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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 "Memory.h"
#include "OutputSections.h"
#include "Relocations.h"
#include "Strings.h"
#include "SymbolTable.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/Support/FileOutputBuffer.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/raw_ostream.h"
#include <climits>
#include <thread>
using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace llvm::support;
using namespace llvm::support::endian;
using namespace lld;
using namespace lld::elf;
namespace {
// The writer writes a SymbolTable result to a file.
template <class ELFT> class Writer {
public:
typedef typename ELFT::uint uintX_t;
typedef typename ELFT::Shdr Elf_Shdr;
typedef typename ELFT::Ehdr Elf_Ehdr;
typedef typename ELFT::Phdr Elf_Phdr;
typedef typename ELFT::Sym Elf_Sym;
typedef typename ELFT::SymRange Elf_Sym_Range;
typedef typename ELFT::Rela Elf_Rela;
void run();
private:
void createSyntheticSections();
void copyLocalSymbols();
void addReservedSymbols();
void addInputSec(InputSectionBase<ELFT> *S);
void createSections();
void forEachRelSec(std::function<void(InputSectionBase<ELFT> &)> Fn);
void sortSections();
void finalizeSections();
void addPredefinedSections();
std::vector<PhdrEntry> createPhdrs();
void removeEmptyPTLoad();
void addPtArmExid(std::vector<PhdrEntry> &Phdrs);
void assignAddresses();
void assignFileOffsets();
void assignFileOffsetsBinary();
void setPhdrs();
void fixHeaders();
void fixSectionAlignments();
void fixAbsoluteSymbols();
void openFile();
void writeHeader();
void writeSections();
void writeSectionsBinary();
void writeBuildId();
std::unique_ptr<FileOutputBuffer> Buffer;
std::vector<OutputSectionBase *> OutputSections;
OutputSectionFactory<ELFT> Factory;
void addRelIpltSymbols();
void addStartEndSymbols();
void addStartStopSymbols(OutputSectionBase *Sec);
uintX_t getEntryAddr();
OutputSectionBase *findSection(StringRef Name);
std::vector<PhdrEntry> Phdrs;
uintX_t FileSize;
uintX_t SectionHeaderOff;
bool AllocateHeader = true;
};
} // anonymous namespace
StringRef elf::getOutputSectionName(StringRef Name) {
if (Config->Relocatable)
return Name;
for (StringRef V :
{".text.", ".rodata.", ".data.rel.ro.", ".data.", ".bss.",
".init_array.", ".fini_array.", ".ctors.", ".dtors.", ".tbss.",
".gcc_except_table.", ".tdata.", ".ARM.exidx."}) {
StringRef Prefix = V.drop_back();
if (Name.startswith(V) || Name == Prefix)
return Prefix;
}
// CommonSection is identified as "COMMON" in linker scripts.
// By default, it should go to .bss section.
if (Name == "COMMON")
return ".bss";
// ".zdebug_" is a prefix for ZLIB-compressed sections.
// Because we decompressed input sections, we want to remove 'z'.
if (Name.startswith(".zdebug_"))
return Saver.save(Twine(".") + Name.substr(2));
return Name;
}
template <class ELFT> void elf::reportDiscarded(InputSectionBase<ELFT> *IS) {
if (!Config->PrintGcSections)
return;
errs() << "removing unused section from '" << IS->Name << "' in file '"
<< IS->getFile()->getName() << "'\n";
}
template <class ELFT> static bool needsInterpSection() {
return !Symtab<ELFT>::X->getSharedFiles().empty() &&
!Config->DynamicLinker.empty() &&
!Script<ELFT>::X->ignoreInterpSection();
}
template <class ELFT> void elf::writeResult() { Writer<ELFT>().run(); }
template <class ELFT> void Writer<ELFT>::removeEmptyPTLoad() {
auto I = std::remove_if(Phdrs.begin(), Phdrs.end(), [&](const PhdrEntry &P) {
if (P.p_type != PT_LOAD)
return false;
if (!P.First)
return true;
uintX_t Size = P.Last->Addr + P.Last->Size - P.First->Addr;
return Size == 0;
});
Phdrs.erase(I, Phdrs.end());
}
// The main function of the writer.
template <class ELFT> void Writer<ELFT>::run() {
// Create linker-synthesized sections such as .got or .plt.
// Such sections are of type input section.
createSyntheticSections();
// We need to create some reserved symbols such as _end. Create them.
if (!Config->Relocatable)
addReservedSymbols();
// Some architectures use small displacements for jump instructions.
// It is linker's responsibility to create thunks containing long
// jump instructions if jump targets are too far. Create thunks.
if (Target->NeedsThunks)
forEachRelSec(createThunks<ELFT>);
// Create output sections.
Script<ELFT>::X->OutputSections = &OutputSections;
if (ScriptConfig->HasSections) {
// If linker script contains SECTIONS commands, let it create sections.
Script<ELFT>::X->processCommands(Factory);
// Linker scripts may have left some input sections unassigned.
// Assign such sections using the default rule.
Script<ELFT>::X->addOrphanSections(Factory);
} else {
// If linker script does not contain SECTIONS commands, create
// output sections by default rules. We still need to give the
// linker script a chance to run, because it might contain
// non-SECTIONS commands such as ASSERT.
createSections();
Script<ELFT>::X->processCommands(Factory);
}
if (Config->Discard != DiscardPolicy::All)
copyLocalSymbols();
// Now that we have a complete set of output sections. This function
// completes section contents. For example, we need to add strings
// to the string table, and add entries to .got and .plt.
// finalizeSections does that.
finalizeSections();
if (ErrorCount)
return;
if (Config->Relocatable) {
assignFileOffsets();
} else {
if (ScriptConfig->HasSections) {
Script<ELFT>::X->assignAddresses(Phdrs);
} else {
fixSectionAlignments();
assignAddresses();
}
// Remove empty PT_LOAD to avoid causing the dynamic linker to try to mmap a
// 0 sized region. This has to be done late since only after assignAddresses
// we know the size of the sections.
removeEmptyPTLoad();
if (!Config->OFormatBinary)
assignFileOffsets();
else
assignFileOffsetsBinary();
setPhdrs();
fixAbsoluteSymbols();
}
// Write the result down to a file.
openFile();
if (ErrorCount)
return;
if (!Config->OFormatBinary) {
writeHeader();
writeSections();
} else {
writeSectionsBinary();
}
// Backfill .note.gnu.build-id section content. This is done at last
// because the content is usually a hash value of the entire output file.
writeBuildId();
if (ErrorCount)
return;
if (auto EC = Buffer->commit())
error(EC, "failed to write to the output file");
// Flush the output streams and exit immediately. A full shutdown
// is a good test that we are keeping track of all allocated memory,
// but actually freeing it is a waste of time in a regular linker run.
if (Config->ExitEarly)
exitLld(0);
}
// Initialize Out<ELFT> members.
template <class ELFT> void Writer<ELFT>::createSyntheticSections() {
// Initialize all pointers with NULL. This is needed because
// you can call lld::elf::main more than once as a library.
memset(&Out<ELFT>::First, 0, sizeof(Out<ELFT>));
// Create singleton output sections.
