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llvm-project/lld/ELF/Symbols.cpp

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//===- Symbols.cpp --------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "Symbols.h"
#include "InputFiles.h"
#include "InputSection.h"
#include "OutputSections.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "Writer.h"
#include "lld/Common/ErrorHandler.h"
#include "lld/Common/Strings.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;
Defined *ElfSym::Bss;
Defined *ElfSym::Etext1;
Defined *ElfSym::Etext2;
Defined *ElfSym::Edata1;
Defined *ElfSym::Edata2;
Defined *ElfSym::End1;
Defined *ElfSym::End2;
Defined *ElfSym::GlobalOffsetTable;
Defined *ElfSym::MipsGp;
Defined *ElfSym::MipsGpDisp;
Defined *ElfSym::MipsLocalGp;
Defined *ElfSym::RelaIpltStart;
Defined *ElfSym::RelaIpltEnd;
static uint64_t getSymVA(const Symbol &Sym, int64_t &Addend) {
switch (Sym.kind()) {
case Symbol::DefinedKind: {
auto &D = cast<Defined>(Sym);
SectionBase *IS = D.Section;
// 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;
IS = IS->Repl;
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;
}
// 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 = IS->getVA(Offset);
// MIPS relocatable files can mix regular and microMIPS code.
// Linker needs to distinguish such code. To do so microMIPS
// symbols has the `STO_MIPS_MICROMIPS` flag in the `st_other`
// field. Unfortunately, the `MIPS::relocateOne()` method has
// a symbol value only. To pass type of the symbol (regular/microMIPS)
// to that routine as well as other places where we write
// a symbol value as-is (.dynamic section, `Elf_Ehdr::e_entry`
// field etc) do the same trick as compiler uses to mark microMIPS
// for CPU - set the less-significant bit.
if (Config->EMachine == EM_MIPS && isMicroMips() &&
((Sym.StOther & STO_MIPS_MICROMIPS) || Sym.NeedsPltAddr))
VA |= 1;
if (D.isTls() && !Config->Relocatable) {
// Use the address of the TLS segment's first section rather than the
// segment's address, because segment addresses aren't initialized until
// after sections are finalized. (e.g. Measuring the size of .rela.dyn
// for Android relocation packing requires knowing TLS symbol addresses
// during section finalization.)
if (!Out::TlsPhdr || !Out::TlsPhdr->FirstSec)
fatal(toString(D.File) +
" has an STT_TLS symbol but doesn't have an SHF_TLS section");
return VA - Out::TlsPhdr->FirstSec->Addr;
}
return VA;
}
case Symbol::SharedKind:
case Symbol::UndefinedKind:
return 0;
case Symbol::LazyArchiveKind:
case Symbol::LazyObjectKind:
assert(Sym.IsUsedInRegularObj && "lazy symbol reached writer");
return 0;
case Symbol::CommonKind:
llvm_unreachable("common symbol reached writer");
case Symbol::PlaceholderKind:
llvm_unreachable("placeholder symbol reached writer");
}
llvm_unreachable("invalid symbol kind");
}
uint64_t Symbol::getVA(int64_t Addend) const {
uint64_t OutVA = getSymVA(*this, Addend);
return OutVA + Addend;
}
uint64_t Symbol::getGotVA() const {
if (GotInIgot)
return In.IgotPlt->getVA() + getGotPltOffset();
return In.Got->getVA() + getGotOffset();
}
uint64_t Symbol::getGotOffset() const {
return GotIndex * Target->GotEntrySize;
}
uint64_t Symbol::getGotPltVA() const {
if (IsInIplt)
return In.IgotPlt->getVA() + getGotPltOffset();
return In.GotPlt->getVA() + getGotPltOffset();
}
uint64_t Symbol::getGotPltOffset() const {
if (IsInIplt)
return PltIndex * Target->GotPltEntrySize;
return (PltIndex + Target->GotPltHeaderEntriesNum) * Target->GotPltEntrySize;
}
uint64_t Symbol::getPPC64LongBranchOffset() const {
assert(PPC64BranchltIndex != 0xffff);
return PPC64BranchltIndex * Target->GotPltEntrySize;
}
uint64_t Symbol::getPltVA() const {
PltSection *Plt = IsInIplt ? In.Iplt : In.Plt;
uint64_t OutVA =
Plt->getVA() + Plt->HeaderSize + PltIndex * Target->PltEntrySize;
// While linking microMIPS code PLT code are always microMIPS
// code. Set the less-significant bit to track that fact.
