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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 "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();
void createSections();
void addPredefinedSections();
bool needsGot();
template <class RelTy>
void scanRelocs(InputSectionBase<ELFT> &C, ArrayRef<RelTy> Rels);
void scanRelocs(InputSection<ELFT> &C);
void scanRelocs(InputSectionBase<ELFT> &S, const Elf_Shdr &RelSec);
void createPhdrs();
void assignAddresses();
void assignFileOffsets();
void setPhdrs();
void fixHeaders();
void fixSectionAlignments();
void fixAbsoluteSymbols();
void 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;
}
template <class RelTy>
void scanRelocsForThunks(const elf::ObjectFile<ELFT> &File,
ArrayRef<RelTy> Rels);
void ensureBss();
void addCommonSymbols(std::vector<DefinedCommon *> &Syms);
void addCopyRelSymbol(SharedSymbol<ELFT> *Sym);
std::unique_ptr<llvm::FileOutputBuffer> Buffer;
BumpPtrAllocator Alloc;
std::vector<OutputSectionBase<ELFT> *> OutputSections;
std::vector<std::unique_ptr<OutputSectionBase<ELFT>>> OwningSections;
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;
typedef typename ELFT::Ehdr Elf_Ehdr;
// 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);
ElfHeader.setSize(sizeof(Elf_Ehdr));
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 == BuildIdKind::Fnv1)
BuildId.reset(new BuildIdFnv1<ELFT>);
else if (Config->BuildId == BuildIdKind::Md5)
BuildId.reset(new BuildIdMd5<ELFT>);
else if (Config->BuildId == BuildIdKind::Sha1)
BuildId.reset(new BuildIdSha1<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();
createSections();
if (HasError)
return;
if (Config->Relocatable) {
assignFileOffsets();
} else {
createPhdrs();
fixHeaders();
fixSectionAlignments();
assignAddresses();
assignFileOffsets();
setPhdrs();
fixAbsoluteSymbols();
}
openFile();
if (HasError)
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) {
typedef typename ELFT::uint uintX_t;
if (Target->pointsToLocalDynamicGotEntry(Type)) {
if (Target->canRelaxTls(Type, nullptr))
return 2;
if (Out<ELFT>::Got->addTlsIndex())
Out<ELFT>::RelaDyn->addReloc({Target->TlsModuleIndexRel, Out<ELFT>::Got,
Out<ELFT>::Got->getTlsIndexOff(), false,
nullptr, 0});
return 1;
}
if (!Body.isTls())
return 0;
if (Target->isTlsGlobalDynamicRel(Type)) {
if (!Target->canRelaxTls(Type, &Body)) {
if (Out<ELFT>::Got->addDynTlsEntry(Body)) {
uintX_t Off = Out<ELFT>::Got->getGlobalDynOffset(Body);
Out<ELFT>::RelaDyn->addReloc(
{Target->TlsModuleIndexRel, Out<ELFT>::Got, Off, false, &Body, 0});
Out<ELFT>::RelaDyn->addReloc({Target->TlsOffsetRel, Out<ELFT>::Got,
Off + (uintX_t)sizeof(uintX_t), false,
&Body, 0});
}
return 1;
}
if (!Body.isPreemptible())
return 2;
if (!Body.isInGot()) {
Out<ELFT>::Got->addEntry(Body);
Out<ELFT>::RelaDyn->addReloc({Target->TlsGotRel, Out<ELFT>::Got,
Body.getGotOffset<ELFT>(), false, &Body,
0});
}
return 2;
}
return 0;
}
// Some targets might require creation of thunks for relocations. Now we
// support only MIPS which requires LA25 thunk to call PIC code from non-PIC
// one. Scan relocations to find each one requires thunk.