Out<ELFT>::Bss =
make<OutputSection<ELFT>>(".bss", SHT_NOBITS, SHF_ALLOC | SHF_WRITE);
In<ELFT>::DynStrTab = make<StringTableSection<ELFT>>(".dynstr", true);
In<ELFT>::Dynamic = make<DynamicSection<ELFT>>();
Out<ELFT>::EhFrame = make<EhOutputSection<ELFT>>();
In<ELFT>::RelaDyn = make<RelocationSection<ELFT>>(
Config->Rela ? ".rela.dyn" : ".rel.dyn", Config->ZCombreloc);
In<ELFT>::ShStrTab = make<StringTableSection<ELFT>>(".shstrtab", false);
Out<ELFT>::ElfHeader = make<OutputSectionBase>("", 0, SHF_ALLOC);
Out<ELFT>::ElfHeader->Size = sizeof(Elf_Ehdr);
Out<ELFT>::ProgramHeaders = make<OutputSectionBase>("", 0, SHF_ALLOC);
Out<ELFT>::ProgramHeaders->updateAlignment(sizeof(uintX_t));
if (needsInterpSection<ELFT>()) {
In<ELFT>::Interp = createInterpSection<ELFT>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::Interp);
} else {
In<ELFT>::Interp = nullptr;
}
if (!Config->Relocatable)
Symtab<ELFT>::X->Sections.push_back(createCommentSection<ELFT>());
if (Config->Strip != StripPolicy::All) {
In<ELFT>::StrTab = make<StringTableSection<ELFT>>(".strtab", false);
In<ELFT>::SymTab = make<SymbolTableSection<ELFT>>(*In<ELFT>::StrTab);
}
if (Config->BuildId != BuildIdKind::None) {
In<ELFT>::BuildId = make<BuildIdSection<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::BuildId);
}
InputSection<ELFT> *Common = createCommonSection<ELFT>();
if (!Common->Data.empty()) {
In<ELFT>::Common = Common;
Symtab<ELFT>::X->Sections.push_back(Common);
}
// Add MIPS-specific sections.
bool HasDynSymTab = !Symtab<ELFT>::X->getSharedFiles().empty() || Config->Pic;
if (Config->EMachine == EM_MIPS) {
if (!Config->Shared && HasDynSymTab) {
In<ELFT>::MipsRldMap = make<MipsRldMapSection<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::MipsRldMap);
}
if (auto *Sec = MipsAbiFlagsSection<ELFT>::create())
Symtab<ELFT>::X->Sections.push_back(Sec);
if (auto *Sec = MipsOptionsSection<ELFT>::create())
Symtab<ELFT>::X->Sections.push_back(Sec);
if (auto *Sec = MipsReginfoSection<ELFT>::create())
Symtab<ELFT>::X->Sections.push_back(Sec);
}
if (HasDynSymTab) {
In<ELFT>::DynSymTab = make<SymbolTableSection<ELFT>>(*In<ELFT>::DynStrTab);
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::DynSymTab);
In<ELFT>::VerSym = make<VersionTableSection<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::VerSym);
if (!Config->VersionDefinitions.empty()) {
In<ELFT>::VerDef = make<VersionDefinitionSection<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::VerDef);
}
In<ELFT>::VerNeed = make<VersionNeedSection<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::VerNeed);
if (Config->GnuHash) {
In<ELFT>::GnuHashTab = make<GnuHashTableSection<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::GnuHashTab);
}
if (Config->SysvHash) {
In<ELFT>::HashTab = make<HashTableSection<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::HashTab);
}
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::Dynamic);
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::DynStrTab);
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::RelaDyn);
}
// Add .got. MIPS' .got is so different from the other archs,
// it has its own class.
if (Config->EMachine == EM_MIPS) {
In<ELFT>::MipsGot = make<MipsGotSection<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::MipsGot);
} else {
In<ELFT>::Got = make<GotSection<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::Got);
}
In<ELFT>::GotPlt = make<GotPltSection<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::GotPlt);
In<ELFT>::IgotPlt = make<IgotPltSection<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::IgotPlt);
if (Config->GdbIndex) {
In<ELFT>::GdbIndex = make<GdbIndexSection<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::GdbIndex);
}
// We always need to add rel[a].plt to output if it has entries.
// Even for static linking it can contain R_[*]_IRELATIVE relocations.
In<ELFT>::RelaPlt = make<RelocationSection<ELFT>>(
Config->Rela ? ".rela.plt" : ".rel.plt", false /*Sort*/);
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::RelaPlt);
// The RelaIplt immediately follows .rel.plt (.rel.dyn for ARM) to ensure
// that the IRelative relocations are processed last by the dynamic loader
In<ELFT>::RelaIplt = make<RelocationSection<ELFT>>(
(Config->EMachine == EM_ARM) ? ".rel.dyn" : In<ELFT>::RelaPlt->Name,
false /*Sort*/);
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::RelaIplt);
In<ELFT>::Plt = make<PltSection<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::Plt);
In<ELFT>::Iplt = make<IpltSection<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::Iplt);
if (Config->EhFrameHdr) {
In<ELFT>::EhFrameHdr = make<EhFrameHeader<ELFT>>();
Symtab<ELFT>::X->Sections.push_back(In<ELFT>::EhFrameHdr);
}
}
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->Discard == DiscardPolicy::None)
return true;
// In ELF assembly .L symbols are normally discarded by the assembler.
// If the assembler fails to do so, the linker discards them if
// * --discard-locals is used.
// * The symbol is in a SHF_MERGE section, which is normally the reason for
// the assembler keeping the .L symbol.
if (!SymName.startswith(".L") && !SymName.empty())
return true;
if (Config->Discard == DiscardPolicy::Locals)
return false;
return !Sec || !(Sec->Flags & SHF_MERGE);
}
template <class ELFT> static bool includeInSymtab(const SymbolBody &B) {
if (!B.isLocal() && !B.symbol()->IsUsedInRegularObj)
return false;
// If --retain-symbols-file is given, we'll keep only symbols listed in that
// file.
if (Config->Discard == DiscardPolicy::RetainFile &&
!Config->RetainSymbolsFile.count(B.getName()))
return false;
if (auto *D = dyn_cast<DefinedRegular<ELFT>>(&B)) {
// Always include absolute symbols.
if (!D->Section)
return true;
// Exclude symbols pointing to garbage-collected sections.
if (!D->Section->Live)
return false;
if (auto *S = dyn_cast<MergeInputSection<ELFT>>(D->Section))
if (!S->getSectionPiece(D->Value)->Live)
return false;
}
return true;
}
// Local symbols are not in the linker's symbol table. This function scans
// each object file's symbol table to copy local symbols to the output.
template <class ELFT> void Writer<ELFT>::copyLocalSymbols() {
if (!In<ELFT>::SymTab)
return;
for (elf::ObjectFile<ELFT> *F : Symtab<ELFT>::X->getObjectFiles()) {
for (SymbolBody *B : F->getLocalSymbols()) {
if (!B->IsLocal)
fatal(toString(F) +
": broken object: getLocalSymbols returns a non-local symbol");
auto *DR = dyn_cast<DefinedRegular<ELFT>>(B);
// No reason to keep local undefined symbol in symtab.
if (!DR)
continue;
if (!includeInSymtab<ELFT>(*B))
continue;
InputSectionBase<ELFT> *Sec = DR->Section;
if (!shouldKeepInSymtab<ELFT>(Sec, B->getName(), *B))
continue;
++In<ELFT>::SymTab->NumLocals;
if (Config->Relocatable)
B->DynsymIndex = In<ELFT>::SymTab->NumLocals;
F->KeptLocalSyms.push_back(std::make_pair(
DR, In<ELFT>::SymTab->StrTabSec.addString(B->getName())));
}
}
}
// 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> bool elf::isRelroSection(const OutputSectionBase *Sec) {
if (!Config->ZRelro)
return false;
uint64_t Flags = Sec->Flags;
if (!(Flags & SHF_ALLOC) || !(Flags & SHF_WRITE))
return false;
if (Flags & SHF_TLS)
return true;
uint32_t Type = Sec->Type;
if (Type == SHT_INIT_ARRAY || Type == SHT_FINI_ARRAY ||
Type == SHT_PREINIT_ARRAY)
return true;
if (Sec == In<ELFT>::GotPlt->OutSec)
return Config->ZNow;
if (Sec == In<ELFT>::Dynamic->OutSec)
return true;
if (In<ELFT>::Got && Sec == In<ELFT>::Got->OutSec)
return true;
if (In<ELFT>::MipsGot && Sec == In<ELFT>::MipsGot->OutSec)
return true;
StringRef S = Sec->getName();
return S == ".data.rel.ro" || S == ".ctors" || S == ".dtors" || S == ".jcr" ||
S == ".eh_frame" || S == ".openbsd.randomdata";
}
template <class ELFT>
static bool compareSectionsNonScript(const OutputSectionBase *A,
const OutputSectionBase *B) {
// Put .interp first because some loaders want to see that section
// on the first page of the executable file when loaded into memory.