// See detailed comment in the `getSymVA` function.
if (Config->EMachine == EM_MIPS && isMicroMips())
OutVA |= 1;
return OutVA;
}
uint64_t Symbol::getPPC64LongBranchTableVA() const {
assert(PPC64BranchltIndex != 0xffff);
return In.PPC64LongBranchTarget->getVA() +
PPC64BranchltIndex * Target->GotPltEntrySize;
}
uint64_t Symbol::getSize() const {
if (const auto *DR = dyn_cast<Defined>(this))
return DR->Size;
return cast<SharedSymbol>(this)->Size;
}
OutputSection *Symbol::getOutputSection() const {
if (auto *S = dyn_cast<Defined>(this)) {
if (auto *Sec = S->Section)
return Sec->Repl->getOutputSection();
return nullptr;
}
return nullptr;
}
// If a symbol name contains '@', the characters after that is
// a symbol version name. This function parses that.
void Symbol::parseSymbolVersion() {
StringRef S = getName();
size_t Pos = S.find('@');
if (Pos == 0 || Pos == StringRef::npos)
return;
StringRef Verstr = S.substr(Pos + 1);
if (Verstr.empty())
return;
// Truncate the symbol name so that it doesn't include the version string.
NameSize = Pos;
// If this is not in this DSO, it is not a definition.
if (!isDefined())
return;
// '@@' 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)
continue;
if (IsDefault)
VersionId = Ver.Id;
else
VersionId = Ver.Id | VERSYM_HIDDEN;
return;
}
// It is an error if the specified version is not defined.
// Usually version script is not provided when linking executable,
// but we may still want to override a versioned symbol from DSO,
// so we do not report error in this case. We also do not error
// if the symbol has a local version as it won't be in the dynamic
// symbol table.
if (Config->Shared && VersionId != VER_NDX_LOCAL)
error(toString(File) + ": symbol " + S + " has undefined version " +
Verstr);
}
void Symbol::fetch() const {
if (auto *Sym = dyn_cast<LazyArchive>(this)) {
cast<ArchiveFile>(Sym->File)->fetch(Sym->Sym);
return;
}
if (auto *Sym = dyn_cast<LazyObject>(this)) {
dyn_cast<LazyObjFile>(Sym->File)->fetch();
return;
}
llvm_unreachable("Symbol::fetch() is called on a non-lazy symbol");
}
MemoryBufferRef LazyArchive::getMemberBuffer() {
Archive::Child C = CHECK(
Sym.getMember(), "could not get the member for symbol " + Sym.getName());
return CHECK(C.getMemoryBufferRef(),
"could not get the buffer for the member defining symbol " +
Sym.getName());
}
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 && isDefined() && !IsPreemptible)
return STB_LOCAL;
if (!Config->GnuUnique && Binding == STB_GNU_UNIQUE)
return STB_GLOBAL;
return Binding;
}
bool Symbol::includeInDynsym() const {
if (!Config->HasDynSymTab)
return false;
if (computeBinding() == STB_LOCAL)
return false;
// If a PIE binary was not linked against any shared libraries, then we can
// safely drop weak undef symbols from .dynsym.
if (isUndefWeak() && Config->Pie && SharedFiles.empty())
return false;
if (!isDefined())
return true;
return ExportDynamic;
}
// Print out a log message for --trace-symbol.
void elf::printTraceSymbol(const Symbol *Sym) {
std::string S;
if (Sym->isUndefined())
S = ": reference to ";
else if (Sym->isLazy())
S = ": lazy definition of ";
else if (Sym->isShared())
S = ": shared definition of ";
else if (Sym->isCommon())
S = ": common definition of ";
else
S = ": definition of ";
message(toString(Sym->File) + S + Sym->getName());
}
void elf::maybeWarnUnorderableSymbol(const Symbol *Sym) {
if (!Config->WarnSymbolOrdering)
return;
// If UnresolvedPolicy::Ignore is used, no "undefined symbol" error/warning
// is emitted. It makes sense to not warn on undefined symbols.
//
// Note, ld.bfd --symbol-ordering-file= does not warn on undefined symbols,
// but we don't have to be compatible here.
if (Sym->isUndefined() &&
Config->UnresolvedSymbols == UnresolvedPolicy::Ignore)
return;
const InputFile *File = Sym->File;
auto *D = dyn_cast<Defined>(Sym);
auto Warn = [&](StringRef S) { warn(toString(File) + S + Sym->getName()); };
if (Sym->isUndefined())
Warn(": unable to order undefined symbol: ");
else if (Sym->isShared())
Warn(": unable to order shared symbol: ");
else if (D && !D->Section)
Warn(": unable to order absolute symbol: ");
else if (D && isa<OutputSection>(D->Section))
Warn(": unable to order synthetic symbol: ");
else if (D && !D->Section->Repl->Live)
Warn(": unable to order discarded symbol: ");
}
// Returns a symbol for an error message.
std::string lld::toString(const Symbol &B) {
if (Config->Demangle)
if (Optional<std::string> S = demangleItanium(B.getName()))
return *S;
return B.getName();
}
static uint8_t getMinVisibility(uint8_t VA, uint8_t VB) {
if (VA == STV_DEFAULT)
return VB;
if (VB == STV_DEFAULT)
return VA;
return std::min(VA, VB);
}
// Merge symbol properties.
