffset, &Body});
}
return 1;
}
if (!Body.isPreemptible())
return 1;
Out<ELFT>::Got->addEntry(Body);
Out<ELFT>::RelaDyn->addReloc(
{Target->TlsGotRel, DynamicReloc<ELFT>::Off_Got, false, &Body});
return 2;
}
return 0;
}
// The reason we have to do this early scan is as follows
// * To mmap the output file, we need to know the size
// * For that, we need to know how many dynamic relocs we will have.
// It might be possible to avoid this by outputting the file with write:
// * Write the allocated output sections, computing addresses.
// * Apply relocations, recording which ones require a dynamic reloc.
// * Write the dynamic relocations.
// * Write the rest of the file.
// This would have some drawbacks. For example, we would only know if .rela.dyn
// is needed after applying relocations. If it is, it will go after rw and rx
// sections. Given that it is ro, we will need an extra PT_LOAD. This
// complicates things for the dynamic linker and means we would have to reserve
// space for the extra PT_LOAD even if we end up not using it.
template <class ELFT>
template <class RelTy>
void Writer<ELFT>::scanRelocs(InputSectionBase<ELFT> &C,
iterator_range<const RelTy *> Rels) {
const elf::ObjectFile<ELFT> &File = *C.getFile();
for (auto I = Rels.begin(), E = Rels.end(); I != E; ++I) {
const RelTy &RI = *I;
uint32_t SymIndex = RI.getSymbol(Config->Mips64EL);
SymbolBody &OrigBody = File.getSymbolBody(SymIndex);
SymbolBody &Body = OrigBody.repl();
uint32_t Type = RI.getType(Config->Mips64EL);
// Ignore "hint" relocation because it is for optional code optimization.
if (Target->isHintRel(Type))
continue;
if (Target->isGotRelative(Type))
HasGotOffRel = true;
// Set "used" bit for --as-needed.
if (OrigBody.isUndefined() && !OrigBody.isWeak())
if (auto *S = dyn_cast<SharedSymbol<ELFT>>(&Body))
S->File->IsUsed = true;
bool Preemptible = Body.isPreemptible();
if (unsigned Processed = handleTlsRelocation<ELFT>(Type, Body, C, RI)) {
I += (Processed - 1);
continue;
}
if (Target->needsDynRelative(Type))
Out<ELFT>::RelaDyn->addReloc({Target->RelativeRel, &C, RI.r_offset, true,
&Body, getAddend<ELFT>(RI)});
// If a symbol in a DSO is referenced directly instead of through GOT,
// we need to create a copy relocation for the symbol.
if (auto *B = dyn_cast<SharedSymbol<ELFT>>(&Body)) {
if (B->needsCopy())
continue;
if (Target->needsCopyRel<ELFT>(Type, *B)) {
B->NeedsCopyOrPltAddr = true;
Out<ELFT>::RelaDyn->addReloc(
{Target->CopyRel, DynamicReloc<ELFT>::Off_Bss, B});
continue;
}
}
// An STT_GNU_IFUNC symbol always uses a PLT entry, and all references
// to the symbol go through the PLT. This is true even for a local
// symbol, although local symbols normally do not require PLT entries.
if (Body.IsGnuIFunc) {
if (Body.isInPlt())
continue;
Out<ELFT>::Plt->addEntry(Body);
if (Target->UseLazyBinding) {
Out<ELFT>::GotPlt->addEntry(Body);
Out<ELFT>::RelaPlt->addReloc(
{Preemptible ? Target->PltRel : Target->IRelativeRel,
DynamicReloc<ELFT>::Off_GotPlt, !Preemptible, &Body});
} else {
Out<ELFT>::Got->addEntry(Body);
Out<ELFT>::RelaDyn->addReloc(
{Preemptible ? Target->PltRel : Target->IRelativeRel,
DynamicReloc<ELFT>::Off_Got, !Preemptible, &Body});
}
continue;
}
// If a relocation needs PLT, we create a PLT and a GOT slot
// for the symbol.
TargetInfo::PltNeed NeedPlt = Target->needsPlt(Type, Body);
if (NeedPlt) {
if (NeedPlt == TargetInfo::Plt_Implicit)
Body.NeedsCopyOrPltAddr = true;
if (Body.isInPlt())
continue;
Out<ELFT>::Plt->addEntry(Body);
if (Target->UseLazyBinding) {
Out<ELFT>::GotPlt->addEntry(Body);
Out<ELFT>::RelaPlt->addReloc(
{Target->PltRel, DynamicReloc<ELFT>::Off_GotPlt, &Body});
} else {
if (Body.isInGot())
continue;
Out<ELFT>::Got->addEntry(Body);
Out<ELFT>::RelaDyn->addReloc(
{Target->GotRel, DynamicReloc<ELFT>::Off_Got, &Body});
}
continue;
}
// If a relocation needs GOT, we create a GOT slot for the symbol.
if (Target->needsGot(Type, Body)) {
if (Body.isInGot())
continue;
Out<ELFT>::Got->addEntry(Body);
if (Config->EMachine == EM_MIPS)
// MIPS ABI has special rules to process GOT entries
// and doesn't require relocation entries for them.
// See "Global Offset Table" in Chapter 5 in the following document
// for detailed description:
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
continue;
bool Dynrel = Config->Pic && !Target->isRelRelative(Type) &&
!Target->isSizeRel(Type);
if (Preemptible || Dynrel) {
uint32_t DynType;
if (Body.IsTls)
DynType = Target->TlsGotRel;
else if (Preemptible)
DynType = Target->GotRel;
else
DynType = Target->RelativeRel;
Out<ELFT>::RelaDyn->addReloc(
{DynType, DynamicReloc<ELFT>::Off_Got, !Preemptible, &Body});
}
continue;
}
if (Config->EMachine == EM_MIPS) {
if (Type == R_MIPS_LO16)
// Ignore R_MIPS_LO16 relocation. If it is a pair for R_MIPS_GOT16 we
// already completed all required action (GOT entry allocation) when
// handle R_MIPS_GOT16a. If it is a pair for R_MIPS_HI16 against
// _gp_disp it does not require dynamic relocation. If its a pair for
// R_MIPS_HI16 against a regular symbol it does not require dynamic
// relocation too because that case is possible for executable file
// linking only.
continue;
if (&Body == Config->MipsGpDisp || &Body == Config->MipsLocalGp)
// MIPS _gp_disp designates offset between start of function and 'gp'
// pointer into GOT. __gnu_local_gp is equal to the current value of
// the 'gp'. Therefore any relocations against them do not require
// dynamic relocation.
continue;
}
if (Preemptible) {
// We don't know anything about the finaly symbol. Just ask the dynamic
// linker to handle the relocation for us.
