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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 "OutputSections.h"
#include "SymbolTable.h"
#include "Target.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/Support/FileOutputBuffer.h"
#include "llvm/Support/StringSaver.h"
using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace lld;
using namespace lld::elf2;
namespace {
// The writer writes a SymbolTable result to a file.
template <class ELFT> class Writer {
public:
typedef typename ELFFile<ELFT>::uintX_t uintX_t;
typedef typename ELFFile<ELFT>::Elf_Shdr Elf_Shdr;
typedef typename ELFFile<ELFT>::Elf_Ehdr Elf_Ehdr;
typedef typename ELFFile<ELFT>::Elf_Phdr Elf_Phdr;
typedef typename ELFFile<ELFT>::Elf_Sym Elf_Sym;
typedef typename ELFFile<ELFT>::Elf_Sym_Range Elf_Sym_Range;
typedef typename ELFFile<ELFT>::Elf_Rela Elf_Rela;
Writer(SymbolTable<ELFT> &S) : Symtab(S) {}
void run();
private:
void copyLocalSymbols();
void createSections();
template <bool isRela>
void scanRelocs(const InputSection<ELFT> &C,
iterator_range<const Elf_Rel_Impl<ELFT, isRela> *> Rels);
void scanRelocs(const InputSection<ELFT> &C);
void assignAddresses();
void openFile(StringRef OutputPath);
void writeHeader();
void writeSections();
bool needsInterpSection() const {
return !Symtab.getSharedFiles().empty() && !Config->DynamicLinker.empty();
}
bool isOutputDynamic() const {
return !Symtab.getSharedFiles().empty() || Config->Shared;
}
uintX_t getEntryAddr() const;
int getPhdrsNum() const;
OutputSection<ELFT> *getBSS();
void addCommonSymbols(std::vector<DefinedCommon<ELFT> *> &Syms);
void addSharedCopySymbols(std::vector<SharedSymbol<ELFT> *> &Syms);
std::unique_ptr<llvm::FileOutputBuffer> Buffer;
SpecificBumpPtrAllocator<OutputSection<ELFT>> SecAlloc;
SpecificBumpPtrAllocator<MergeOutputSection<ELFT>> MSecAlloc;
BumpPtrAllocator Alloc;
std::vector<OutputSectionBase<ELFT> *> OutputSections;
unsigned getNumSections() const { return OutputSections.size() + 1; }
void addStartStopSymbols(OutputSectionBase<ELFT> *Sec);
void setPhdr(Elf_Phdr *PH, uint32_t Type, uint32_t Flags, uintX_t FileOff,
uintX_t VA, uintX_t Size, uintX_t Align);
void copyPhdr(Elf_Phdr *PH, OutputSectionBase<ELFT> *From);
SymbolTable<ELFT> &Symtab;
std::vector<Elf_Phdr> Phdrs;
uintX_t FileSize;
uintX_t SectionHeaderOff;
};
} // anonymous namespace
template <class ELFT> void lld::elf2::writeResult(SymbolTable<ELFT> *Symtab) {
// Initialize output sections that are handled by Writer specially.
// Don't reorder because the order of initialization matters.
InterpSection<ELFT> Interp;
Out<ELFT>::Interp = &Interp;
StringTableSection<ELFT> ShStrTab(".shstrtab", false);
Out<ELFT>::ShStrTab = &ShStrTab;
StringTableSection<ELFT> StrTab(".strtab", false);
if (!Config->StripAll)
Out<ELFT>::StrTab = &StrTab;
StringTableSection<ELFT> DynStrTab(".dynstr", true);
Out<ELFT>::DynStrTab = &DynStrTab;
GotSection<ELFT> Got;
Out<ELFT>::Got = &Got;
GotPltSection<ELFT> GotPlt;
if (Target->supportsLazyRelocations())
Out<ELFT>::GotPlt = &GotPlt;
PltSection<ELFT> Plt;
Out<ELFT>::Plt = &Plt;
std::unique_ptr<SymbolTableSection<ELFT>> SymTab;
if (!Config->StripAll) {
SymTab.reset(new SymbolTableSection<ELFT>(*Symtab, *Out<ELFT>::StrTab));
Out<ELFT>::SymTab = SymTab.get();
}
SymbolTableSection<ELFT> DynSymTab(*Symtab, *Out<ELFT>::DynStrTab);
Out<ELFT>::DynSymTab = &DynSymTab;
HashTableSection<ELFT> HashTab;
if (Config->SysvHash)
Out<ELFT>::HashTab = &HashTab;
GnuHashTableSection<ELFT> GnuHashTab;
if (Config->GnuHash)
Out<ELFT>::GnuHashTab = &GnuHashTab;
bool IsRela = Symtab->shouldUseRela();
RelocationSection<ELFT> RelaDyn(IsRela ? ".rela.dyn" : ".rel.dyn", IsRela);
Out<ELFT>::RelaDyn = &RelaDyn;
RelocationSection<ELFT> RelaPlt(IsRela ? ".rela.plt" : ".rel.plt", IsRela);
if (Target->supportsLazyRelocations())
Out<ELFT>::RelaPlt = &RelaPlt;
DynamicSection<ELFT> Dynamic(*Symtab);
Out<ELFT>::Dynamic = &Dynamic;
Writer<ELFT>(*Symtab).run();
}
// The main function of the writer.