//
// When we have many symbols of the same name, we choose one of them,
// and that's the result of symbol resolution. However, symbols that
// were not chosen still affect some symbol properties.
void Symbol::mergeProperties(const Symbol &Other) {
if (Other.ExportDynamic)
ExportDynamic = true;
if (Other.IsUsedInRegularObj)
IsUsedInRegularObj = true;
// DSO symbols do not affect visibility in the output.
if (!Other.isShared())
Visibility = getMinVisibility(Visibility, Other.Visibility);
}
void Symbol::resolve(const Symbol &Other) {
mergeProperties(Other);
if (isPlaceholder()) {
replace(Other);
return;
}
switch (Other.kind()) {
case Symbol::UndefinedKind:
resolveUndefined(cast<Undefined>(Other));
break;
case Symbol::CommonKind:
resolveCommon(cast<CommonSymbol>(Other));
break;
case Symbol::DefinedKind:
resolveDefined(cast<Defined>(Other));
break;
case Symbol::LazyArchiveKind:
resolveLazy(cast<LazyArchive>(Other));
break;
case Symbol::LazyObjectKind:
resolveLazy(cast<LazyObject>(Other));
break;
case Symbol::SharedKind:
resolveShared(cast<SharedSymbol>(Other));
break;
case Symbol::PlaceholderKind:
llvm_unreachable("bad symbol kind");
}
}
void Symbol::resolveUndefined(const Undefined &Other) {
// An undefined symbol with non default visibility must be satisfied
// in the same DSO.
//
// If this is a non-weak defined symbol in a discarded section, override the
// existing undefined symbol for better error message later.
if ((isShared() && Other.Visibility != STV_DEFAULT) ||
(isUndefined() && Other.Binding != STB_WEAK && Other.DiscardedSecIdx)) {
replace(Other);
return;
}
if (Traced)
printTraceSymbol(&Other);
if (isShared() || isLazy() || (isUndefined() && Other.Binding != STB_WEAK))
Binding = Other.Binding;
if (isLazy()) {
// An undefined weak will not fetch archive members. See comment on Lazy in
// Symbols.h for the details.
if (Other.Binding == STB_WEAK) {
Type = Other.Type;
return;
}
// Do extra check for --warn-backrefs.
//
// --warn-backrefs is an option to prevent an undefined reference from
// fetching an archive member written earlier in the command line. It can be
// used to keep compatibility with GNU linkers to some degree.
// I'll explain the feature and why you may find it useful in this comment.
//
// lld's symbol resolution semantics is more relaxed than traditional Unix
// linkers. For example,
//
// ld.lld foo.a bar.o
//
// succeeds even if bar.o contains an undefined symbol that has to be
// resolved by some object file in foo.a. Traditional Unix linkers don't
// allow this kind of backward reference, as they visit each file only once
// from left to right in the command line while resolving all undefined
// symbols at the moment of visiting.
//
// In the above case, since there's no undefined symbol when a linker visits
// foo.a, no files are pulled out from foo.a, and because the linker forgets
// about foo.a after visiting, it can't resolve undefined symbols in bar.o
// that could have been resolved otherwise.
//
// That lld accepts more relaxed form means that (besides it'd make more
// sense) you can accidentally write a command line or a build file that
// works only with lld, even if you have a plan to distribute it to wider
// users who may be using GNU linkers. With --warn-backrefs, you can detect
// a library order that doesn't work with other Unix linkers.
//
// The option is also useful to detect cyclic dependencies between static
// archives. Again, lld accepts
//
// ld.lld foo.a bar.a
//
// even if foo.a and bar.a depend on each other. With --warn-backrefs, it is
// handled as an error.
//
// Here is how the option works. We assign a group ID to each file. A file
// with a smaller group ID can pull out object files from an archive file
// with an equal or greater group ID. Otherwise, it is a reverse dependency
// and an error.
//
// A file outside --{start,end}-group gets a fresh ID when instantiated. All
// files within the same --{start,end}-group get the same group ID. E.g.
//
// ld.lld A B --start-group C D --end-group E
//
// A forms group 0. B form group 1. C and D (including their member object
// files) form group 2. E forms group 3. I think that you can see how this
// group assignment rule simulates the traditional linker's semantics.
bool Backref = Config->WarnBackrefs && Other.File &&
File->GroupId < Other.File->GroupId;
fetch();
// We don't report backward references to weak symbols as they can be
// overridden later.
if (Backref && !isWeak())
warn("backward reference detected: " + Other.getName() + " in " +
toString(Other.File) + " refers to " + toString(File));
}
}
// Using .symver foo,foo@@VER unfortunately creates two symbols: foo and
// foo@@VER. We want to effectively ignore foo, so give precedence to
// foo@@VER.