Out<ELFT>::RelaDyn->addReloc({Target->getDynRel(Type), &C, RI.r_offset,
false, &Body, getAddend<ELFT>(RI)});
continue;
}
// We know that this is the final symbol. If the program being produced
// is position independent, the final value is still not known.
// If the relocation depends on the symbol value (not the size or distances
// in the output), we still need some help from the dynamic linker.
// We can however do better than just copying the incoming relocation. We
// can process some of it and and just ask the dynamic linker to add the
// load address.
if (!Config->Pic || Target->isRelRelative(Type) || Target->isSizeRel(Type))
continue;
uintX_t Addend = getAddend<ELFT>(RI);
if (Config->EMachine == EM_PPC64 && RI.getType(false) == R_PPC64_TOC) {
Out<ELFT>::RelaDyn->addReloc({R_PPC64_RELATIVE, &C, RI.r_offset, false,
nullptr,
(uintX_t)getPPC64TocBase() + Addend});
continue;
}
Out<ELFT>::RelaDyn->addReloc(
{Target->RelativeRel, &C, RI.r_offset, true, &Body, Addend});
}
}
template <class ELFT> void Writer<ELFT>::scanRelocs(InputSection<ELFT> &C) {
if (C.getSectionHdr()->sh_flags & SHF_ALLOC)
for (const Elf_Shdr *RelSec : C.RelocSections)
scanRelocs(C, *RelSec);
}
template <class ELFT>
void Writer<ELFT>::scanRelocs(InputSectionBase<ELFT> &S,
const Elf_Shdr &RelSec) {
ELFFile<ELFT> &EObj = S.getFile()->getObj();
if (RelSec.sh_type == SHT_RELA)
scanRelocs(S, EObj.relas(&RelSec));
else
scanRelocs(S, EObj.rels(&RelSec));
}
template <class ELFT>
static void reportUndefined(SymbolTable<ELFT> &Symtab, SymbolBody *Sym) {
if ((Config->Relocatable || Config->Shared) && !Config->NoUndefined)
return;
std::string Msg = "undefined symbol: " + Sym->getName().str();
if (InputFile *File = Symtab.findFile(Sym))
Msg += " in " + File->getName().str();
if (Config->NoinhibitExec)
warning(Msg);
else
error(Msg);
}
template <class ELFT>
static bool shouldKeepInSymtab(const elf::ObjectFile<ELFT> &File,
StringRef SymName,
const typename ELFT::Sym &Sym) {
if (Sym.getType() == STT_FILE)
return false;
// We keep sections in symtab for relocatable output.
if (Sym.getType() == STT_SECTION)
return Config->Relocatable;
InputSectionBase<ELFT> *Sec = File.getSection(Sym);
// If sym references a section in a discarded group, don't keep it.
if (Sec == InputSection<ELFT>::Discarded)
return false;
if (Config->DiscardNone)
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->DiscardLocals)
return false;
return !(Sec->getSectionHdr()->sh_flags & SHF_MERGE);
}
// 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 (!Out<ELFT>::SymTab)
return;
for (const std::unique_ptr<elf::ObjectFile<ELFT>> &F :
Symtab.getObjectFiles()) {
for (SymbolBody *B : F->getLocalSymbols()) {
const Elf_Sym &Sym = cast<DefinedRegular<ELFT>>(B)->Sym;
StringRef SymName = check(Sym.getName(F->getStringTable()));
if (!shouldKeepInSymtab<ELFT>(*F, SymName, Sym))
continue;
if (Sym.st_shndx != SHN_ABS)
if (!F->getSection(Sym)->Live)
continue;
++Out<ELFT>::SymTab->NumLocals;
if (Config->Relocatable)
B->DynsymIndex = Out<ELFT>::SymTab->NumLocals;
F->KeptLocalSyms.push_back(std::make_pair(
&Sym, Out<ELFT>::SymTab->StrTabSec.addString(SymName)));
}
}
}
// 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);
}
template <class ELFT> static bool isRelroSection(OutputSectionBase<ELFT> *Sec) {
if (!Config->ZRelro)
return false;
typename OutputSectionBase<ELFT>::uintX_t Flags = Sec->getFlags();
if (!(Flags & SHF_ALLOC) || !(Flags & SHF_WRITE))
return false;
if (Flags & SHF_TLS)
return true;
uint32_t Type = Sec->getType();
if (Type == SHT_INIT_ARRAY || Type == SHT_FINI_ARRAY ||
Type == SHT_PREINIT_ARRAY)
return true;
if (Sec == Out<ELFT>::GotPlt)
return Config->ZNow;
if (Sec == Out<ELFT>::Dynamic || Sec == Out<ELFT>::Got)
return true;
StringRef S = Sec->getName();
return S == ".data.rel.ro" || S == ".ctors" || S == ".dtors" || S == ".jcr" ||
S == ".eh_frame";
}
// Output section ordering is determined by this function.
template <class ELFT>
static bool compareSections(OutputSectionBase<ELFT> *A,
OutputSectionBase<ELFT> *B) {
typedef typename ELFT::uint uintX_t;
int Comp = Script->compareSections(A->getName(), B->getName());
if (Comp != 0)
return Comp < 0;
uintX_t AFlags = A->getFlags();
uintX_t BFlags = B->getFlags();
// Allocatable sections go first to reduce the total PT_LOAD size and
// so debug info doesn't change addresses in actual code.
bool AIsAlloc = AFlags & SHF_ALLOC;
bool BIsAlloc = BFlags & 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 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 = AFlags & SHF_WRITE;
bool BIsWritable = BFlags & SHF_WRITE;
if (AIsWritable != BIsWritable)
return BIsWritable;
// For a corresponding reason, put non exec sections first (the program
// header PT_LOAD is not executable).
bool AIsExec = AFlags & SHF_EXECINSTR;
bool BIsExec = BFlags & SHF_EXECINSTR;
if (AIsExec != BIsExec)
return BIsExec;
// If we got here we know that both A and B are in the same PT_LOAD.