template <class ELFT> void Writer<ELFT>::run() {
if (!Config->DiscardAll)
copyLocalSymbols();
createSections();
assignAddresses();
openFile(Config->OutputFile);
writeHeader();
writeSections();
error(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 EntSize;
};
}
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.EntSize);
}
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.EntSize == RHS.EntSize;
}
};
}
// 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.
template <class ELFT>
template <bool isRela>
void Writer<ELFT>::scanRelocs(
const InputSection<ELFT> &C,
iterator_range<const Elf_Rel_Impl<ELFT, isRela> *> Rels) {
typedef Elf_Rel_Impl<ELFT, isRela> RelType;
const ObjectFile<ELFT> &File = *C.getFile();
for (const RelType &RI : Rels) {
uint32_t SymIndex = RI.getSymbol(Config->Mips64EL);
SymbolBody *Body = File.getSymbolBody(SymIndex);
uint32_t Type = RI.getType(Config->Mips64EL);
if (Type == Target->getTlsLocalDynamicReloc()) {
if (Out<ELFT>::LocalModuleTlsIndexOffset == uint32_t(-1)) {
Out<ELFT>::LocalModuleTlsIndexOffset =
Out<ELFT>::Got->addLocalModuleTlsIndex();
Out<ELFT>::RelaDyn->addReloc({C, RI});
}
continue;
}
// Set "used" bit for --as-needed.
if (Body && Body->isUndefined() && !Body->isWeak())
if (auto *S = dyn_cast<SharedSymbol<ELFT>>(Body->repl()))
S->File->IsUsed = true;
if (Body)
Body = Body->repl();
bool NeedsGot = false;
bool NeedsPlt = false;
if (Body) {
if (auto *E = dyn_cast<SharedSymbol<ELFT>>(Body)) {
if (E->needsCopy())
continue;
if (Target->relocNeedsCopy(Type, *Body))
E->OffsetInBSS = 0;
}
NeedsPlt = Target->relocNeedsPlt(Type, *Body);
if (NeedsPlt) {
if (Body->isInPlt())
continue;
Out<ELFT>::Plt->addEntry(Body);
}
NeedsGot = Target->relocNeedsGot(Type, *Body);
if (NeedsGot) {
if (NeedsPlt && Target->supportsLazyRelocations()) {
Out<ELFT>::GotPlt->addEntry(Body);
} else {
if (Body->isInGot())
continue;
Out<ELFT>::Got->addEntry(Body);
}
}
}
if (Config->EMachine == EM_MIPS && NeedsGot) {
// 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
Body->setUsedInDynamicReloc();
continue;
}
bool CBP = canBePreempted(Body, NeedsGot);
if (!CBP && (!Config->Shared || Target->isRelRelative(Type)))
continue;
if (CBP)
Body->setUsedInDynamicReloc();
if (NeedsPlt && Target->supportsLazyRelocations())
Out<ELFT>::RelaPlt->addReloc({C, RI});
else
Out<ELFT>::RelaDyn->addReloc({C, RI});
}
}
template <class ELFT>
void Writer<ELFT>::scanRelocs(const InputSection<ELFT> &C) {
ObjectFile<ELFT> *File = C.getFile();
ELFFile<ELFT> &EObj = File->getObj();
if (!(C.getSectionHdr()->sh_flags & SHF_ALLOC))
return;
for (const Elf_Shdr *RelSec : C.RelocSections) {
if (RelSec->sh_type == SHT_RELA)
scanRelocs(C, EObj.relas(RelSec));
else
scanRelocs(C, EObj.rels(RelSec));
}
}
template <class ELFT>
static void reportUndefined(const SymbolTable<ELFT> &S, const SymbolBody &Sym) {
typedef typename ELFFile<ELFT>::Elf_Sym Elf_Sym;
typedef typename ELFFile<ELFT>::Elf_Sym_Range Elf_Sym_Range;
if (Config->Shared && !Config->NoUndefined)
return;