// FIXME: If users can transition to using
// .symver foo,foo@@@VER
// we can delete this hack.
static int compareVersion(StringRef A, StringRef B) {
bool X = A.contains("@@");
bool Y = B.contains("@@");
if (!X && Y)
return 1;
if (X && !Y)
return -1;
return 0;
}
// Compare two symbols. Return 1 if the new symbol should win, -1 if
// the new symbol should lose, or 0 if there is a conflict.
int Symbol::compare(const Symbol *Other) const {
assert(Other->isDefined() || Other->isCommon());
if (!isDefined() && !isCommon())
return 1;
if (int Cmp = compareVersion(getName(), Other->getName()))
return Cmp;
if (Other->isWeak())
return -1;
if (isWeak())
return 1;
if (isCommon() && Other->isCommon()) {
if (Config->WarnCommon)
warn("multiple common of " + getName());
return 0;
}
if (isCommon()) {
if (Config->WarnCommon)
warn("common " + getName() + " is overridden");
return 1;
}
if (Other->isCommon()) {
if (Config->WarnCommon)
warn("common " + getName() + " is overridden");
return -1;
}
auto *OldSym = cast<Defined>(this);
auto *NewSym = cast<Defined>(Other);
if (Other->File && isa<BitcodeFile>(Other->File))
return 0;
if (!OldSym->Section && !NewSym->Section && OldSym->Value == NewSym->Value &&
NewSym->Binding == STB_GLOBAL)
return -1;
return 0;
}
static void reportDuplicate(Symbol *Sym, InputFile *NewFile,
InputSectionBase *ErrSec, uint64_t ErrOffset) {
if (Config->AllowMultipleDefinition)
return;
Defined *D = cast<Defined>(Sym);
if (!D->Section || !ErrSec) {
error("duplicate symbol: " + toString(*Sym) + "\n>>> defined in " +
toString(Sym->File) + "\n>>> defined in " + toString(NewFile));
return;
}
// Construct and print an error message in the form of:
//
// ld.lld: error: duplicate symbol: foo
// >>> defined at bar.c:30
// >>> bar.o (/home/alice/src/bar.o)
// >>> defined at baz.c:563
// >>> baz.o in archive libbaz.a
auto *Sec1 = cast<InputSectionBase>(D->Section);
std::string Src1 = Sec1->getSrcMsg(*Sym, D->Value);
std::string Obj1 = Sec1->getObjMsg(D->Value);
std::string Src2 = ErrSec->getSrcMsg(*Sym, ErrOffset);
std::string Obj2 = ErrSec->getObjMsg(ErrOffset);
std::string Msg = "duplicate symbol: " + toString(*Sym) + "\n>>> defined at ";
if (!Src1.empty())
Msg += Src1 + "\n>>> ";
Msg += Obj1 + "\n>>> defined at ";
if (!Src2.empty())
Msg += Src2 + "\n>>> ";
Msg += Obj2;
error(Msg);
}
void Symbol::resolveCommon(const CommonSymbol &Other) {
int Cmp = compare(&Other);
if (Cmp < 0)
return;
if (Cmp > 0) {
replace(Other);
return;
}
CommonSymbol *OldSym = cast<CommonSymbol>(this);
OldSym->Alignment = std::max(OldSym->Alignment, Other.Alignment);
if (OldSym->Size < Other.Size) {
OldSym->File = Other.File;
OldSym->Size = Other.Size;
}
}
void Symbol::resolveDefined(const Defined &Other) {
int Cmp = compare(&Other);
if (Cmp > 0)
replace(Other);
else if (Cmp == 0)
reportDuplicate(this, Other.File,
dyn_cast_or_null<InputSectionBase>(Other.Section),
Other.Value);
}
template <class LazyT> void Symbol::resolveLazy(const LazyT &Other) {
if (!isUndefined())
return;
// An undefined weak will not fetch archive members. See comment on Lazy in
// Symbols.h for the details.
if (isWeak()) {
uint8_t Ty = Type;
replace(Other);
Type = Ty;
Binding = STB_WEAK;
return;
}
Other.fetch();
}
void Symbol::resolveShared(const SharedSymbol &Other) {
if (Visibility == STV_DEFAULT && (isUndefined() || isLazy())) {
// An undefined symbol with non default visibility must be satisfied
// in the same DSO.
uint8_t Bind = Binding;
replace(Other);
Binding = Bind;
}
}
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