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//===- Symbols.cpp --------------------------------------------------------===//
// The LLVM Linker
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
#include "Symbols.h"
#include "Error.h"
#include "InputFiles.h"
#include "InputSection.h"
#include "OutputSections.h"
#include "Strings.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "Writer.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/Path.h"
#include <cstring>
using namespace llvm;
using namespace llvm::object;
using namespace llvm::ELF;
using namespace lld;
using namespace lld::elf;
DefinedRegular *ElfSym::Bss;
DefinedRegular *ElfSym::Etext1;
DefinedRegular *ElfSym::Etext2;
DefinedRegular *ElfSym::Edata1;
DefinedRegular *ElfSym::Edata2;
DefinedRegular *ElfSym::End1;
DefinedRegular *ElfSym::End2;
DefinedRegular *ElfSym::MipsGp;
DefinedRegular *ElfSym::MipsGpDisp;
DefinedRegular *ElfSym::MipsLocalGp;
static uint64_t getSymVA(const SymbolBody &Body, int64_t &Addend) {
switch (Body.kind()) {
case SymbolBody::DefinedRegularKind: {
auto &D = cast<DefinedRegular>(Body);
SectionBase *IS = D.Section;
if (auto *ISB = dyn_cast_or_null<InputSectionBase>(IS))
IS = ISB->Repl;
// According to the ELF spec reference to a local symbol from outside
// the group are not allowed. Unfortunately .eh_frame breaks that rule
// and must be treated specially. For now we just replace the symbol with
// 0.
if (IS == &InputSection::Discarded)
return 0;
// This is an absolute symbol.
if (!IS)
return D.Value;
uint64_t Offset = D.Value;
// An object in an SHF_MERGE section might be referenced via a
// section symbol (as a hack for reducing the number of local
// symbols).
// Depending on the addend, the reference via a section symbol
// refers to a different object in the merge section.
// Since the objects in the merge section are not necessarily
// contiguous in the output, the addend can thus affect the final
// VA in a non-linear way.
// To make this work, we incorporate the addend into the section
// offset (and zero out the addend for later processing) so that
// we find the right object in the section.
if (D.isSection()) {
Offset += Addend;
Addend = 0;
const OutputSection *OutSec = IS->getOutputSection();
// In the typical case, this is actually very simple and boils
// down to adding together 3 numbers:
// 1. The address of the output section.
// 2. The offset of the input section within the output section.
// 3. The offset within the input section (this addition happens
// inside InputSection::getOffset).
// If you understand the data structures involved with this next
// line (and how they get built), then you have a pretty good
// understanding of the linker.
uint64_t VA = (OutSec ? OutSec->Addr : 0) + IS->getOffset(Offset);
if (D.isTls() && !Config->Relocatable) {
if (!Out::TlsPhdr)
fatal(toString(D.File) +
" has a STT_TLS symbol but doesn't have a PT_TLS section");
return VA - Out::TlsPhdr->p_vaddr;
return VA;
case SymbolBody::DefinedCommonKind:
if (!Config->DefineCommon)
return 0;
return InX::Common->OutSec->Addr + InX::Common->OutSecOff +
case SymbolBody::SharedKind: {
auto &SS = cast<SharedSymbol>(Body);
if (SS.NeedsCopy)
return SS.CopyRelSec->OutSec->Addr + SS.CopyRelSec->OutSecOff +
if (SS.NeedsPltAddr)
return Body.getPltVA();
return 0;
case SymbolBody::UndefinedKind:
return 0;
case SymbolBody::LazyArchiveKind:
case SymbolBody::LazyObjectKind:
assert(Body.symbol()->IsUsedInRegularObj && "lazy symbol reached writer");
return 0;
llvm_unreachable("invalid symbol kind");
SymbolBody::SymbolBody(Kind K, StringRefZ Name, bool IsLocal, uint8_t StOther,
uint8_t Type)
: SymbolKind(K), NeedsCopy(false), NeedsPltAddr(false), IsLocal(IsLocal),
IsInGlobalMipsGot(false), Is32BitMipsGot(false), IsInIplt(false),
IsInIgot(false), Type(Type), StOther(StOther), Name(Name) {}
// Returns true if a symbol can be replaced at load-time by a symbol
// with the same name defined in other ELF executable or DSO.