const Elf_Sym &SymE = cast<ELFSymbolBody<ELFT>>(Sym).Sym;
ELFFileBase<ELFT> *SymFile = nullptr;
for (const std::unique_ptr<ObjectFile<ELFT>> &File : S.getObjectFiles()) {
Elf_Sym_Range Syms = File->getObj().symbols(File->getSymbolTable());
if (&SymE > Syms.begin() && &SymE < Syms.end())
SymFile = File.get();
}
std::string Message = "undefined symbol: " + Sym.getName().str();
if (SymFile)
Message += " in " + SymFile->getName().str();
if (Config->NoInhibitExec)
warning(Message);
else
error(Message);
}
// 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() {
for (const std::unique_ptr<ObjectFile<ELFT>> &F : Symtab.getObjectFiles()) {
for (const Elf_Sym &Sym : F->getLocalSymbols()) {
ErrorOr<StringRef> SymNameOrErr = Sym.getName(F->getStringTable());
error(SymNameOrErr);
StringRef SymName = *SymNameOrErr;
if (!shouldKeepInSymtab<ELFT>(*F, SymName, Sym))
continue;
if (Out<ELFT>::SymTab)
Out<ELFT>::SymTab->addLocalSymbol(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);
}
// Output section ordering is determined by this function.
template <class ELFT>
static bool compareOutputSections(OutputSectionBase<ELFT> *A,
OutputSectionBase<ELFT> *B) {
typedef typename ELFFile<ELFT>::uintX_t uintX_t;
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;
// 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;
}
template <class ELFT> OutputSection<ELFT> *Writer<ELFT>::getBSS() {
if (!Out<ELFT>::Bss) {
Out<ELFT>::Bss = new (SecAlloc.Allocate())
OutputSection<ELFT>(".bss", SHT_NOBITS, SHF_ALLOC | SHF_WRITE);
OutputSections.push_back(Out<ELFT>::Bss);
}
return 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<ELFT> *> &Syms) {
typedef typename ELFFile<ELFT>::uintX_t uintX_t;
typedef typename ELFFile<ELFT>::Elf_Sym Elf_Sym;
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<ELFT> *A, const DefinedCommon<ELFT> *B) {
return A->MaxAlignment > B->MaxAlignment;
});
uintX_t Off = getBSS()->getSize();
for (DefinedCommon<ELFT> *C : Syms) {
const Elf_Sym &Sym = C->Sym;
uintX_t Align = C->MaxAlignment;
Off = RoundUpToAlignment(Off, Align);
C->OffsetInBSS = Off;
Off += Sym.st_size;
}
Out<ELFT>::Bss->setSize(Off);
}
template <class ELFT>
void Writer<ELFT>::addSharedCopySymbols(
std::vector<SharedSymbol<ELFT> *> &Syms) {
typedef typename ELFFile<ELFT>::uintX_t uintX_t;
typedef typename ELFFile<ELFT>::Elf_Sym Elf_Sym;
typedef typename ELFFile<ELFT>::Elf_Shdr Elf_Shdr;
if (Syms.empty())
return;
uintX_t Off = getBSS()->getSize();
for (SharedSymbol<ELFT> *C : Syms) {
const Elf_Sym &Sym = C->Sym;
const Elf_Shdr *Sec = C->File->getSection(Sym);
uintX_t SecAlign = Sec->sh_addralign;
uintX_t Align = Sym.st_value % SecAlign;
if (Align == 0)
Align = SecAlign;
Out<ELFT>::Bss->updateAlign(Align);
Off = RoundUpToAlignment(Off, Align);
C->OffsetInBSS = Off;
Off += Sym.st_size;
}
Out<ELFT>::Bss->setSize(Off);
}
static StringRef getOutputName(StringRef S) {
if (S.startswith(".text."))
return ".text";
if (S.startswith(".rodata."))
return ".rodata";
if (S.startswith(".data."))
return ".data";
if (S.startswith(".bss."))