// The TLS initialization block needs to be a single contiguous block in a R/W
// PT_LOAD, so stick TLS sections directly before R/W sections. The TLS NOBITS
// sections are placed here as they don't take up virtual address space in the
// PT_LOAD.
bool AIsTls = AFlags & SHF_TLS;
bool BIsTls = BFlags & SHF_TLS;
if (AIsTls != BIsTls)
return AIsTls;
// The next requirement we have is to put 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 AIsNoBits = A->getType() == SHT_NOBITS;
bool BIsNoBits = B->getType() == SHT_NOBITS;
if (AIsNoBits != BIsNoBits)
return BIsNoBits;
// We place RelRo section before plain r/w ones.
bool AIsRelRo = isRelroSection(A);
bool BIsRelRo = isRelroSection(B);
if (AIsRelRo != BIsRelRo)
return AIsRelRo;
// Some architectures have additional ordering restrictions for sections
// within the same PT_LOAD.
if (Config->EMachine == EM_PPC64)
return getPPC64SectionRank(A->getName()) <
getPPC64SectionRank(B->getName());
return false;
}
// The .bss section does not exist if no input file has a .bss section.
// This function creates one if that's the case.
template <class ELFT> void Writer<ELFT>::ensureBss() {
if (Out<ELFT>::Bss)
return;
Out<ELFT>::Bss =
new OutputSection<ELFT>(".bss", SHT_NOBITS, SHF_ALLOC | SHF_WRITE);
OwningSections.emplace_back(Out<ELFT>::Bss);
OutputSections.push_back(Out<ELFT>::Bss);
}
// Until this function is called, common symbols do not belong to any section.
// This function adds them to end of BSS section.
template <class ELFT>
void Writer<ELFT>::addCommonSymbols(std::vector<DefinedCommon *> &Syms) {
if (Syms.empty())
return;
// Sort the common symbols by alignment as an heuristic to pack them better.
std::stable_sort(Syms.begin(), Syms.end(),
[](const DefinedCommon *A, const DefinedCommon *B) {
return A->Alignment > B->Alignment;
});
ensureBss();
uintX_t Off = Out<ELFT>::Bss->getSize();
for (DefinedCommon *C : Syms) {
Off = alignTo(Off, C->Alignment);
Out<ELFT>::Bss->updateAlign(C->Alignment);
C->OffsetInBss = Off;
Off += C->Size;
}
Out<ELFT>::Bss->setSize(Off);
}
template <class ELFT> static uint32_t getAlignment(SharedSymbol<ELFT> *SS) {
typedef typename ELFFile<ELFT>::uintX_t uintX_t;
uintX_t SecAlign = SS->File->getSection(SS->Sym)->sh_addralign;
uintX_t SymValue = SS->Sym.st_value;
int TrailingZeros = std::min(countTrailingZeros(SecAlign),
countTrailingZeros(SymValue));
return 1 << TrailingZeros;
}
// Reserve space in .bss for copy relocations.
template <class ELFT>
void Writer<ELFT>::addCopyRelSymbols(std::vector<SharedSymbol<ELFT> *> &Syms) {
if (Syms.empty())
return;
ensureBss();
uintX_t Off = Out<ELFT>::Bss->getSize();
uintX_t MaxAlign = Out<ELFT>::Bss->getAlign();
for (SharedSymbol<ELFT> *SS : Syms) {
uintX_t Align = getAlignment(SS);
Off = alignTo(Off, Align);
SS->OffsetInBss = Off;
Off += SS->Sym.st_size;
MaxAlign = std::max(MaxAlign, Align);
}
Out<ELFT>::Bss->setSize(Off);
Out<ELFT>::Bss->updateAlign(MaxAlign);
}
template <class ELFT>
StringRef Writer<ELFT>::getOutputSectionName(InputSectionBase<ELFT> *S) const {
StringRef Dest = Script->getOutputSection<ELFT>(S);
if (!Dest.empty())
return Dest;
StringRef Name = S->getSectionName();
for (StringRef V : {".text.", ".rodata.", ".data.rel.ro.", ".data.", ".bss.",
".init_array.", ".fini_array.", ".ctors.", ".dtors.",
".tbss.", ".gcc_except_table.", ".tdata."})
if (Name.startswith(V))
return V.drop_back();
return Name;
}
template <class ELFT>
void reportDiscarded(InputSectionBase<ELFT> *IS,
const std::unique_ptr<elf::ObjectFile<ELFT>> &File) {
if (!Config->PrintGcSections || !IS || IS->Live)
return;
llvm::errs() << "removing unused section from '" << IS->getSectionName()
<< "' in file '" << File->getName() << "'\n";
}
template <class ELFT>
bool Writer<ELFT>::isDiscarded(InputSectionBase<ELFT> *S) const {
return !S || S == InputSection<ELFT>::Discarded || !S->Live ||
Script->isDiscarded(S);
}
// 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 (isOutputDynamic() || !Out<ELFT>::RelaPlt)
return;
StringRef S = Config->Rela ? "__rela_iplt_start" : "__rel_iplt_start";
if (Symtab.find(S))
Symtab.addAbsolute(S, ElfSym<ELFT>::RelaIpltStart);
S = Config->Rela ? "__rela_iplt_end" : "__rel_iplt_end";
if (Symtab.find(S))
Symtab.addAbsolute(S, ElfSym<ELFT>::RelaIpltEnd);
}
template <class ELFT> static bool includeInSymtab(const SymbolBody &B) {
if (!B.isUsedInRegularObj())
return false;
if (auto *D = dyn_cast<DefinedRegular<ELFT>>(&B)) {
// Don't include synthetic symbols like __init_array_start in every output.
if (&D->Sym == &ElfSym<ELFT>::Ignored)
return false;
// Exclude symbols pointing to garbage-collected sections.