bool SymbolBody::isPreemptible() const {
if (isLocal())
return false;
// Shared symbols resolve to the definition in the DSO. The exceptions are
// symbols with copy relocations (which resolve to .bss) or preempt plt
// entries (which resolve to that plt entry).
if (isShared())
return !NeedsCopy && !NeedsPltAddr;
// That's all that can be preempted in a non-DSO.
if (!Config->Shared)
return false;
// Only symbols that appear in dynsym can be preempted.
if (!symbol()->includeInDynsym())
return false;
// Only default visibility symbols can be preempted.
if (symbol()->Visibility != STV_DEFAULT)
return false;
// -Bsymbolic means that definitions are not preempted.
if (Config->Bsymbolic || (Config->BsymbolicFunctions && isFunc()))
return !isDefined();
return true;
uint64_t SymbolBody::getVA(int64_t Addend) const {
uint64_t OutVA = getSymVA(*this, Addend);
return OutVA + Addend;
uint64_t SymbolBody::getGotVA() const {
return InX::Got->getVA() + getGotOffset();
uint64_t SymbolBody::getGotOffset() const {
return GotIndex * Target->GotEntrySize;
uint64_t SymbolBody::getGotPltVA() const {
if (this->IsInIgot)
return InX::IgotPlt->getVA() + getGotPltOffset();
return InX::GotPlt->getVA() + getGotPltOffset();
uint64_t SymbolBody::getGotPltOffset() const {
return GotPltIndex * Target->GotPltEntrySize;
uint64_t SymbolBody::getPltVA() const {
if (this->IsInIplt)
return InX::Iplt->getVA() + PltIndex * Target->PltEntrySize;
return InX::Plt->getVA() + Target->PltHeaderSize +
PltIndex * Target->PltEntrySize;
template <class ELFT> typename ELFT::uint SymbolBody::getSize() const {
if (const auto *C = dyn_cast<DefinedCommon>(this))
return C->Size;
if (const auto *DR = dyn_cast<DefinedRegular>(this))
return DR->Size;
if (const auto *S = dyn_cast<SharedSymbol>(this))
return S->getSize<ELFT>();
return 0;
OutputSection *SymbolBody::getOutputSection() const {
if (auto *S = dyn_cast<DefinedRegular>(this)) {
if (S->Section)
return S->Section->getOutputSection();
return nullptr;
if (auto *S = dyn_cast<SharedSymbol>(this)) {
if (S->NeedsCopy)
return S->CopyRelSec->OutSec;
return nullptr;
if (isa<DefinedCommon>(this)) {
if (Config->DefineCommon)
return InX::Common->OutSec;
return nullptr;
return nullptr;
// If a symbol name contains '@', the characters after that is
// a symbol version name. This function parses that.
void SymbolBody::parseSymbolVersion() {
StringRef S = getName();
size_t Pos = S.find('@');
if (Pos == 0 || Pos == StringRef::npos)
StringRef Verstr = S.substr(Pos + 1);
if (Verstr.empty())
// Truncate the symbol name so that it doesn't include the version string.
Name = {, Pos};
// If this is not in this DSO, it is not a definition.
if (!isInCurrentDSO())
// '@@' in a symbol name means the default version.
// It is usually the most recent one.
bool IsDefault = (Verstr[0] == '@');
if (IsDefault)
Verstr = Verstr.substr(1);
for (VersionDefinition &Ver : Config->VersionDefinitions) {
if (Ver.Name != Verstr)
if (IsDefault)
symbol()->VersionId = Ver.Id;
symbol()->VersionId = Ver.Id | VERSYM_HIDDEN;
// It is an error if the specified version is not defined.