return ".bss";
return S;
}
// Create output section objects and add them to OutputSections.
template <class ELFT> void Writer<ELFT>::createSections() {
// .interp needs to be on the first page in the output file.
if (needsInterpSection())
OutputSections.push_back(Out<ELFT>::Interp);
SmallDenseMap<SectionKey<ELFT::Is64Bits>, OutputSectionBase<ELFT> *> Map;
std::vector<OutputSectionBase<ELFT> *> RegularSections;
for (const std::unique_ptr<ObjectFile<ELFT>> &F : Symtab.getObjectFiles()) {
for (InputSectionBase<ELFT> *C : F->getSections()) {
if (!C || !C->isLive() || C == &InputSection<ELFT>::Discarded)
continue;
const Elf_Shdr *H = C->getSectionHdr();
uintX_t OutFlags = H->sh_flags & ~SHF_GROUP;
// For SHF_MERGE we create different output sections for each sh_entsize.
// This makes each output section simple and keeps a single level
// mapping from input to output.
auto *IS = dyn_cast<InputSection<ELFT>>(C);
uintX_t EntSize = IS ? 0 : H->sh_entsize;
SectionKey<ELFT::Is64Bits> Key{getOutputName(C->getSectionName()),
H->sh_type, OutFlags, EntSize};
OutputSectionBase<ELFT> *&Sec = Map[Key];
if (!Sec) {
if (IS)
Sec = new (SecAlloc.Allocate())
OutputSection<ELFT>(Key.Name, Key.Type, Key.Flags);
else
Sec = new (MSecAlloc.Allocate())
MergeOutputSection<ELFT>(Key.Name, Key.Type, Key.Flags);
OutputSections.push_back(Sec);
RegularSections.push_back(Sec);
}
if (IS)
static_cast<OutputSection<ELFT> *>(Sec)->addSection(IS);
else
static_cast<MergeOutputSection<ELFT> *>(Sec)
->addSection(cast<MergeInputSection<ELFT>>(C));
}
}
Out<ELFT>::Bss = static_cast<OutputSection<ELFT> *>(
Map[{".bss", SHT_NOBITS, SHF_ALLOC | SHF_WRITE, 0}]);
Out<ELFT>::Dynamic->PreInitArraySec = Map.lookup(
{".preinit_array", SHT_PREINIT_ARRAY, SHF_WRITE | SHF_ALLOC, 0});
Out<ELFT>::Dynamic->InitArraySec =
Map.lookup({".init_array", SHT_INIT_ARRAY, SHF_WRITE | SHF_ALLOC, 0});
Out<ELFT>::Dynamic->FiniArraySec =
Map.lookup({".fini_array", SHT_FINI_ARRAY, SHF_WRITE | SHF_ALLOC, 0});
auto AddStartEnd = [&](StringRef Start, StringRef End,
OutputSectionBase<ELFT> *OS) {
if (OS) {
Symtab.addSyntheticSym(Start, *OS, 0);
Symtab.addSyntheticSym(End, *OS, OS->getSize());
} else {
Symtab.addIgnoredSym(Start);
Symtab.addIgnoredSym(End);
}
};
AddStartEnd("__preinit_array_start", "__preinit_array_end",
Out<ELFT>::Dynamic->PreInitArraySec);
AddStartEnd("__init_array_start", "__init_array_end",
Out<ELFT>::Dynamic->InitArraySec);
AddStartEnd("__fini_array_start", "__fini_array_end",
Out<ELFT>::Dynamic->FiniArraySec);
for (OutputSectionBase<ELFT> *Sec : RegularSections)
addStartStopSymbols(Sec);
// __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.addIgnoredSym("__tls_get_addr");
// 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<ObjectFile<ELFT>> &F : Symtab.getObjectFiles())
for (InputSectionBase<ELFT> *B : F->getSections())
if (auto *S = dyn_cast_or_null<InputSection<ELFT>>(B))
if (S != &InputSection<ELFT>::Discarded && S->isLive())
scanRelocs(*S);
std::vector<DefinedCommon<ELFT> *> CommonSymbols;
std::vector<SharedSymbol<ELFT> *> SharedCopySymbols;
for (auto &P : Symtab.getSymbols()) {
SymbolBody *Body = P.second->Body;
if (auto *U = dyn_cast<Undefined<ELFT>>(Body))
if (!U->isWeak() && !U->canKeepUndefined())
reportUndefined<ELFT>(Symtab, *Body);
if (auto *C = dyn_cast<DefinedCommon<ELFT>>(Body))
CommonSymbols.push_back(C);
if (auto *SC = dyn_cast<SharedSymbol<ELFT>>(Body))
if (SC->needsCopy())
SharedCopySymbols.push_back(SC);
if (!includeInSymtab<ELFT>(*Body))
continue;
if (Out<ELFT>::SymTab)
Out<ELFT>::SymTab->addSymbol(Body);
if (isOutputDynamic() && includeInDynamicSymtab(*Body))
Out<ELFT>::DynSymTab->addSymbol(Body);
}
addCommonSymbols(CommonSymbols);
addSharedCopySymbols(SharedCopySymbols);
// This order is not the same as the final output order
// because we sort the sections using their attributes below.