if (D->Section && !D->Section->Live)
return false;
}
return true;
}
static bool includeInDynsym(const SymbolBody &B) {
uint8_t V = B.getVisibility();
if (V != STV_DEFAULT && V != STV_PROTECTED)
return false;
if (Config->ExportDynamic || Config->Shared)
return true;
return B.MustBeInDynSym;
}
// This class knows how to create an output section for a given
// input section. Output section type is determined by various
// factors, including input section's sh_flags, sh_type and
// linker scripts.
namespace {
template <class ELFT> class OutputSectionFactory {
typedef typename ELFT::Shdr Elf_Shdr;
typedef typename ELFT::uint uintX_t;
public:
std::pair<OutputSectionBase<ELFT> *, bool> create(InputSectionBase<ELFT> *C,
StringRef OutsecName);
OutputSectionBase<ELFT> *lookup(StringRef Name, uint32_t Type, uintX_t Flags);
private:
SectionKey<ELFT::Is64Bits> createKey(InputSectionBase<ELFT> *C,
StringRef OutsecName);
SmallDenseMap<SectionKey<ELFT::Is64Bits>, OutputSectionBase<ELFT> *> Map;
};
}
template <class ELFT>
std::pair<OutputSectionBase<ELFT> *, bool>
OutputSectionFactory<ELFT>::create(InputSectionBase<ELFT> *C,
StringRef OutsecName) {
SectionKey<ELFT::Is64Bits> Key = createKey(C, OutsecName);
OutputSectionBase<ELFT> *&Sec = Map[Key];
if (Sec)
return {Sec, false};
switch (C->SectionKind) {
case InputSectionBase<ELFT>::Regular:
Sec = new OutputSection<ELFT>(Key.Name, Key.Type, Key.Flags);
break;
case InputSectionBase<ELFT>::EHFrame:
Sec = new EHOutputSection<ELFT>(Key.Name, Key.Type, Key.Flags);
break;
case InputSectionBase<ELFT>::Merge:
Sec = new MergeOutputSection<ELFT>(Key.Name, Key.Type, Key.Flags,
Key.Alignment);
break;
case InputSectionBase<ELFT>::MipsReginfo:
Sec = new MipsReginfoOutputSection<ELFT>();
break;
}
return {Sec, true};
}
template <class ELFT>
OutputSectionBase<ELFT> *OutputSectionFactory<ELFT>::lookup(StringRef Name,
uint32_t Type,
uintX_t Flags) {
return Map.lookup({Name, Type, Flags, 0});
}
template <class ELFT>
SectionKey<ELFT::Is64Bits>
OutputSectionFactory<ELFT>::createKey(InputSectionBase<ELFT> *C,
StringRef OutsecName) {
const Elf_Shdr *H = C->getSectionHdr();
uintX_t Flags = H->sh_flags & ~SHF_GROUP;
// For SHF_MERGE we create different output sections for each alignment.
// This makes each output section simple and keeps a single level mapping from
// input to output.
uintX_t Alignment = 0;
if (isa<MergeInputSection<ELFT>>(C)) {
Alignment = H->sh_addralign;
if (H->sh_entsize > Alignment)
Alignment = H->sh_entsize;
}
// GNU as can give .eh_frame secion type SHT_PROGBITS or SHT_X86_64_UNWIND
// depending on the construct. We want to canonicalize it so that
// there is only one .eh_frame in the end.
uint32_t Type = H->sh_type;
if (Type == SHT_PROGBITS && Config->EMachine == EM_X86_64 &&
isa<EHInputSection<ELFT>>(C))
Type = SHT_X86_64_UNWIND;
return SectionKey<ELFT::Is64Bits>{OutsecName, Type, Flags, Alignment};
}
// 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() {
// __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.
if (!isOutputDynamic())
Symtab.addIgnored("__tls_get_addr");
auto Define = [this](StringRef S, Elf_Sym &Sym) {
if (Symtab.find(S))
Symtab.addAbsolute(S, Sym);
// The name without the underscore is not a reserved name,
// so it is defined only when there is a reference against it.
assert(S.startswith("_"));
S = S.substr(1);
if (SymbolBody *B = Symtab.find(S))
if (B->isUndefined())
Symtab.addAbsolute(S, Sym);
};
Define("_end", ElfSym<ELFT>::End);
Define("_etext", ElfSym<ELFT>::Etext);
Define("_edata", ElfSym<ELFT>::Edata);
}
// Sort input sections by section name suffixes for
// __attribute__((init_priority(N))).
template <class ELFT> static void sortInitFini(OutputSectionBase<ELFT> *S) {
if (S)
reinterpret_cast<OutputSection<ELFT> *>(S)->sortInitFini();
}
// Sort input sections by the special rule for .ctors and .dtors.
template <class ELFT> static void sortCtorsDtors(OutputSectionBase<ELFT> *S) {
if (S)
reinterpret_cast<OutputSection<ELFT> *>(S)->sortCtorsDtors();
}
// Create output section objects and add them to OutputSections.
template <class ELFT> bool Writer<ELFT>::createSections() {
OutputSections.push_back(Out<ELFT>::ElfHeader);
if (!Config->Relocatable)
OutputSections.push_back(Out<ELFT>::ProgramHeaders);
// Add .interp first because some loaders want to see that section
// on the first page of the executable file when loaded into memory.
if (needsInterpSection())
OutputSections.push_back(Out<ELFT>::Interp);
// A core file does not usually contain unmodified segments except
// the first page of the executable. Add the build ID section now
// so that the section is included in the first page.
if (Out<ELFT>::BuildId)
OutputSections.push_back(Out<ELFT>::BuildId);
// Create output sections for input object file sections.
std::vector<OutputSectionBase<ELFT> *> RegularSections;
OutputSectionFactory<ELFT> Factory;
for (const std::unique_ptr<elf::ObjectFile<ELFT>> &F :
Symtab.getObjectFiles()) {
for (InputSectionBase<ELFT> *C : F->getSections()) {
if (isDiscarded(C)) {
reportDiscarded(C, F);
continue;
}
OutputSectionBase<ELFT> *Sec;
bool IsNew;
std::tie(Sec, IsNew) = Factory.create(C, getOutputSectionName(C));
if (IsNew) {
OwningSections.emplace_back(Sec);
OutputSections.push_back(Sec);
RegularSections.push_back(Sec);
}
Sec->addSection(C);
}
}
Out<ELFT>::Bss = static_cast<OutputSection<ELFT> *>(
Factory.lookup(".bss", SHT_NOBITS, SHF_ALLOC | SHF_WRITE));
// If we have a .opd section (used under PPC64 for function descriptors),
// store a pointer to it here so that we can use it later when processing
// relocations.