bool AIsInterp = A->getName() == ".interp";
bool BIsInterp = B->getName() == ".interp";
if (AIsInterp != BIsInterp)
return AIsInterp;
// Allocatable sections go first to reduce the total PT_LOAD size and
// so debug info doesn't change addresses in actual code.
bool AIsAlloc = A->Flags & SHF_ALLOC;
bool BIsAlloc = B->Flags & SHF_ALLOC;
if (AIsAlloc != BIsAlloc)
return AIsAlloc;
// We don't have any special requirements for the relative order of two non
// allocatable sections.
if (!AIsAlloc)
return false;
// We want to put section specified by -T option first, so we
// can start assigning VA starting from them later.
auto AAddrSetI = Config->SectionStartMap.find(A->getName());
auto BAddrSetI = Config->SectionStartMap.find(B->getName());
bool AHasAddrSet = AAddrSetI != Config->SectionStartMap.end();
bool BHasAddrSet = BAddrSetI != Config->SectionStartMap.end();
if (AHasAddrSet != BHasAddrSet)
return AHasAddrSet;
if (AHasAddrSet)
return AAddrSetI->second < BAddrSetI->second;
// We want the read only sections first so that they go in the PT_LOAD
// covering the program headers at the start of the file.
bool AIsWritable = A->Flags & SHF_WRITE;
bool BIsWritable = B->Flags & SHF_WRITE;
if (AIsWritable != BIsWritable)
return BIsWritable;
if (!Config->SingleRoRx) {
// For a corresponding reason, put non exec sections first (the program
// header PT_LOAD is not executable).
// We only do that if we are not using linker scripts, since with linker
// scripts ro and rx sections are in the same PT_LOAD, so their relative
// order is not important. The same applies for -no-rosegment.
bool AIsExec = A->Flags & SHF_EXECINSTR;
bool BIsExec = B->Flags & SHF_EXECINSTR;
if (AIsExec != BIsExec)
return BIsExec;
}
// If we got here we know that both A and B are in the same PT_LOAD.
// 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 = A->Flags & SHF_TLS;
bool BIsTls = B->Flags & 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->Type == SHT_NOBITS;
bool BIsNoBits = B->Type == SHT_NOBITS;
if (AIsNoBits != BIsNoBits)
return BIsNoBits;
// We place RelRo section before plain r/w ones.
bool AIsRelRo = isRelroSection<ELFT>(A);
bool BIsRelRo = isRelroSection<ELFT>(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;
}
// Output section ordering is determined by this function.
template <class ELFT>
static bool compareSections(const OutputSectionBase *A,
const OutputSectionBase *B) {
// For now, put sections mentioned in a linker script first.
int AIndex = Script<ELFT>::X->getSectionIndex(A->getName());
int BIndex = Script<ELFT>::X->getSectionIndex(B->getName());
bool AInScript = AIndex != INT_MAX;
bool BInScript = BIndex != INT_MAX;
if (AInScript != BInScript)
return AInScript;
// If both are in the script, use that order.
if (AInScript)
return AIndex < BIndex;
return compareSectionsNonScript<ELFT>(A, B);
}
// Program header entry
PhdrEntry::PhdrEntry(unsigned Type, unsigned Flags) {
p_type = Type;
p_flags = Flags;
}
void PhdrEntry::add(OutputSectionBase *Sec) {
Last = Sec;
if (!First)
First = Sec;
p_align = std::max(p_align, Sec->Addralign);
if (p_type == PT_LOAD)
Sec->FirstInPtLoad = First;
}
template <class ELFT>
static Symbol *addOptionalSynthetic(StringRef Name, OutputSectionBase *Sec,
typename ELFT::uint Val,
uint8_t StOther = STV_HIDDEN) {
SymbolBody *S = Symtab<ELFT>::X->find(Name);
if (!S)
return nullptr;
if (!S->isUndefined() && !S->isShared())
return S->symbol();
return Symtab<ELFT>::X->addSynthetic(Name, Sec, Val, StOther);
}
template <class ELFT>
static Symbol *addRegular(StringRef Name, InputSectionBase<ELFT> *Sec,
typename ELFT::uint Value) {
// The linker generated symbols are added as STB_WEAK to allow user defined
// ones to override them.
return Symtab<ELFT>::X->addRegular(Name, STV_HIDDEN, STT_NOTYPE, Value,
/*Size=*/0, STB_WEAK, Sec,
/*File=*/nullptr);
}
template <class ELFT>
static Symbol *addOptionalRegular(StringRef Name, InputSectionBase<ELFT> *IS,
typename ELFT::uint Value) {
SymbolBody *S = Symtab<ELFT>::X->find(Name);
if (!S)
return nullptr;
if (!S->isUndefined() && !S->isShared())
return S->symbol();
return addRegular(Name, IS, Value);
}
// The beginning and the ending of .rel[a].plt section are marked
// with __rel[a]_iplt_{start,end} symbols if it is a statically linked
// executable. The runtime needs these symbols in order to resolve
// all IRELATIVE relocs on startup. For dynamic executables, we don't
// need these symbols, since IRELATIVE relocs are resolved through GOT
// and PLT. For details, see http://www.airs.com/blog/archives/403.
template <class ELFT> void Writer<ELFT>::addRelIpltSymbols() {
if (In<ELFT>::DynSymTab)
return;
StringRef S = Config->Rela ? "__rela_iplt_start" : "__rel_iplt_start";
addOptionalRegular<ELFT>(S, In<ELFT>::RelaIplt, 0);
S = Config->Rela ? "__rela_iplt_end" : "__rel_iplt_end";
addOptionalRegular<ELFT>(S, In<ELFT>::RelaIplt, -1);
}
// The linker is expected to define some symbols depending on
// the linking result. This function defines such symbols.
template <class ELFT> void Writer<ELFT>::addReservedSymbols() {
if (Config->EMachine == EM_MIPS) {
// Define _gp for MIPS. st_value of _gp symbol will be updated by Writer
// so that it points to an absolute address which by default is relative
// to GOT. Default offset is 0x7ff0.
// See "Global Data Symbols" in Chapter 6 in the following document:
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
ElfSym<ELFT>::MipsGp =
Symtab<ELFT>::X->addAbsolute("_gp", STV_HIDDEN, STB_LOCAL);
// On MIPS O32 ABI, _gp_disp is a magic symbol designates offset between
// start of function and 'gp' pointer into GOT. To simplify relocation
// calculation we assign _gp value to it and calculate corresponding
// relocations as relative to this value.
if (Symtab<ELFT>::X->find("_gp_disp"))
ElfSym<ELFT>::MipsGpDisp =
Symtab<ELFT>::X->addAbsolute("_gp_disp", STV_HIDDEN, STB_LOCAL);
// The __gnu_local_gp is a magic symbol equal to the current value of 'gp'
// pointer. This symbol is used in the code generated by .cpload pseudo-op
// in case of using -mno-shared option.
// https://sourceware.org/ml/binutils/2004-12/msg00094.html
if (Symtab<ELFT>::X->find("__gnu_local_gp"))
ElfSym<ELFT>::MipsLocalGp =
Symtab<ELFT>::X->addAbsolute("__gnu_local_gp", STV_HIDDEN, STB_LOCAL);
}
// In the assembly for 32 bit x86 the _GLOBAL_OFFSET_TABLE_ symbol
// is magical and is used to produce a R_386_GOTPC relocation.
// The R_386_GOTPC relocation value doesn't actually depend on the
// symbol value, so it could use an index of STN_UNDEF which, according
// to the spec, means the symbol value is 0.