if (Out<ELFT>::SymTab)
OutputSections.push_back(Out<ELFT>::SymTab);
OutputSections.push_back(Out<ELFT>::ShStrTab);
if (Out<ELFT>::StrTab)
OutputSections.push_back(Out<ELFT>::StrTab);
if (isOutputDynamic()) {
OutputSections.push_back(Out<ELFT>::DynSymTab);
if (Out<ELFT>::GnuHashTab)
OutputSections.push_back(Out<ELFT>::GnuHashTab);
if (Out<ELFT>::HashTab)
OutputSections.push_back(Out<ELFT>::HashTab);
OutputSections.push_back(Out<ELFT>::Dynamic);
OutputSections.push_back(Out<ELFT>::DynStrTab);
if (Out<ELFT>::RelaDyn->hasRelocs())
OutputSections.push_back(Out<ELFT>::RelaDyn);
if (Out<ELFT>::RelaPlt && Out<ELFT>::RelaPlt->hasRelocs())
OutputSections.push_back(Out<ELFT>::RelaPlt);
}
if (!Out<ELFT>::Got->empty())
OutputSections.push_back(Out<ELFT>::Got);
if (Out<ELFT>::GotPlt && !Out<ELFT>::GotPlt->empty())
OutputSections.push_back(Out<ELFT>::GotPlt);
if (!Out<ELFT>::Plt->empty())
OutputSections.push_back(Out<ELFT>::Plt);
std::stable_sort(OutputSections.begin(), OutputSections.end(),
compareOutputSections<ELFT>);
for (unsigned I = 0, N = OutputSections.size(); I < N; ++I)
OutputSections[I]->SectionIndex = I + 1;
for (OutputSectionBase<ELFT> *Sec : OutputSections)
Out<ELFT>::ShStrTab->add(Sec->getName());
// Finalizers fix each section's size.
// .dynamic section's finalizer may add strings to .dynstr,
// so finalize that early.
// Likewise, .dynsym is finalized early since that may fill up .gnu.hash.
Out<ELFT>::Dynamic->finalize();
if (isOutputDynamic())
Out<ELFT>::DynSymTab->finalize();
// Fill other section headers.
for (OutputSectionBase<ELFT> *Sec : OutputSections)
Sec->finalize();
// 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 = Map.lookup({".opd", SHT_PROGBITS, SHF_WRITE | SHF_ALLOC, 0});
}
static bool isAlpha(char C) {
return ('a' <= C && C <= 'z') || ('A' <= C && C <= 'Z') || C == '_';
}
static bool isAlnum(char C) { return isAlpha(C) || ('0' <= C && C <= '9'); }
// Returns true if S is valid as a C language identifier.
static bool isValidCIdentifier(StringRef S) {
if (S.empty() || !isAlpha(S[0]))
return false;
return std::all_of(S.begin() + 1, S.end(), isAlnum);
}
// 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 (Symtab.isUndefined(Start))
Symtab.addSyntheticSym(Start, *Sec, 0);
if (Symtab.isUndefined(Stop))
Symtab.addSyntheticSym(Stop, *Sec, Sec->getSize());
}
template <class ELFT> static bool needsPhdr(OutputSectionBase<ELFT> *Sec) {
return Sec->getFlags() & SHF_ALLOC;
}
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;
}
// Visits all sections to create PHDRs and to assign incremental,
// non-overlapping addresses to output sections.
template <class ELFT> void Writer<ELFT>::assignAddresses() {
uintX_t VA = Target->getVAStart() + sizeof(Elf_Ehdr);
uintX_t FileOff = sizeof(Elf_Ehdr);
// Calculate and reserve the space for the program header first so that
// the first section can start right after the program header.