template <class ELFT>
template <class RelTy>
void Writer<ELFT>::scanRelocsForThunks(const elf::ObjectFile<ELFT> &File,
ArrayRef<RelTy> Rels) {
for (const RelTy &RI : Rels) {
uint32_t Type = RI.getType(Config->Mips64EL);
uint32_t SymIndex = RI.getSymbol(Config->Mips64EL);
SymbolBody &Body = File.getSymbolBody(SymIndex).repl();
if (Body.hasThunk() || !Target->needsThunk(Type, File, Body))
continue;
auto *D = cast<DefinedRegular<ELFT>>(&Body);
auto *S = cast<InputSection<ELFT>>(D->Section);
S->addThunk(Body);
}
}
// 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, ArrayRef<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;
uintX_t Offset = C.getOffset(RI.r_offset);
if (Offset == (uintX_t)-1)
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.OutSec, 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 (Target->needsCopyRel<ELFT>(Type, Body)) {
if (!B->needsCopy())
addCopyRelSymbol(B);
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);
uint32_t Rel;
if (Body.isGnuIFunc())
Rel = Preemptible ? Target->PltRel : Target->IRelativeRel;
else
Rel = Target->UseLazyBinding ? Target->PltRel : Target->GotRel;
if (Target->UseLazyBinding) {
Out<ELFT>::GotPlt->addEntry(Body);
Out<ELFT>::RelaPlt->addReloc({Rel, Out<ELFT>::GotPlt,
Body.getGotPltOffset<ELFT>(),
!Preemptible, &Body, 0});
} else {
if (Body.isInGot())
continue;
Out<ELFT>::Got->addEntry(Body);
Out<ELFT>::RelaDyn->addReloc({Rel, Out<ELFT>::Got,
Body.getGotOffset<ELFT>(), !Preemptible,
&Body, 0});
}
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;
if (Preemptible || Config->Pic) {
uint32_t DynType;
if (Body.isTls())
DynType = Target->TlsGotRel;
else if (Preemptible)
DynType = Target->GotRel;
else
DynType = Target->RelativeRel;
Out<ELFT>::RelaDyn->addReloc({DynType, Out<ELFT>::Got,
Body.getGotOffset<ELFT>(), !Preemptible,
&Body, 0});
}
continue;
}
// 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.
if (Config->EMachine == EM_MIPS && (&Body == ElfSym<ELFT>::MipsGpDisp ||
&Body == ElfSym<ELFT>::MipsLocalGp))
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.OutSec, 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.OutSec, Offset, false,
nullptr,
(uintX_t)getPPC64TocBase() + Addend});
continue;
}
Out<ELFT>::RelaDyn->addReloc(
{Target->RelativeRel, C.OutSec, Offset, true, &Body, Addend});
}
// Scan relocations for necessary thunks.
if (Config->EMachine == EM_MIPS)
scanRelocsForThunks(File, Rels);
}
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(InputSectionBase<ELFT> *Sec, StringRef SymName,
const SymbolBody &B) {
if (B.isFile())
return false;
// We keep sections in symtab for relocatable output.
if (B.isSection())
return Config->Relocatable;
// 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()) {
const char *StrTab = F->getStringTable().data();
for (SymbolBody *B : F->getLocalSymbols()) {
auto *DR = dyn_cast<DefinedRegular<ELFT>>(B);
// No reason to keep local undefined symbol in symtab.
if (!DR)
continue;
StringRef SymName(StrTab + B->getNameOffset());
InputSectionBase<ELFT> *Sec = DR->Section;
if (!shouldKeepInSymtab<ELFT>(Sec, SymName, *B))
continue;
if (Sec && !Sec->Live)
continue;
++Out<ELFT>::SymTab->NumLocals;
if (Config->Relocatable)
B->DynsymIndex = Out<ELFT>::SymTab->NumLocals;
F->KeptLocalSyms.push_back(
std::make_pair(DR, 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 relocation.
template <class ELFT>
void Writer<ELFT>::addCopyRelSymbol(SharedSymbol<ELFT> *SS) {
ensureBss();
uintX_t Off = Out<ELFT>::Bss->getSize();
uintX_t Align = getAlignment(SS);
Off = alignTo(Off, Align);
Out<ELFT>::Bss->setSize(Off + SS->template getSize<ELFT>());
Out<ELFT>::Bss->updateAlign(Align);
uintX_t Shndx = SS->Sym.st_shndx;
uintX_t Value = SS->Sym.st_value;
// Look through the DSO's dynamic symbol for aliases and create a dynamic
// symbol for each one. This causes the copy relocation to correctly interpose
// any aliases.