Out<ELFT>::Opd = Factory.lookup(".opd", SHT_PROGBITS, SHF_WRITE | SHF_ALLOC);
Out<ELFT>::Dynamic->PreInitArraySec = Factory.lookup(
".preinit_array", SHT_PREINIT_ARRAY, SHF_WRITE | SHF_ALLOC);
Out<ELFT>::Dynamic->InitArraySec =
Factory.lookup(".init_array", SHT_INIT_ARRAY, SHF_WRITE | SHF_ALLOC);
Out<ELFT>::Dynamic->FiniArraySec =
Factory.lookup(".fini_array", SHT_FINI_ARRAY, SHF_WRITE | SHF_ALLOC);
// Sort section contents for __attribute__((init_priority(N)).
sortInitFini(Out<ELFT>::Dynamic->InitArraySec);
sortInitFini(Out<ELFT>::Dynamic->FiniArraySec);
sortCtorsDtors(Factory.lookup(".ctors", SHT_PROGBITS, SHF_WRITE | SHF_ALLOC));
sortCtorsDtors(Factory.lookup(".dtors", SHT_PROGBITS, SHF_WRITE | SHF_ALLOC));
// The linker needs to define SECNAME_start, SECNAME_end and SECNAME_stop
// symbols for sections, so that the runtime can get the start and end
// addresses of each section by section name. Add such symbols.
if (!Config->Relocatable) {
addStartEndSymbols();
for (OutputSectionBase<ELFT> *Sec : RegularSections)
addStartStopSymbols(Sec);
}
// Add _DYNAMIC symbol. Unlike GNU gold, our _DYNAMIC symbol has no type.
// It should be okay as no one seems to care about the type.
// Even the author of gold doesn't remember why gold behaves that way.
// https://sourceware.org/ml/binutils/2002-03/msg00360.html
if (isOutputDynamic())
Symtab.addSynthetic("_DYNAMIC", *Out<ELFT>::Dynamic, 0, STV_HIDDEN);
// Define __rel[a]_iplt_{start,end} symbols if needed.
addRelIpltSymbols();
// Scan relocations. This must be done after every symbol is declared so that
// we can correctly decide if a dynamic relocation is needed.
for (const std::unique_ptr<elf::ObjectFile<ELFT>> &F :
Symtab.getObjectFiles()) {
for (InputSectionBase<ELFT> *C : F->getSections()) {
if (isDiscarded(C))
continue;
if (auto *S = dyn_cast<InputSection<ELFT>>(C))
scanRelocs(*S);
else if (auto *S = dyn_cast<EHInputSection<ELFT>>(C))
if (S->RelocSection)
scanRelocs(*S, *S->RelocSection);
}
}
// Now that we have defined all possible symbols including linker-
// synthesized ones. Visit all symbols to give the finishing touches.
std::vector<DefinedCommon *> CommonSymbols;
std::vector<SharedSymbol<ELFT> *> CopyRelSymbols;
for (auto &P : Symtab.getSymbols()) {
SymbolBody *Body = P.second->Body;
if (auto *U = dyn_cast<Undefined>(Body))
if (!U->isWeak() && !U->canKeepUndefined())
reportUndefined<ELFT>(Symtab, Body);
if (auto *C = dyn_cast<DefinedCommon>(Body))
CommonSymbols.push_back(C);
if (auto *SC = dyn_cast<SharedSymbol<ELFT>>(Body))
if (SC->needsCopy())
CopyRelSymbols.push_back(SC);
if (!includeInSymtab<ELFT>(*Body))
continue;
if (Out<ELFT>::SymTab)
Out<ELFT>::SymTab->addSymbol(Body);
if (isOutputDynamic() && includeInDynsym(*Body))
Out<ELFT>::DynSymTab->addSymbol(Body);
}
// Do not proceed if there was an undefined symbol.
if (HasError)
return false;
addCommonSymbols(CommonSymbols);
addCopyRelSymbols(CopyRelSymbols);
// So far we have added sections from input object files.
// This function adds linker-created Out<ELFT>::* sections.
addPredefinedSections();
std::stable_sort(OutputSections.begin(), OutputSections.end(),
compareSections<ELFT>);
for (unsigned I = dummySectionsNum(), N = OutputSections.size(); I < N; ++I)
OutputSections[I]->SectionIndex = I + 1 - dummySectionsNum();
for (OutputSectionBase<ELFT> *Sec : getSections())
Sec->setSHName(Out<ELFT>::ShStrTab->addString(Sec->getName()));
// Finalizers fix each section's size.
// .dynsym is finalized early since that may fill up .gnu.hash.
if (isOutputDynamic())
Out<ELFT>::DynSymTab->finalize();
// Fill other section headers. The dynamic table is finalized
// at the end because some tags like RELSZ depend on result
// of finalizing other sections. The dynamic string table is
// finalized once the .dynamic finalizer has added a few last
// strings. See DynamicSection::finalize()
for (OutputSectionBase<ELFT> *Sec : OutputSections)
if (Sec != Out<ELFT>::DynStrTab && Sec != Out<ELFT>::Dynamic)
Sec->finalize();
if (isOutputDynamic())
Out<ELFT>::Dynamic->finalize();
return true;
}
template <class ELFT> bool Writer<ELFT>::needsGot() {
if (!Out<ELFT>::Got->empty())
return true;
// We add the .got section to the result for dynamic MIPS target because
// its address and properties are mentioned in the .dynamic section.
if (Config->EMachine == EM_MIPS && isOutputDynamic())
return true;
// If we have a relocation that is relative to GOT (such as GOTOFFREL),
// we need to emit a GOT even if it's empty.
return HasGotOffRel;
}
// This function add Out<ELFT>::* sections to OutputSections.
template <class ELFT> void Writer<ELFT>::addPredefinedSections() {
auto Add = [&](OutputSectionBase<ELFT> *C) {
if (C)
OutputSections.push_back(C);
};
// This order is not the same as the final output order
// because we sort the sections using their attributes below.