Phdrs.resize(getPhdrsNum());
size_t PhdrSize = sizeof(Elf_Phdr) * Phdrs.size();
// The first phdr entry is PT_PHDR which describes the program header itself.
setPhdr(&Phdrs[0], PT_PHDR, PF_R, FileOff, VA, PhdrSize, /*Align=*/8);
FileOff += PhdrSize;
VA += PhdrSize;
// PT_INTERP must be the second entry if exists.
int PhdrIdx = 0;
Elf_Phdr *Interp = nullptr;
if (needsInterpSection())
Interp = &Phdrs[++PhdrIdx];
// Add the first PT_LOAD segment for regular output sections.
setPhdr(&Phdrs[++PhdrIdx], PT_LOAD, PF_R, 0, Target->getVAStart(), FileOff,
Target->getPageSize());
Elf_Phdr TlsPhdr{};
uintX_t ThreadBSSOffset = 0;
// Create phdrs as we assign VAs and file offsets to all output sections.
for (OutputSectionBase<ELFT> *Sec : OutputSections) {
if (needsPhdr<ELFT>(Sec)) {
uintX_t Flags = toPhdrFlags(Sec->getFlags());
if (Phdrs[PhdrIdx].p_flags != Flags) {
// Flags changed. Create a new PT_LOAD.
VA = RoundUpToAlignment(VA, Target->getPageSize());
FileOff = RoundUpToAlignment(FileOff, Target->getPageSize());
Elf_Phdr *PH = &Phdrs[++PhdrIdx];
setPhdr(PH, PT_LOAD, Flags, FileOff, VA, 0, Target->getPageSize());
}
if (Sec->getFlags() & SHF_TLS) {
if (!TlsPhdr.p_vaddr)
setPhdr(&TlsPhdr, PT_TLS, PF_R, FileOff, VA, 0, Sec->getAlign());
if (Sec->getType() != SHT_NOBITS)
VA = RoundUpToAlignment(VA, Sec->getAlign());
uintX_t TVA = RoundUpToAlignment(VA + ThreadBSSOffset, Sec->getAlign());
Sec->setVA(TVA);
TlsPhdr.p_memsz += Sec->getSize();
if (Sec->getType() == SHT_NOBITS) {
ThreadBSSOffset = TVA - VA + Sec->getSize();
} else {
TlsPhdr.p_filesz += Sec->getSize();
VA += Sec->getSize();
}
TlsPhdr.p_align = std::max<uintX_t>(TlsPhdr.p_align, Sec->getAlign());
} else {
VA = RoundUpToAlignment(VA, Sec->getAlign());
Sec->setVA(VA);
VA += Sec->getSize();
}
}
FileOff = RoundUpToAlignment(FileOff, Sec->getAlign());
Sec->setFileOffset(FileOff);
if (Sec->getType() != SHT_NOBITS)
FileOff += Sec->getSize();
if (needsPhdr<ELFT>(Sec)) {
Elf_Phdr *Cur = &Phdrs[PhdrIdx];
Cur->p_filesz = FileOff - Cur->p_offset;
Cur->p_memsz = VA - Cur->p_vaddr;
}
}
if (TlsPhdr.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.
TlsPhdr.p_memsz = RoundUpToAlignment(TlsPhdr.p_memsz, TlsPhdr.p_align);
Phdrs[++PhdrIdx] = TlsPhdr;
Out<ELFT>::TlsPhdr = &Phdrs[PhdrIdx];
}
// Add an entry for .dynamic.
if (isOutputDynamic()) {
Elf_Phdr *PH = &Phdrs[++PhdrIdx];
PH->p_type = PT_DYNAMIC;
copyPhdr(PH, Out<ELFT>::Dynamic);
}
// Fix up PT_INTERP as we now know the address of .interp section.
if (Interp) {
Interp->p_type = PT_INTERP;
copyPhdr(Interp, Out<ELFT>::Interp);
}
// Add space for section headers.