for (SharedSymbol<ELFT> &S : SS->File->getSharedSymbols()) {
if (S.Sym.st_shndx != Shndx || S.Sym.st_value != Value)
continue;
S.OffsetInBss = Off;
S.NeedsCopyOrPltAddr = true;
S.setUsedInRegularObj();
S.MustBeInDynSym = true;
}
Out<ELFT>::RelaDyn->addReloc(
{Target->CopyRel, Out<ELFT>::Bss, SS->OffsetInBss, false, SS, 0});
}
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);
}
template <class ELFT>
static SymbolBody *
addOptionalSynthetic(SymbolTable<ELFT> &Table, StringRef Name,
OutputSectionBase<ELFT> &Sec, typename ELFT::uint Val,
uint8_t Visibility) {
if (!Table.find(Name))
return nullptr;
return Table.addSynthetic(Name, Sec, Val, Visibility);
}
// 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";
ElfSym<ELFT>::RelaIpltStart =
addOptionalSynthetic(Symtab, S, *Out<ELFT>::RelaPlt, 0, STV_HIDDEN);
S = Config->Rela ? "__rela_iplt_end" : "__rel_iplt_end";
ElfSym<ELFT>::RelaIpltEnd =
addOptionalSynthetic(Symtab, S, *Out<ELFT>::RelaPlt,
DefinedSynthetic<ELFT>::SectionEnd, STV_HIDDEN);
}
template <class ELFT> static bool includeInSymtab(const SymbolBody &B) {
if (!B.isUsedInRegularObj())
return false;
if (auto *D = dyn_cast<DefinedRegular<ELFT>>(&B)) {
// Exclude symbols pointing to garbage-collected sections.
if (D->Section && !D->Section->Live)
return false;
}
return true;
}
static bool includeInDynsym(const SymbolBody &B) {
if (B.MustBeInDynSym)
return true;
uint8_t V = B.getVisibility();
if (V != STV_DEFAULT && V != STV_PROTECTED)
return false;
if (Config->ExportDynamic || Config->Shared)
return true;
return false;
}
// 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() {
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 is relative to GOT.
// See "Global Data Symbols" in Chapter 6 in the following document:
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
ElfSym<ELFT>::MipsGp =
Symtab.addSynthetic("_gp", *Out<ELFT>::Got, MipsGPOffset, STV_DEFAULT);
// On MIPS O32 ABI, _gp_disp is a magic symbol designates offset between
// start of function and 'gp' pointer into GOT.
ElfSym<ELFT>::MipsGpDisp = addOptionalSynthetic(
Symtab, "_gp_disp", *Out<ELFT>::Got, MipsGPOffset, STV_HIDDEN);
// 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
ElfSym<ELFT>::MipsLocalGp = addOptionalSynthetic(
Symtab, "__gnu_local_gp", *Out<ELFT>::Got, MipsGPOffset, STV_HIDDEN);
}
// 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.
if (!Config->Relocatable)
Symtab.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.
if (!isOutputDynamic())
Symtab.addIgnored("__tls_get_addr");
auto Define = [this](StringRef S, DefinedRegular<ELFT> *&Sym,
DefinedRegular<ELFT> *&Sym2) {
Sym = Symtab.addIgnored(S, STV_DEFAULT);
// 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())
Sym2 = Symtab.addAbsolute(S, STV_DEFAULT);
};
Define("_end", ElfSym<ELFT>::End, ElfSym<ELFT>::End2);
Define("_etext", ElfSym<ELFT>::Etext, ElfSym<ELFT>::Etext2);
Define("_edata", ElfSym<ELFT>::Edata, ElfSym<ELFT>::Edata2);
}
// Sort input sections by section name suffixes for
// __attribute__((init_priority(N))).