// Unfortunately both gas and MC keep the _GLOBAL_OFFSET_TABLE_ symbol in
// the object file.
// The situation is even stranger on x86_64 where the assembly doesn't
// need the magical symbol, but gas still puts _GLOBAL_OFFSET_TABLE_ as
// an undefined symbol in the .o files.
// Given that the symbol is effectively unused, we just create a dummy
// hidden one to avoid the undefined symbol error.
Symtab<ELFT>::X->addIgnored("_GLOBAL_OFFSET_TABLE_");
// __tls_get_addr is defined by the dynamic linker for dynamic ELFs. For
// static linking the linker is required to optimize away any references to
// __tls_get_addr, so it's not defined anywhere. Create a hidden definition
// to avoid the undefined symbol error. As usual special cases are ARM and
// MIPS - the libc for these targets defines __tls_get_addr itself because
// there are no TLS optimizations for these targets.
if (!In<ELFT>::DynSymTab &&
(Config->EMachine != EM_MIPS && Config->EMachine != EM_ARM))
Symtab<ELFT>::X->addIgnored("__tls_get_addr");
// If linker script do layout we do not need to create any standart symbols.
if (ScriptConfig->HasSections)
return;
ElfSym<ELFT>::EhdrStart = Symtab<ELFT>::X->addIgnored("__ehdr_start");
auto Define = [this](StringRef S, DefinedRegular<ELFT> *&Sym1,
DefinedRegular<ELFT> *&Sym2) {
Sym1 = Symtab<ELFT>::X->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<ELFT>::X->find(S))
if (B->isUndefined())
Sym2 = Symtab<ELFT>::X->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))).
template <class ELFT> static void sortInitFini(OutputSectionBase *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 *S) {
if (S)
reinterpret_cast<OutputSection<ELFT> *>(S)->sortCtorsDtors();
}
// Sort input sections using the list provided by --symbol-ordering-file.
template <class ELFT>
static void sortBySymbolsOrder(ArrayRef<OutputSectionBase *> OutputSections) {
if (Config->SymbolOrderingFile.empty())
return;
// Build a map from symbols to their priorities. Symbols that didn't
// appear in the symbol ordering file have the lowest priority 0.
// All explicitly mentioned symbols have negative (higher) priorities.
DenseMap<StringRef, int> SymbolOrder;
int Priority = -Config->SymbolOrderingFile.size();
for (StringRef S : Config->SymbolOrderingFile)
SymbolOrder.insert({S, Priority++});
// Build a map from sections to their priorities.
DenseMap<InputSectionBase<ELFT> *, int> SectionOrder;
for (elf::ObjectFile<ELFT> *File : Symtab<ELFT>::X->getObjectFiles()) {
for (SymbolBody *Body : File->getSymbols()) {
auto *D = dyn_cast<DefinedRegular<ELFT>>(Body);
if (!D || !D->Section)
continue;
int &Priority = SectionOrder[D->Section];
Priority = std::min(Priority, SymbolOrder.lookup(D->getName()));
}
}
// Sort sections by priority.
for (OutputSectionBase *Base : OutputSections)
if (auto *Sec = dyn_cast<OutputSection<ELFT>>(Base))
Sec->sort([&](InputSection<ELFT> *S) { return SectionOrder.lookup(S); });
}
template <class ELFT>
void Writer<ELFT>::forEachRelSec(
std::function<void(InputSectionBase<ELFT> &)> Fn) {
for (InputSectionBase<ELFT> *IS : Symtab<ELFT>::X->Sections) {
if (!IS->Live)
continue;
// Scan all relocations. Each relocation goes through a series
// of tests to determine if it needs special treatment, such as
// creating GOT, PLT, copy relocations, etc.
// Note that relocations for non-alloc sections are directly
// processed by InputSection::relocateNonAlloc.
if (!(IS->Flags & SHF_ALLOC))
continue;
if (isa<InputSection<ELFT>>(IS) || isa<EhInputSection<ELFT>>(IS))
Fn(*IS);
}
}
template <class ELFT>
void Writer<ELFT>::addInputSec(InputSectionBase<ELFT> *IS) {
if (!IS)
return;
if (!IS->Live) {
reportDiscarded(IS);
return;
}
OutputSectionBase *Sec;
bool IsNew;
StringRef OutsecName = getOutputSectionName(IS->Name);
std::tie(Sec, IsNew) = Factory.create(IS, OutsecName);
if (IsNew)
OutputSections.push_back(Sec);
Sec->addSection(IS);
}
template <class ELFT> void Writer<ELFT>::createSections() {
for (InputSectionBase<ELFT> *IS : Symtab<ELFT>::X->Sections)
addInputSec(IS);
sortBySymbolsOrder<ELFT>(OutputSections);
sortInitFini<ELFT>(findSection(".init_array"));
sortInitFini<ELFT>(findSection(".fini_array"));
sortCtorsDtors<ELFT>(findSection(".ctors"));
sortCtorsDtors<ELFT>(findSection(".dtors"));
for (OutputSectionBase *Sec : OutputSections)
Sec->assignOffsets();
}
template <class ELFT>
static bool canSharePtLoad(const OutputSectionBase &S1,
const OutputSectionBase &S2) {
if (!(S1.Flags & SHF_ALLOC) || !(S2.Flags & SHF_ALLOC))
return false;
bool S1IsWrite = S1.Flags & SHF_WRITE;
bool S2IsWrite = S2.Flags & SHF_WRITE;
if (S1IsWrite != S2IsWrite)
return false;
if (!S1IsWrite)
return true; // RO and RX share a PT_LOAD with linker scripts.
return (S1.Flags & SHF_EXECINSTR) == (S2.Flags & SHF_EXECINSTR);
}
template <class ELFT> void Writer<ELFT>::sortSections() {
// Don't sort if using -r. It is not necessary and we want to preserve the
// relative order for SHF_LINK_ORDER sections.
if (Config->Relocatable)
return;
if (!ScriptConfig->HasSections) {
std::stable_sort(OutputSections.begin(), OutputSections.end(),
compareSectionsNonScript<ELFT>);
return;
}
Script<ELFT>::X->adjustSectionsBeforeSorting();
// The order of the sections in the script is arbitrary and may not agree with
// compareSectionsNonScript. This means that we cannot easily define a
// strict weak ordering. To see why, consider a comparison of a section in the
// script and one not in the script. We have a two simple options:
// * Make them equivalent (a is not less than b, and b is not less than a).
// The problem is then that equivalence has to be transitive and we can
// have sections a, b and c with only b in a script and a less than c
// which breaks this property.
// * Use compareSectionsNonScript. Given that the script order doesn't have
// to match, we can end up with sections a, b, c, d where b and c are in the
// script and c is compareSectionsNonScript less than b. In which case d
// can be equivalent to c, a to b and d < a. As a concrete example:
// .a (rx) # not in script
// .b (rx) # in script
// .c (ro) # in script
// .d (ro) # not in script
//
// The way we define an order then is:
// * First put script sections at the start and sort the script and
// non-script sections independently.