Add(Out<ELFT>::SymTab);
Add(Out<ELFT>::ShStrTab);
Add(Out<ELFT>::StrTab);
if (isOutputDynamic()) {
Add(Out<ELFT>::DynSymTab);
Add(Out<ELFT>::GnuHashTab);
Add(Out<ELFT>::HashTab);
Add(Out<ELFT>::Dynamic);
Add(Out<ELFT>::DynStrTab);
if (Out<ELFT>::RelaDyn->hasRelocs())
Add(Out<ELFT>::RelaDyn);
Add(Out<ELFT>::MipsRldMap);
}
// We always need to add rel[a].plt to output if it has entries.
// Even during static linking it can contain R_[*]_IRELATIVE relocations.
if (Out<ELFT>::RelaPlt && Out<ELFT>::RelaPlt->hasRelocs()) {
Add(Out<ELFT>::RelaPlt);
Out<ELFT>::RelaPlt->Static = !isOutputDynamic();
}
if (needsGot())
Add(Out<ELFT>::Got);
if (Out<ELFT>::GotPlt && !Out<ELFT>::GotPlt->empty())
Add(Out<ELFT>::GotPlt);
if (!Out<ELFT>::Plt->empty())
Add(Out<ELFT>::Plt);
if (Out<ELFT>::EhFrameHdr->Live)
Add(Out<ELFT>::EhFrameHdr);
}
// 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() {
auto Define = [&](StringRef Start, StringRef End,
OutputSectionBase<ELFT> *OS) {
if (OS) {
Symtab.addSynthetic(Start, *OS, 0, STV_DEFAULT);
Symtab.addSynthetic(End, *OS, OS->getSize(), STV_DEFAULT);
} else {
Symtab.addIgnored(Start);
Symtab.addIgnored(End);
}
};
Define("__preinit_array_start", "__preinit_array_end",
Out<ELFT>::Dynamic->PreInitArraySec);
Define("__init_array_start", "__init_array_end",
Out<ELFT>::Dynamic->InitArraySec);
Define("__fini_array_start", "__fini_array_end",
Out<ELFT>::Dynamic->FiniArraySec);
}
// 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(OutputSectionBase<ELFT> *Sec) {
StringRef S = Sec->getName();
if (!isValidCIdentifier(S))
return;
StringSaver Saver(Alloc);
StringRef Start = Saver.save("__start_" + S);
StringRef Stop = Saver.save("__stop_" + S);
if (SymbolBody *B = Symtab.find(Start))
if (B->isUndefined())
Symtab.addSynthetic(Start, *Sec, 0, STV_DEFAULT);
if (SymbolBody *B = Symtab.find(Stop))
if (B->isUndefined())
Symtab.addSynthetic(Stop, *Sec, Sec->getSize(), STV_DEFAULT);
}
template <class ELFT> static bool needsPtLoad(OutputSectionBase<ELFT> *Sec) {
if (!(Sec->getFlags() & 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->getFlags() & SHF_TLS && Sec->getType() == SHT_NOBITS)
return false;
return true;
}
static uint32_t toPhdrFlags(uint64_t Flags) {
uint32_t Ret = PF_R;
if (Flags & SHF_WRITE)
Ret |= PF_W;
if (Flags & SHF_EXECINSTR)
Ret |= PF_X;
return Ret;
}
// Decide which program headers to create and which sections to include in each
// one.
template <class ELFT> void Writer<ELFT>::createPhdrs() {
auto AddHdr = [this](unsigned Type, unsigned Flags) {
return &*Phdrs.emplace(Phdrs.end(), Type, Flags);
};
auto AddSec = [](Phdr &Hdr, OutputSectionBase<ELFT> *Sec) {
Hdr.Last = Sec;
if (!Hdr.First)
Hdr.First = Sec;
Hdr.H.p_align = std::max<uintX_t>(Hdr.H.p_align, Sec->getAlign());
};
// The first phdr entry is PT_PHDR which describes the program header itself.
Phdr &Hdr = *AddHdr(PT_PHDR, PF_R);
AddSec(Hdr, Out<ELFT>::ProgramHeaders);
// PT_INTERP must be the second entry if exists.
if (needsInterpSection()) {
Phdr &Hdr = *AddHdr(PT_INTERP, toPhdrFlags(Out<ELFT>::Interp->getFlags()));
AddSec(Hdr, Out<ELFT>::Interp);
}
// Add the first PT_LOAD segment for regular output sections.
uintX_t Flags = PF_R;
Phdr *Load = AddHdr(PT_LOAD, Flags);
AddSec(*Load, Out<ELFT>::ElfHeader);
Phdr TlsHdr(PT_TLS, PF_R);
Phdr RelRo(PT_GNU_RELRO, PF_R);
Phdr Note(PT_NOTE, PF_R);
for (OutputSectionBase<ELFT> *Sec : OutputSections) {
if (!(Sec->getFlags() & SHF_ALLOC))
break;
// If we meet TLS section then we create TLS header
// and put all TLS sections inside for futher use when
// assign addresses.
if (Sec->getFlags() & SHF_TLS)
AddSec(TlsHdr, Sec);
if (!needsPtLoad<ELFT>(Sec))
continue;
// If flags changed then we want new load segment.
uintX_t NewFlags = toPhdrFlags(Sec->getFlags());
if (Flags != NewFlags) {
Load = AddHdr(PT_LOAD, NewFlags);
Flags = NewFlags;
}
AddSec(*Load, Sec);
if (isRelroSection(Sec))
AddSec(RelRo, Sec);
if (Sec->getType() == SHT_NOTE)
AddSec(Note, Sec);
}
// Add the TLS segment unless it's empty.