error(toString(File) + ": symbol " + S + " has undefined version " + Verstr);
Defined::Defined(Kind K, StringRefZ Name, bool IsLocal, uint8_t StOther,
uint8_t Type)
: SymbolBody(K, Name, IsLocal, StOther, Type) {}
template <class ELFT> bool DefinedRegular::isMipsPIC() const {
if (!Section || !isFunc())
return false;
return (this->StOther & STO_MIPS_MIPS16) == STO_MIPS_PIC ||
->template getFile<ELFT>()
->e_flags &
Undefined::Undefined(StringRefZ Name, bool IsLocal, uint8_t StOther,
uint8_t Type, InputFile *File)
: SymbolBody(SymbolBody::UndefinedKind, Name, IsLocal, StOther, Type) {
this->File = File;
DefinedCommon::DefinedCommon(StringRef Name, uint64_t Size, uint32_t Alignment,
uint8_t StOther, uint8_t Type, InputFile *File)
: Defined(SymbolBody::DefinedCommonKind, Name, /*IsLocal=*/false, StOther,
Alignment(Alignment), Size(Size) {
this->File = File;
// If a shared symbol is referred via a copy relocation, its alignment
// becomes part of the ABI. This function returns a symbol alignment.
// Because symbols don't have alignment attributes, we need to infer that.
template <class ELFT> uint32_t SharedSymbol::getAlignment() const {
auto *File = cast<SharedFile<ELFT>>(this->File);
uint32_t SecAlign = File->getSection(getSym<ELFT>())->sh_addralign;
uint64_t SymValue = getSym<ELFT>().st_value;
uint32_t SymAlign = uint32_t(1) << countTrailingZeros(SymValue);
return std::min(SecAlign, SymAlign);
InputFile *Lazy::fetch() {
if (auto *S = dyn_cast<LazyArchive>(this))
return S->fetch();
return cast<LazyObject>(this)->fetch();
LazyArchive::LazyArchive(ArchiveFile &File,
const llvm::object::Archive::Symbol S, uint8_t Type)
: Lazy(LazyArchiveKind, S.getName(), Type), Sym(S) {
this->File = &File;
LazyObject::LazyObject(StringRef Name, LazyObjectFile &File, uint8_t Type)
: Lazy(LazyObjectKind, Name, Type) {
this->File = &File;
InputFile *LazyArchive::fetch() {
std::pair<MemoryBufferRef, uint64_t> MBInfo = file()->getMember(&Sym);
// getMember returns an empty buffer if the member was already
// read from the library.
if (MBInfo.first.getBuffer().empty())
return nullptr;
return createObjectFile(MBInfo.first, file()->getName(), MBInfo.second);
InputFile *LazyObject::fetch() { return file()->fetch(); }
uint8_t Symbol::computeBinding() const {
if (Config->Relocatable)
return Binding;
if (Visibility != STV_DEFAULT && Visibility != STV_PROTECTED)
return STB_LOCAL;
if (VersionId == VER_NDX_LOCAL && body()->isInCurrentDSO())
return STB_LOCAL;
if (Config->NoGnuUnique && Binding == STB_GNU_UNIQUE)
return STB_GLOBAL;
return Binding;
bool Symbol::includeInDynsym() const {
if (computeBinding() == STB_LOCAL)
return false;
return ExportDynamic || body()->isShared() ||
(body()->isUndefined() && Config->Shared);
// Print out a log message for --trace-symbol.
void elf::printTraceSymbol(Symbol *Sym) {
SymbolBody *B = Sym->body();
std::string S;
if (B->isUndefined())
S = ": reference to ";
else if (B->isCommon())
S = ": common definition of ";
S = ": definition of ";
message(toString(B->File) + S + B->getName());
// Returns a symbol for an error message.
std::string lld::toString(const SymbolBody &B) {
if (Config->Demangle)
if (Optional<std::string> S = demangle(B.getName()))
return *S;
return B.getName();
template uint32_t SymbolBody::template getSize<ELF32LE>() const;
template uint32_t SymbolBody::template getSize<ELF32BE>() const;
template uint64_t SymbolBody::template getSize<ELF64LE>() const;
template uint64_t SymbolBody::template getSize<ELF64BE>() const;
template bool DefinedRegular::template isMipsPIC<ELF32LE>() const;
template bool DefinedRegular::template isMipsPIC<ELF32BE>() const;
template bool DefinedRegular::template isMipsPIC<ELF64LE>() const;
template bool DefinedRegular::template isMipsPIC<ELF64BE>() const;
template uint32_t SharedSymbol::template getAlignment<ELF32LE>() const;
template uint32_t SharedSymbol::template getAlignment<ELF32BE>() const;
template uint32_t SharedSymbol::template getAlignment<ELF64LE>() const;
template uint32_t SharedSymbol::template getAlignment<ELF64BE>() const;