// * Move each non-script section to its preferred position. We try
// to put each section in the last position where it it can share
// a PT_LOAD.
std::stable_sort(OutputSections.begin(), OutputSections.end(),
compareSections<ELFT>);
auto I = OutputSections.begin();
auto E = OutputSections.end();
auto NonScriptI =
std::find_if(OutputSections.begin(), E, [](OutputSectionBase *S) {
return Script<ELFT>::X->getSectionIndex(S->getName()) == INT_MAX;
});
while (NonScriptI != E) {
auto BestPos = std::max_element(
I, NonScriptI, [&](OutputSectionBase *&A, OutputSectionBase *&B) {
bool ACanSharePtLoad = canSharePtLoad<ELFT>(**NonScriptI, *A);
bool BCanSharePtLoad = canSharePtLoad<ELFT>(**NonScriptI, *B);
if (ACanSharePtLoad != BCanSharePtLoad)
return BCanSharePtLoad;
bool ACmp = compareSectionsNonScript<ELFT>(*NonScriptI, A);
bool BCmp = compareSectionsNonScript<ELFT>(*NonScriptI, B);
if (ACmp != BCmp)
return BCmp; // FIXME: missing test
size_t PosA = &A - &OutputSections[0];
size_t PosB = &B - &OutputSections[0];
return ACmp ? PosA > PosB : PosA < PosB;
});
// max_element only returns NonScriptI if the range is empty. If the range
// is not empty we should consider moving the the element forward one
// position.
if (BestPos != NonScriptI &&
!compareSectionsNonScript<ELFT>(*NonScriptI, *BestPos))
++BestPos;
std::rotate(BestPos, NonScriptI, NonScriptI + 1);
++NonScriptI;
}
Script<ELFT>::X->adjustSectionsAfterSorting();
}
template <class ELFT>
static void
finalizeSynthetic(const std::vector<SyntheticSection<ELFT> *> &Sections) {
for (SyntheticSection<ELFT> *SS : Sections)
if (SS && SS->OutSec && !SS->empty()) {
SS->finalize();
SS->OutSec->Size = 0;
SS->OutSec->assignOffsets();
}
}
// We need to add input synthetic sections early in createSyntheticSections()
// to make them visible from linkescript side. But not all sections are always
// required to be in output. For example we don't need dynamic section content
// sometimes. This function filters out such unused sections from output.
template <class ELFT>
static void removeUnusedSyntheticSections(std::vector<OutputSectionBase *> &V) {
// Input synthetic sections are placed after all regular ones. We iterate over
// them all and exit at first non-synthetic.
for (InputSectionBase<ELFT> *S : llvm::reverse(Symtab<ELFT>::X->Sections)) {
SyntheticSection<ELFT> *SS = dyn_cast<SyntheticSection<ELFT>>(S);
if (!SS)
return;
if (!SS->empty() || !SS->OutSec)
continue;
OutputSection<ELFT> *OutSec = cast<OutputSection<ELFT>>(SS->OutSec);
OutSec->Sections.erase(
std::find(OutSec->Sections.begin(), OutSec->Sections.end(), SS));
// If there is no other sections in output section, remove it from output.
if (OutSec->Sections.empty())
V.erase(std::find(V.begin(), V.end(), OutSec));
}
}
// Create output section objects and add them to OutputSections.
template <class ELFT> void Writer<ELFT>::finalizeSections() {
Out<ELFT>::DebugInfo = findSection(".debug_info");
Out<ELFT>::PreinitArray = findSection(".preinit_array");
Out<ELFT>::InitArray = findSection(".init_array");
Out<ELFT>::FiniArray = findSection(".fini_array");
// The linker needs to define SECNAME_start, SECNAME_end and SECNAME_stop
// symbols for sections, so that the runtime can get the start and end
// addresses of each section by section name. Add such symbols.
if (!Config->Relocatable) {
addStartEndSymbols();
for (OutputSectionBase *Sec : OutputSections)
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 (In<ELFT>::DynSymTab)
addRegular("_DYNAMIC", In<ELFT>::Dynamic, 0);
// Define __rel[a]_iplt_{start,end} symbols if needed.
addRelIpltSymbols();
if (!Out<ELFT>::EhFrame->empty()) {
OutputSections.push_back(Out<ELFT>::EhFrame);
Out<ELFT>::EhFrame->finalize();
}
// Scan relocations. This must be done after every symbol is declared so that
// we can correctly decide if a dynamic relocation is needed.
forEachRelSec(scanRelocations<ELFT>);
// Now that we have defined all possible symbols including linker-
// synthesized ones. Visit all symbols to give the finishing touches.
for (Symbol *S : Symtab<ELFT>::X->getSymbols()) {
SymbolBody *Body = S->body();
if (!includeInSymtab<ELFT>(*Body))
continue;
if (In<ELFT>::SymTab)
In<ELFT>::SymTab->addSymbol(Body);
if (In<ELFT>::DynSymTab && S->includeInDynsym()) {
In<ELFT>::DynSymTab->addSymbol(Body);
if (auto *SS = dyn_cast<SharedSymbol<ELFT>>(Body))
if (SS->file()->isNeeded())
In<ELFT>::VerNeed->addSymbol(SS);
}
}
// Do not proceed if there was an undefined symbol.
if (ErrorCount)
return;
// So far we have added sections from input object files.
// This function adds linker-created Out<ELFT>::* sections.
addPredefinedSections();
removeUnusedSyntheticSections<ELFT>(OutputSections);
sortSections();
unsigned I = 1;
for (OutputSectionBase *Sec : OutputSections) {
Sec->SectionIndex = I++;
Sec->ShName = In<ELFT>::ShStrTab->addString(Sec->getName());
}
// Binary and relocatable output does not have PHDRS.
// The headers have to be created before finalize as that can influence the
// image base and the dynamic section on mips includes the image base.
if (!Config->Relocatable && !Config->OFormatBinary) {
Phdrs = Script<ELFT>::X->hasPhdrsCommands() ? Script<ELFT>::X->createPhdrs()
: createPhdrs();
addPtArmExid(Phdrs);
fixHeaders();
}
// Fill other section headers. The dynamic table is finalized
// at the end because some tags like RELSZ depend on result
// of finalizing other sections.
for (OutputSectionBase *Sec : OutputSections)
Sec->finalize();
// Dynamic section must be the last one in this list and dynamic
// symbol table section (DynSymTab) must be the first one.
finalizeSynthetic<ELFT>(
{In<ELFT>::DynSymTab, In<ELFT>::GnuHashTab, In<ELFT>::HashTab,
In<ELFT>::SymTab, In<ELFT>::ShStrTab, In<ELFT>::StrTab,
In<ELFT>::VerDef, In<ELFT>::DynStrTab, In<ELFT>::GdbIndex,
In<ELFT>::Got, In<ELFT>::MipsGot, In<ELFT>::IgotPlt,
In<ELFT>::GotPlt, In<ELFT>::RelaDyn, In<ELFT>::RelaIplt,
In<ELFT>::RelaPlt, In<ELFT>::Plt, In<ELFT>::Iplt,
In<ELFT>::Plt, In<ELFT>::EhFrameHdr, In<ELFT>::VerSym,
In<ELFT>::VerNeed, In<ELFT>::Dynamic});
}
template <class ELFT> void Writer<ELFT>::addPredefinedSections() {
if (Out<ELFT>::Bss->Size > 0)
OutputSections.push_back(Out<ELFT>::Bss);
auto OS = dyn_cast_or_null<OutputSection<ELFT>>(findSection(".ARM.exidx"));
if (OS && !OS->Sections.empty() && !Config->Relocatable)
OS->addSection(make<ARMExidxSentinelSection<ELFT>>());
addInputSec(In<ELFT>::SymTab);
addInputSec(In<ELFT>::ShStrTab);
addInputSec(In<ELFT>::StrTab);
}
// 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 *OS) {
// These symbols resolve to the image base if the section does not exist.