if (TlsHdr.First)
Phdrs.push_back(std::move(TlsHdr));
// Add an entry for .dynamic.
if (isOutputDynamic()) {
Phdr &H = *AddHdr(PT_DYNAMIC, toPhdrFlags(Out<ELFT>::Dynamic->getFlags()));
AddSec(H, Out<ELFT>::Dynamic);
}
// PT_GNU_RELRO includes all sections that should be marked as
// read-only by dynamic linker after proccessing relocations.
if (RelRo.First)
Phdrs.push_back(std::move(RelRo));
// PT_GNU_EH_FRAME is a special section pointing on .eh_frame_hdr.
if (Out<ELFT>::EhFrameHdr->Live) {
Phdr &Hdr = *AddHdr(PT_GNU_EH_FRAME,
toPhdrFlags(Out<ELFT>::EhFrameHdr->getFlags()));
AddSec(Hdr, Out<ELFT>::EhFrameHdr);
}
// PT_GNU_STACK is a special section to tell the loader to make the
// pages for the stack non-executable.
if (!Config->ZExecStack)
AddHdr(PT_GNU_STACK, PF_R | PF_W);
if (Note.First)
Phdrs.push_back(std::move(Note));
}
// Used for relocatable output (-r). In this case we create only ELF file
// header, do not create program headers. Also assign of section addresses
// is very straightforward: we just put all sections sequentually to the file.
template <class ELFT> void Writer<ELFT>::assignAddressesRelocatable() {
Out<ELFT>::ElfHeader->setSize(sizeof(Elf_Ehdr));
uintX_t FileOff = 0;
for (OutputSectionBase<ELFT> *Sec : OutputSections) {
if (Sec->getType() != SHT_NOBITS)
FileOff = alignTo(FileOff, Sec->getAlign());
Sec->setFileOffset(FileOff);
if (Sec->getType() != SHT_NOBITS)
FileOff += Sec->getSize();
}
SectionHeaderOff = alignTo(FileOff, sizeof(uintX_t));
FileSize = SectionHeaderOff + getNumSections() * sizeof(Elf_Shdr);
}
// Visits all headers in PhdrTable and assigns the adresses to
// the output sections. Also creates common and special headers.
template <class ELFT> void Writer<ELFT>::assignAddresses() {
Out<ELFT>::ElfHeader->setSize(sizeof(Elf_Ehdr));
size_t PhdrSize = sizeof(Elf_Phdr) * Phdrs.size();
Out<ELFT>::ProgramHeaders->setSize(PhdrSize);
// The first section of each PT_LOAD and the first section after PT_GNU_RELRO
// have to be page aligned so that the dynamic linker can set the permissions.
SmallPtrSet<OutputSectionBase<ELFT> *, 4> PageAlign;
for (const Phdr &P : Phdrs) {
if (P.H.p_type == PT_GNU_RELRO) {
// Find the first section after PT_GNU_RELRO. If it is in a PT_LOAD we
// have to align it to a page.
auto I = std::find(OutputSections.begin(), OutputSections.end(), P.Last);
++I;
if (I != OutputSections.end() && needsPtLoad(*I))
PageAlign.insert(*I);
}
if (P.H.p_type == PT_LOAD)
PageAlign.insert(P.First);
}
uintX_t ThreadBssOffset = 0;
uintX_t VA = Target->getVAStart();
uintX_t FileOff = 0;
for (OutputSectionBase<ELFT> *Sec : OutputSections) {
uintX_t Align = Sec->getAlign();
if (PageAlign.count(Sec))
Align = std::max<uintX_t>(Align, Target->PageSize);
if (Sec->getType() != SHT_NOBITS)
FileOff = alignTo(FileOff, Align);
Sec->setFileOffset(FileOff);
if (Sec->getType() != SHT_NOBITS)
FileOff += Sec->getSize();
// We only assign VAs to allocated sections.
if (needsPtLoad<ELFT>(Sec)) {
VA = alignTo(VA, Align);
Sec->setVA(VA);
VA += Sec->getSize();
} else if (Sec->getFlags() & SHF_TLS && Sec->getType() == SHT_NOBITS) {
uintX_t TVA = VA + ThreadBssOffset;
TVA = alignTo(TVA, Align);
Sec->setVA(TVA);
ThreadBssOffset = TVA - VA + Sec->getSize();
}
}
// Add space for section headers.
SectionHeaderOff = alignTo(FileOff, sizeof(uintX_t));
FileSize = SectionHeaderOff + getNumSections() * sizeof(Elf_Shdr);
// Update "_end" and "end" symbols so that they
// point to the end of the data segment.
ElfSym<ELFT>::End.st_value = VA;
for (Phdr &PHdr : Phdrs) {
Elf_Phdr &H = PHdr.H;
if (PHdr.First) {
OutputSectionBase<ELFT> *Last = PHdr.Last;
H.p_filesz = Last->getFileOff() - PHdr.First->getFileOff();
if (Last->getType() != SHT_NOBITS)
H.p_filesz += Last->getSize();
H.p_memsz = Last->getVA() + Last->getSize() - PHdr.First->getVA();
H.p_offset = PHdr.First->getFileOff();
H.p_vaddr = PHdr.First->getVA();
}
if (H.p_type == PT_LOAD)
H.p_align = Target->PageSize;
else if (H.p_type == PT_GNU_RELRO)
H.p_align = 1;
H.p_paddr = H.p_vaddr;
// The TLS pointer goes after PT_TLS. At least glibc will align it,
// so round up the size to make sure the offsets are correct.
if (H.p_type == PT_TLS) {
Out<ELFT>::TlsPhdr = &H;
H.p_memsz = alignTo(H.p_memsz, H.p_align);
}
}
}
static uint32_t getMipsEFlags() {
// FIXME: In fact ELF flags depends on ELF flags of input object files
// and selected emulation. For now just use hard coded values.