SectionHeaderOff = RoundUpToAlignment(FileOff, ELFT::Is64Bits ? 8 : 4);
FileSize = SectionHeaderOff + getNumSections() * sizeof(Elf_Shdr);
// Update MIPS _gp absolute symbol so that it points to the static data.
if (Config->EMachine == EM_MIPS)
DefinedAbsolute<ELFT>::MipsGp.st_value = getMipsGpAddr<ELFT>();
}
// Returns the number of PHDR entries.
template <class ELFT> int Writer<ELFT>::getPhdrsNum() const {
bool Tls = false;
int I = 2; // 2 for PT_PHDR and the first PT_LOAD
if (needsInterpSection())
++I;
if (isOutputDynamic())
++I;
uintX_t Last = PF_R;
for (OutputSectionBase<ELFT> *Sec : OutputSections) {
if (!needsPhdr<ELFT>(Sec))
continue;
if (Sec->getFlags() & SHF_TLS)
Tls = true;
uintX_t Flags = toPhdrFlags(Sec->getFlags());
if (Last != Flags) {
Last = Flags;
++I;
}
}
if (Tls)
++I;
return I;
}
template <class ELFT> void Writer<ELFT>::writeHeader() {
uint8_t *Buf = Buffer->getBufferStart();
memcpy(Buf, "\177ELF", 4);
// 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] = ELFT::TargetEndianness == llvm::support::little
? ELFDATA2LSB
: ELFDATA2MSB;
EHdr->e_ident[EI_VERSION] = EV_CURRENT;
auto &FirstObj = cast<ELFFileBase<ELFT>>(*Config->FirstElf);
EHdr->e_ident[EI_OSABI] = FirstObj.getOSABI();
EHdr->e_type = Config->Shared ? ET_DYN : ET_EXEC;
EHdr->e_machine = FirstObj.getEMachine();
EHdr->e_version = EV_CURRENT;
EHdr->e_entry = getEntryAddr();
EHdr->e_phoff = sizeof(Elf_Ehdr);
EHdr->e_shoff = SectionHeaderOff;
EHdr->e_ehsize = sizeof(Elf_Ehdr);
EHdr->e_phentsize = sizeof(Elf_Phdr);
EHdr->e_phnum = Phdrs.size();
EHdr->e_shentsize = sizeof(Elf_Shdr);
EHdr->e_shnum = getNumSections();
EHdr->e_shstrndx = Out<ELFT>::ShStrTab->SectionIndex;
// Write the program header table.
memcpy(Buf + EHdr->e_phoff, &Phdrs[0], Phdrs.size() * sizeof(Phdrs[0]));
// 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 : OutputSections)
Sec->writeHeaderTo(++SHdrs);
}
template <class ELFT> void Writer<ELFT>::openFile(StringRef Path) {
ErrorOr<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
FileOutputBuffer::create(Path, FileSize, FileOutputBuffer::F_executable);
error(BufferOrErr, Twine("failed to open ") + Path);
Buffer = std::move(*BufferOrErr);
}
// 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>
typename ELFFile<ELFT>::uintX_t Writer<ELFT>::getEntryAddr() const {
if (Config->EntrySym) {
if (auto *E = dyn_cast<ELFSymbolBody<ELFT>>(Config->EntrySym->repl()))
return getSymVA<ELFT>(*E);
return 0;
}
if (Config->EntryAddr != uint64_t(-1))
return Config->EntryAddr;
return 0;
}
template <class ELFT>
void Writer<ELFT>::setPhdr(Elf_Phdr *PH, uint32_t Type, uint32_t Flags,
uintX_t FileOff, uintX_t VA, uintX_t Size,
uintX_t Align) {
PH->p_type = Type;
PH->p_flags = Flags;
PH->p_offset = FileOff;
PH->p_vaddr = VA;
PH->p_paddr = VA;
PH->p_filesz = Size;
PH->p_memsz = Size;
PH->p_align = Align;
}
template <class ELFT>
void Writer<ELFT>::copyPhdr(Elf_Phdr *PH, OutputSectionBase<ELFT> *From) {
PH->p_flags = toPhdrFlags(From->getFlags());
PH->p_offset = From->getFileOff();
PH->p_vaddr = From->getVA();
PH->p_paddr = From->getVA();
PH->p_filesz = From->getSize();
PH->p_memsz = From->getSize();
PH->p_align = From->getAlign();
}
template void lld::elf2::writeResult<ELF32LE>(SymbolTable<ELF32LE> *Symtab);
template void lld::elf2::writeResult<ELF32BE>(SymbolTable<ELF32BE> *Symtab);
template void lld::elf2::writeResult<ELF64LE>(SymbolTable<ELF64LE> *Symtab);
template void lld::elf2::writeResult<ELF64BE>(SymbolTable<ELF64BE> *Symtab);
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