// A special value -1 indicates end of the section.
addOptionalSynthetic<ELFT>(Start, OS, 0);
addOptionalSynthetic<ELFT>(End, OS, OS ? -1 : 0);
};
Define("__preinit_array_start", "__preinit_array_end",
Out<ELFT>::PreinitArray);
Define("__init_array_start", "__init_array_end", Out<ELFT>::InitArray);
Define("__fini_array_start", "__fini_array_end", Out<ELFT>::FiniArray);
if (OutputSectionBase *Sec = findSection(".ARM.exidx"))
Define("__exidx_start", "__exidx_end", Sec);
}
// If a section name is valid as a C identifier (which is rare because of
// the leading '.'), linkers are expected to define __start_<secname> and
// __stop_<secname> symbols. They are at beginning and end of the section,
// respectively. This is not requested by the ELF standard, but GNU ld and
// gold provide the feature, and used by many programs.
template <class ELFT>
void Writer<ELFT>::addStartStopSymbols(OutputSectionBase *Sec) {
StringRef S = Sec->getName();
if (!isValidCIdentifier(S))
return;
addOptionalSynthetic<ELFT>(Saver.save("__start_" + S), Sec, 0, STV_DEFAULT);
addOptionalSynthetic<ELFT>(Saver.save("__stop_" + S), Sec, -1, STV_DEFAULT);
}
template <class ELFT>
OutputSectionBase *Writer<ELFT>::findSection(StringRef Name) {
for (OutputSectionBase *Sec : OutputSections)
if (Sec->getName() == Name)
return Sec;
return nullptr;
}
template <class ELFT> static bool needsPtLoad(OutputSectionBase *Sec) {
if (!(Sec->Flags & SHF_ALLOC))
return false;
// Don't allocate VA space for TLS NOBITS sections. The PT_TLS PHDR is
// responsible for allocating space for them, not the PT_LOAD that
// contains the TLS initialization image.
if (Sec->Flags & SHF_TLS && Sec->Type == SHT_NOBITS)
return false;
return true;
}
// Linker scripts are responsible for aligning addresses. Unfortunately, most
// linker scripts are designed for creating two PT_LOADs only, one RX and one
// RW. This means that there is no alignment in the RO to RX transition and we
// cannot create a PT_LOAD there.
template <class ELFT>
static typename ELFT::uint computeFlags(typename ELFT::uint F) {
if (Config->OMagic)
return PF_R | PF_W | PF_X;
if (Config->SingleRoRx && !(F & PF_W))
return F | PF_X;
return F;
}
// Decide which program headers to create and which sections to include in each
// one.
template <class ELFT> std::vector<PhdrEntry> Writer<ELFT>::createPhdrs() {
std::vector<PhdrEntry> Ret;
auto AddHdr = [&](unsigned Type, unsigned Flags) -> PhdrEntry * {
Ret.emplace_back(Type, Flags);
return &Ret.back();
};
// The first phdr entry is PT_PHDR which describes the program header itself.
PhdrEntry &Hdr = *AddHdr(PT_PHDR, PF_R);
Hdr.add(Out<ELFT>::ProgramHeaders);
// PT_INTERP must be the second entry if exists.
if (OutputSectionBase *Sec = findSection(".interp")) {
PhdrEntry &Hdr = *AddHdr(PT_INTERP, Sec->getPhdrFlags());
Hdr.add(Sec);
}
// Add the first PT_LOAD segment for regular output sections.
uintX_t Flags = computeFlags<ELFT>(PF_R);
PhdrEntry *Load = AddHdr(PT_LOAD, Flags);
PhdrEntry TlsHdr(PT_TLS, PF_R);
PhdrEntry RelRo(PT_GNU_RELRO, PF_R);
PhdrEntry Note(PT_NOTE, PF_R);
for (OutputSectionBase *Sec : OutputSections) {
if (!(Sec->Flags & SHF_ALLOC))
break;
// If we meet TLS section then we create TLS header
// and put all TLS sections inside for further use when
// assign addresses.
if (Sec->Flags & SHF_TLS)
TlsHdr.add(Sec);
if (!needsPtLoad<ELFT>(Sec))
continue;
// Segments are contiguous memory regions that has the same attributes
// (e.g. executable or writable). There is one phdr for each segment.
// Therefore, we need to create a new phdr when the next section has
// different flags or is loaded at a discontiguous address using AT linker
// script command.
uintX_t NewFlags = computeFlags<ELFT>(Sec->getPhdrFlags());
if (Script<ELFT>::X->hasLMA(Sec->getName()) || Flags != NewFlags) {
Load = AddHdr(PT_LOAD, NewFlags);
Flags = NewFlags;
}
Load->add(Sec);
if (isRelroSection<ELFT>(Sec))
RelRo.add(Sec);
if (Sec->Type == SHT_NOTE)
Note.add(Sec);
}
// Add the TLS segment unless it's empty.
if (TlsHdr.First)
Ret.push_back(std::move(TlsHdr));
// Add an entry for .dynamic.
if (In<ELFT>::DynSymTab) {
PhdrEntry &H =
*AddHdr(PT_DYNAMIC, In<ELFT>::Dynamic->OutSec->getPhdrFlags());
H.add(In<ELFT>::Dynamic->OutSec);
}
// PT_GNU_RELRO includes all sections that should be marked as
// read-only by dynamic linker after proccessing relocations.
if (RelRo.First)
Ret.push_back(std::move(RelRo));
// PT_GNU_EH_FRAME is a special section pointing on .eh_frame_hdr.
if (!Out<ELFT>::EhFrame->empty() && In<ELFT>::EhFrameHdr) {
PhdrEntry &Hdr =
*AddHdr(PT_GNU_EH_FRAME, In<ELFT>::EhFrameHdr->OutSec->getPhdrFlags());
Hdr.add(In<ELFT>::EhFrameHdr->OutSec);
}
// PT_OPENBSD_RANDOMIZE specifies the location and size of a part of the
// memory image of the program that must be filled with random data before any
// code in the object is executed.
if (OutputSectionBase *Sec = findSection(".openbsd.randomdata")) {
PhdrEntry &Hdr = *AddHdr(PT_OPENBSD_RANDOMIZE, Sec->getPhdrFlags());
Hdr.add(Sec);
}
// PT_GNU_STACK is a special section to tell the loader to make the
// pages for the stack non-executable.
if (!Config->ZExecstack) {
PhdrEntry &Hdr = *AddHdr(PT_GNU_STACK, PF_R | PF_W);
if (Config->ZStackSize != uint64_t(-1))
Hdr.p_memsz = Config->ZStackSize;
}
// PT_OPENBSD_WXNEEDED is a OpenBSD-specific header to mark the executable
// is expected to perform W^X violations, such as calling mprotect(2) or
// mmap(2) with PROT_WRITE | PROT_EXEC, which is prohibited by default on
// OpenBSD.
if (Config->ZWxneeded)
AddHdr(PT_OPENBSD_WXNEEDED, PF_X);
if (Note.First)
Ret.push_back(std::move(Note));
return Ret;
}
template <class ELFT>
void Writer<ELFT>::addPtArmExid(std::vector<PhdrEntry> &Phdrs) {
if (Config->EMachine != EM_ARM)
return;
auto I = std::find_if(
OutputSections.begin(), OutputSections.end(),
[](OutputSectionBase *Sec) { return Sec->Type == SHT_ARM_EXIDX; });
if (I == OutputSections.end())
return;
// PT_ARM_EXIDX is the ARM EHABI equivalent of PT_GNU_EH_FRAME
PhdrEntry ARMExidx(PT_ARM_EXIDX, PF_R);
ARMExidx.add(*I);
Phdrs.push_back(ARMExidx);
}
// 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.
template <class ELFT> void Writer<ELFT>::fixSectionAlignments() {
for (const PhdrEntry &P : Phdrs)
if (P.p_type == PT_LOAD && P.First)
P.First->PageAlign = true;
for (const PhdrEntry &P : Phdrs) {
if (P.p_type != PT_GNU_RELRO)
continue;
// Find the first section after PT_GNU_RELRO. If it is in a PT_LOAD we
// have to align it to a page.
auto End = OutputSections.end();
auto I = std::find(OutputSections.begin(), End, P.Last);
if (I == End || (I + 1) == End)
continue;
OutputSectionBase *Sec = *(I + 1);
if (needsPtLoad<ELFT>(Sec))
Sec->PageAlign = true;
}
}
template <class ELFT>
void elf::allocateHeaders(MutableArrayRef<PhdrEntry> Phdrs,
ArrayRef<OutputSectionBase *> OutputSections) {
auto FirstPTLoad =
std::find_if(Phdrs.begin(), Phdrs.end(),
[](const PhdrEntry &E) { return E.p_type == PT_LOAD; });
if (FirstPTLoad == Phdrs.end())
return;
if (FirstPTLoad->First)
for (OutputSectionBase *Sec : OutputSections)
if (Sec->FirstInPtLoad == FirstPTLoad->First)
Sec->FirstInPtLoad = Out<ELFT>::ElfHeader;
FirstPTLoad->First = Out<ELFT>::ElfHeader;
if (!FirstPTLoad->Last)
FirstPTLoad->Last = Out<ELFT>::ProgramHeaders;
}
// We should set file offsets and VAs for elf header and program headers
// sections. These are special, we do not include them into output sections
// list, but have them to simplify the code.