uint32_t V = EF_MIPS_ABI_O32 | EF_MIPS_CPIC | EF_MIPS_ARCH_32R2;
if (Config->Shared)
V |= EF_MIPS_PIC;
return V;
}
template <class ELFT> static typename ELFT::uint getEntryAddr() {
if (SymbolBody *B = Config->EntrySym)
return B->repl().getVA<ELFT>();
if (Config->EntryAddr != uint64_t(-1))
return Config->EntryAddr;
return 0;
}
template <class ELFT> static uint8_t getELFEncoding() {
if (ELFT::TargetEndianness == llvm::support::little)
return ELFDATA2LSB;
return ELFDATA2MSB;
}
static uint16_t getELFType() {
if (Config->Pic)
return ET_DYN;
if (Config->Relocatable)
return ET_REL;
return ET_EXEC;
}
// This function is called after we have assigned address and size
// to each section. This function fixes some predefined absolute
// symbol values that depend on section address and size.
template <class ELFT> void Writer<ELFT>::fixAbsoluteSymbols() {
// Update __rel[a]_iplt_{start,end} symbols so that they point
// to beginning or ending of .rela.plt section, respectively.
if (Out<ELFT>::RelaPlt) {
uintX_t Start = Out<ELFT>::RelaPlt->getVA();
ElfSym<ELFT>::RelaIpltStart.st_value = Start;
ElfSym<ELFT>::RelaIpltEnd.st_value = Start + Out<ELFT>::RelaPlt->getSize();
}
// Update MIPS _gp absolute symbol so that it points to the static data.
if (Config->EMachine == EM_MIPS)
ElfSym<ELFT>::MipsGp.st_value = getMipsGpAddr<ELFT>();
// _etext is the first location after the last read-only loadable segment.
// _edata is the first location after the last read-write loadable segment.
for (Phdr &PHdr : Phdrs) {
if (PHdr.H.p_type != PT_LOAD)
continue;
uintX_t Val = PHdr.H.p_vaddr + PHdr.H.p_filesz;
if (PHdr.H.p_flags & PF_W)
ElfSym<ELFT>::Edata.st_value = Val;
else
ElfSym<ELFT>::Etext.st_value = Val;
}
}
template <class ELFT> void Writer<ELFT>::writeHeader() {
uint8_t *Buf = Buffer->getBufferStart();
memcpy(Buf, "\177ELF", 4);
auto &FirstObj = cast<ELFFileBase<ELFT>>(*Config->FirstElf);
// Write the ELF header.
auto *EHdr = reinterpret_cast<Elf_Ehdr *>(Buf);
EHdr->e_ident[EI_CLASS] = ELFT::Is64Bits ? ELFCLASS64 : ELFCLASS32;
EHdr->e_ident[EI_DATA] = getELFEncoding<ELFT>();
EHdr->e_ident[EI_VERSION] = EV_CURRENT;
EHdr->e_ident[EI_OSABI] = FirstObj.getOSABI();
EHdr->e_type = getELFType();
EHdr->e_machine = FirstObj.getEMachine();
EHdr->e_version = EV_CURRENT;
EHdr->e_entry = getEntryAddr<ELFT>();
EHdr->e_shoff = SectionHeaderOff;
EHdr->e_ehsize = sizeof(Elf_Ehdr);
EHdr->e_phnum = Phdrs.size();
EHdr->e_shentsize = sizeof(Elf_Shdr);
EHdr->e_shnum = getNumSections();
EHdr->e_shstrndx = Out<ELFT>::ShStrTab->SectionIndex;
if (Config->EMachine == EM_MIPS)
EHdr->e_flags = getMipsEFlags();
if (!Config->Relocatable) {
EHdr->e_phoff = sizeof(Elf_Ehdr);
EHdr->e_phentsize = sizeof(Elf_Phdr);
}
// Write the program header table.
auto *HBuf = reinterpret_cast<Elf_Phdr *>(Buf + EHdr->e_phoff);
for (Phdr &P : Phdrs)
*HBuf++ = P.H;
// Write the section header table. Note that the first table entry is null.
auto *SHdrs = reinterpret_cast<Elf_Shdr *>(Buf + EHdr->e_shoff);
for (OutputSectionBase<ELFT> *Sec : getSections())
Sec->writeHeaderTo(++SHdrs);
}
template <class ELFT> bool Writer<ELFT>::openFile() {
ErrorOr<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
FileOutputBuffer::create(Config->OutputFile, FileSize,
FileOutputBuffer::F_executable);
if (!BufferOrErr) {
error(BufferOrErr, "failed to open " + Config->OutputFile);
return false;
}
Buffer = std::move(*BufferOrErr);
return true;
}
// Write section contents to a mmap'ed file.
template <class ELFT> void Writer<ELFT>::writeSections() {
uint8_t *Buf = Buffer->getBufferStart();
// PPC64 needs to process relocations in the .opd section before processing
// relocations in code-containing sections.
if (OutputSectionBase<ELFT> *Sec = Out<ELFT>::Opd) {
Out<ELFT>::OpdBuf = Buf + Sec->getFileOff();
Sec->writeTo(Buf + Sec->getFileOff());
}
for (OutputSectionBase<ELFT> *Sec : OutputSections)
if (Sec != Out<ELFT>::Opd)
Sec->writeTo(Buf + Sec->getFileOff());
}
template <class ELFT> void Writer<ELFT>::writeBuildId() {
BuildIdSection<ELFT> *S = Out<ELFT>::BuildId;
if (!S)
return;
// Compute a hash of all sections except .debug_* sections.
// We skip debug sections because they tend to be very large
// and their contents are very likely to be the same as long as
// other sections are the same.
uint8_t *Start = Buffer->getBufferStart();
uint8_t *Last = Start;
for (OutputSectionBase<ELFT> *Sec : OutputSections) {
uint8_t *End = Start + Sec->getFileOff();
if (!Sec->getName().startswith(".debug_"))
S->update({Last, End});
Last = End;
}
S->update({Last, Start + FileSize});
// Fill the hash value field in the .note.gnu.build-id section.
S->writeBuildId();
}
template void elf::writeResult<ELF32LE>(SymbolTable<ELF32LE> *Symtab);
template void elf::writeResult<ELF32BE>(SymbolTable<ELF32BE> *Symtab);
template void elf::writeResult<ELF64LE>(SymbolTable<ELF64LE> *Symtab);
template void elf::writeResult<ELF64BE>(SymbolTable<ELF64BE> *Symtab);
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