template <class ELFT> void Writer<ELFT>::fixHeaders() {
Out<ELFT>::ProgramHeaders->Size = sizeof(Elf_Phdr) * Phdrs.size();
// If the script has SECTIONS, assignAddresses will compute the values.
if (ScriptConfig->HasSections)
return;
uintX_t HeaderSize = getHeaderSize<ELFT>();
// When -T<section> option is specified, lower the base to make room for those
// sections.
if (!Config->SectionStartMap.empty()) {
uint64_t Min = -1;
for (const auto &P : Config->SectionStartMap)
Min = std::min(Min, P.second);
if (HeaderSize < Min)
Min -= HeaderSize;
else
AllocateHeader = false;
if (Min < Config->ImageBase)
Config->ImageBase = alignDown(Min, Config->MaxPageSize);
}
if (AllocateHeader)
allocateHeaders<ELFT>(Phdrs, OutputSections);
uintX_t BaseVA = Config->ImageBase;
Out<ELFT>::ElfHeader->Addr = BaseVA;
Out<ELFT>::ProgramHeaders->Addr = BaseVA + Out<ELFT>::ElfHeader->Size;
}
// Assign VAs (addresses at run-time) to output sections.
template <class ELFT> void Writer<ELFT>::assignAddresses() {
uintX_t VA = Config->ImageBase;
if (AllocateHeader)
VA += getHeaderSize<ELFT>();
uintX_t ThreadBssOffset = 0;
for (OutputSectionBase *Sec : OutputSections) {
uintX_t Alignment = Sec->Addralign;
if (Sec->PageAlign)
Alignment = std::max<uintX_t>(Alignment, Config->MaxPageSize);
auto I = Config->SectionStartMap.find(Sec->getName());
if (I != Config->SectionStartMap.end())
VA = I->second;
// We only assign VAs to allocated sections.
if (needsPtLoad<ELFT>(Sec)) {
VA = alignTo(VA, Alignment);
Sec->Addr = VA;
VA += Sec->Size;
} else if (Sec->Flags & SHF_TLS && Sec->Type == SHT_NOBITS) {
uintX_t TVA = VA + ThreadBssOffset;
TVA = alignTo(TVA, Alignment);
Sec->Addr = TVA;
ThreadBssOffset = TVA - VA + Sec->Size;
}
}
}
// Adjusts the file alignment for a given output section and returns
// its new file offset. The file offset must be the same with its
// virtual address (modulo the page size) so that the loader can load
// executables without any address adjustment.
template <class ELFT, class uintX_t>
static uintX_t getFileAlignment(uintX_t Off, OutputSectionBase *Sec) {
OutputSectionBase *First = Sec->FirstInPtLoad;
// If the section is not in a PT_LOAD, we just have to align it.
if (!First)
return alignTo(Off, Sec->Addralign);
// The first section in a PT_LOAD has to have congruent offset and address
// module the page size.
if (Sec == First)
return alignTo(Off, Config->MaxPageSize, Sec->Addr);
// If two sections share the same PT_LOAD the file offset is calculated
// using this formula: Off2 = Off1 + (VA2 - VA1).
return First->Offset + Sec->Addr - First->Addr;
}
template <class ELFT, class uintX_t>
void setOffset(OutputSectionBase *Sec, uintX_t &Off) {
if (Sec->Type == SHT_NOBITS) {
Sec->Offset = Off;
return;
}
Off = getFileAlignment<ELFT>(Off, Sec);
Sec->Offset = Off;
Off += Sec->Size;
}
template <class ELFT> void Writer<ELFT>::assignFileOffsetsBinary() {
uintX_t Off = 0;
for (OutputSectionBase *Sec : OutputSections)
if (Sec->Flags & SHF_ALLOC)
setOffset<ELFT>(Sec, Off);
FileSize = alignTo(Off, sizeof(uintX_t));
}
// Assign file offsets to output sections.
template <class ELFT> void Writer<ELFT>::assignFileOffsets() {
uintX_t Off = 0;
setOffset<ELFT>(Out<ELFT>::ElfHeader, Off);
setOffset<ELFT>(Out<ELFT>::ProgramHeaders, Off);
for (OutputSectionBase *Sec : OutputSections)
setOffset<ELFT>(Sec, Off);
SectionHeaderOff = alignTo(Off, sizeof(uintX_t));
FileSize = SectionHeaderOff + (OutputSections.size() + 1) * sizeof(Elf_Shdr);
}
// Finalize the program headers. We call this function after we assign
// file offsets and VAs to all sections.
template <class ELFT> void Writer<ELFT>::setPhdrs() {
for (PhdrEntry &P : Phdrs) {
OutputSectionBase *First = P.First;
OutputSectionBase *Last = P.Last;
if (First) {
P.p_filesz = Last->Offset - First->Offset;
if (Last->Type != SHT_NOBITS)
P.p_filesz += Last->Size;
P.p_memsz = Last->Addr + Last->Size - First->Addr;
P.p_offset = First->Offset;
P.p_vaddr = First->Addr;
if (!P.HasLMA)
P.p_paddr = First->getLMA();
}
if (P.p_type == PT_LOAD)
P.p_align = Config->MaxPageSize;
else if (P.p_type == PT_GNU_RELRO)
P.p_align = 1;
// 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 (P.p_type == PT_TLS) {
Out<ELFT>::TlsPhdr = &P;
if (P.p_memsz)
P.p_memsz = alignTo(P.p_memsz, P.p_align);
}
}
}
// The entry point address is chosen in the following ways.
//
// 1. the '-e' entry command-line option;
// 2. the ENTRY(symbol) command in a linker control script;
// 3. the value of the symbol start, if present;
// 4. the address of the first byte of the .text section, if present;
// 5. the address 0.
template <class ELFT> typename ELFT::uint Writer<ELFT>::getEntryAddr() {
// Case 1, 2 or 3. As a special case, if the symbol is actually
// a number, we'll use that number as an address.
if (SymbolBody *B = Symtab<ELFT>::X->find(Config->Entry))
return B->getVA<ELFT>();
uint64_t Addr;
if (!Config->Entry.getAsInteger(0, Addr))
return Addr;
// Case 4
if (OutputSectionBase *Sec = findSection(".text")) {
if (Config->WarnMissingEntry)
warn("cannot find entry symbol " + Config->Entry + "; defaulting to 0x" +
utohexstr(Sec->Addr));
return Sec->Addr;
}
// Case 5
if (Config->WarnMissingEntry)
warn("cannot find entry symbol " + Config->Entry +
"; not setting start address");
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() {
// __ehdr_start is the location of program headers.
if (ElfSym<ELFT>::EhdrStart)
ElfSym<ELFT>::EhdrStart->Value = Out<ELFT>::ProgramHeaders->Addr;
auto Set = [](DefinedRegular<ELFT> *S1, DefinedRegular<ELFT> *S2, uintX_t V) {
if (S1)
S1->Value = V;
if (S2)
S2->Value = V;
};
// _etext is the first location after the last read-only loadable segment.
// _edata is the first location after the last read-write loadable segment.
// _end is the first location after the uninitialized data region.
for (PhdrEntry &P : Phdrs) {
if (P.p_type != PT_LOAD)
continue;
Set(ElfSym<ELFT>::End, ElfSym<ELFT>::End2, P.p_vaddr + P.p_memsz);
uintX_t Val = P.p_vaddr + P.p_filesz;
if (P.p_flags & PF_W)
Set(ElfSym<ELFT>::Edata, ElfSym<ELFT>::Edata2, Val);
else
Set(ElfSym<ELFT>::Etext, ElfSym<ELFT>::Etext2, Val);
}
// Setup MIPS _gp_disp/__gnu_local_gp symbols which should
// be equal to the _gp symbol's value.
if (Config->EMachine == EM_MIPS) {
if (!ElfSym<ELFT>::MipsGp->Value) {
// Find GP-relative section with the lowest address
// and use this address to calculate default _gp value.
uintX_t Gp = -1;
for (const OutputSectionBase * OS : OutputSections)
if ((OS->Flags & SHF_MIPS_GPREL) && OS->Addr < Gp)
Gp = OS->Addr;
if (Gp != (uintX_t)-1)
ElfSym<ELFT>::MipsGp->Value = Gp + 0x7ff0;
}
if (ElfSym<ELFT>::MipsGpDisp)
ElfSym<ELFT>::MipsGpDisp->Value = ElfSym<ELFT>::MipsGp->Value;
if (ElfSym<ELFT>::MipsLocalGp)
ElfSym<ELFT>::MipsLocalGp->Value = ElfSym<ELFT>::MipsGp->Value;
}
}
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] = getELFEncoding<ELFT>();
EHdr->e_ident[EI_VERSION] = EV_CURRENT;
EHdr->e_ident[EI_OSABI] = Config->OSABI;
EHdr->e_type = getELFType();
EHdr->e_machine = Config->EMachine;
EHdr->e_version = EV_CURRENT;
EHdr->e_entry = getEntryAddr();
EHdr->e_shoff = SectionHeaderOff;
EHdr->e_ehsize = sizeof(Elf_Ehdr);
EHdr->e_phnum = Phdrs.size();
EHdr->e_shentsize = sizeof(Elf_Shdr);
EHdr->e_shnum = OutputSections.size() + 1;
EHdr->e_shstrndx = In<ELFT>::ShStrTab->OutSec->SectionIndex;
if (Config->EMachine == EM_ARM)
// We don't currently use any features incompatible with EF_ARM_EABI_VER5,
// but we don't have any firm guarantees of conformance. Linux AArch64
// kernels (as of 2016) require an EABI version to be set.
EHdr->e_flags = EF_ARM_EABI_VER5;
else if (Config->EMachine == EM_MIPS)
EHdr->e_flags = getMipsEFlags<ELFT>();
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 (PhdrEntry &P : Phdrs) {
HBuf->p_type = P.p_type;
HBuf->p_flags = P.p_flags;
HBuf->p_offset = P.p_offset;
HBuf->p_vaddr = P.p_vaddr;
HBuf->p_paddr = P.p_paddr;
HBuf->p_filesz = P.p_filesz;
HBuf->p_memsz = P.p_memsz;
HBuf->p_align = P.p_align;
++HBuf;
}
// 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 *Sec : OutputSections)
Sec->writeHeaderTo<ELFT>(++SHdrs);
}
// Removes a given file asynchronously. This is a performance hack,
// so remove this when operating systems are improved.
//
// On Linux (and probably on other Unix-like systems), unlink(2) is a
// noticeably slow system call. As of 2016, unlink takes 250
// milliseconds to remove a 1 GB file on ext4 filesystem on my machine.
//
// To create a new result file, we first remove existing file. So, if
// you repeatedly link a 1 GB program in a regular compile-link-debug
// cycle, every cycle wastes 250 milliseconds only to remove a file.
// Since LLD can link a 1 GB binary in about 5 seconds, that waste
// actually counts.
//
// This function spawns a background thread to call unlink.
// The calling thread returns almost immediately.
static void unlinkAsync(StringRef Path) {
if (!Config->Threads || !sys::fs::exists(Config->OutputFile))
return;
// First, rename Path to avoid race condition. We cannot remomve
// Path from a different thread because we are now going to create
// Path as a new file. If we do that in a different thread, the new
// thread can remove the new file.
SmallString<128> TempPath;
if (auto EC = sys::fs::createUniqueFile(Path + "tmp%%%%%%%%", TempPath))
fatal(EC, "createUniqueFile failed");
if (auto EC = sys::fs::rename(Path, TempPath))
fatal(EC, "rename failed");
// Remove TempPath in background.
std::thread([=] { ::remove(TempPath.str().str().c_str()); }).detach();
}
// Open a result file.
template <class ELFT> void Writer<ELFT>::openFile() {
unlinkAsync(Config->OutputFile);
ErrorOr<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
FileOutputBuffer::create(Config->OutputFile, FileSize,
FileOutputBuffer::F_executable);
if (auto EC = BufferOrErr.getError())
error(EC, "failed to open " + Config->OutputFile);
else
Buffer = std::move(*BufferOrErr);
}
template <class ELFT> void Writer<ELFT>::writeSectionsBinary() {
uint8_t *Buf = Buffer->getBufferStart();
for (OutputSectionBase *Sec : OutputSections)
if (Sec->Flags & SHF_ALLOC)
Sec->writeTo(Buf + Sec->Offset);
}
// 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.
Out<ELFT>::Opd = findSection(".opd");
if (Out<ELFT>::Opd) {
Out<ELFT>::OpdBuf = Buf + Out<ELFT>::Opd->Offset;
Out<ELFT>::Opd->writeTo(Buf + Out<ELFT>::Opd->Offset);
}
OutputSectionBase *EhFrameHdr =
In<ELFT>::EhFrameHdr ? In<ELFT>::EhFrameHdr->OutSec : nullptr;
for (OutputSectionBase *Sec : OutputSections)
if (Sec != Out<ELFT>::Opd && Sec != EhFrameHdr)
Sec->writeTo(Buf + Sec->Offset);
// The .eh_frame_hdr depends on .eh_frame section contents, therefore
// it should be written after .eh_frame is written.
if (!Out<ELFT>::EhFrame->empty() && EhFrameHdr)
EhFrameHdr->writeTo(Buf + EhFrameHdr->Offset);
}
template <class ELFT> void Writer<ELFT>::writeBuildId() {
if (!In<ELFT>::BuildId || !In<ELFT>::BuildId->OutSec)
return;
// Compute a hash of all sections of the output file.
uint8_t *Start = Buffer->getBufferStart();
uint8_t *End = Start + FileSize;
In<ELFT>::BuildId->writeBuildId({Start, End});
}
template void elf::writeResult<ELF32LE>();
template void elf::writeResult<ELF32BE>();
template void elf::writeResult<ELF64LE>();
template void elf::writeResult<ELF64BE>();
template void elf::allocateHeaders<ELF32LE>(MutableArrayRef<PhdrEntry>,
ArrayRef<OutputSectionBase *>);
template void elf::allocateHeaders<ELF32BE>(MutableArrayRef<PhdrEntry>,
ArrayRef<OutputSectionBase *>);
template void elf::allocateHeaders<ELF64LE>(MutableArrayRef<PhdrEntry>,
ArrayRef<OutputSectionBase *>);
template void elf::allocateHeaders<ELF64BE>(MutableArrayRef<PhdrEntry>,
ArrayRef<OutputSectionBase *>);
template bool elf::isRelroSection<ELF32LE>(const OutputSectionBase *);
template bool elf::isRelroSection<ELF32BE>(const OutputSectionBase *);
template bool elf::isRelroSection<ELF64LE>(const OutputSectionBase *);
template bool elf::isRelroSection<ELF64BE>(const OutputSectionBase *);
template void elf::reportDiscarded<ELF32LE>(InputSectionBase<ELF32LE> *);
template void elf::reportDiscarded<ELF32BE>(InputSectionBase<ELF32BE> *);
template void elf::reportDiscarded<ELF64LE>(InputSectionBase<ELF64LE> *);
template void elf::reportDiscarded<ELF64BE>(InputSectionBase<ELF64BE> *);
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