lld/ELF/SymbolTable.cpp

//===- SymbolTable.cpp ----------------------------------------------------===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#include "SymbolTable.h"
#include "Config.h"
#include "Error.h"
#include "Symbols.h"
#include "Target.h"

using namespace llvm;
using namespace llvm::object;
using namespace llvm::ELF;

using namespace lld;
using namespace lld::elf2;

SymbolTable::SymbolTable() {}

bool SymbolTable::shouldUseRela() const {
  ELFKind K = getFirstELF()->getELFKind();
  return K == ELF64LEKind || K == ELF64BEKind;
}

void SymbolTable::addFile(std::unique_ptr<InputFile> File) {
  if (auto *AF = dyn_cast<ArchiveFile>(File.get())) {
    File.release();
    ArchiveFiles.emplace_back(AF);
    if (Config->WholeArchive) {
      for (MemoryBufferRef &MBRef : AF->getMembers())
        addFile(createELFFile<ObjectFile>(MBRef));
      return;
    }
    AF->parse();
    for (Lazy &Sym : AF->getLazySymbols())
      addLazy(&Sym);
    return;
  }
  if (auto *S = dyn_cast<SharedFileBase>(File.get())) {
    S->parseSoName();
    if (!IncludedSoNames.insert(S->getSoName()).second)
      return;
  }
  File->parse();
  addELFFile(cast<ELFFileBase>(File.release()));
}

static TargetInfo *createTarget(uint16_t EMachine) {
  switch (EMachine) {
  case EM_386:
    return new X86TargetInfo();
  case EM_AARCH64:
    return new AArch64TargetInfo();
  case EM_ARM:
    return new ARMTargetInfo();
  case EM_MIPS:
    return new MipsTargetInfo();
  case EM_PPC:
    return new PPCTargetInfo();
  case EM_PPC64:
    return new PPC64TargetInfo();
  case EM_X86_64:
    return new X86_64TargetInfo();
  }
  error("Unknown target machine");
}

void SymbolTable::addUndefinedSym(StringRef Name) {
  switch (getFirstELF()->getELFKind()) {
  case ELF32LEKind:
    addUndefinedSym<ELF32LE>(Name);
    break;
  case ELF32BEKind:
    addUndefinedSym<ELF32BE>(Name);
    break;
  case ELF64LEKind:
    addUndefinedSym<ELF64LE>(Name);
    break;
  case ELF64BEKind:
    addUndefinedSym<ELF64BE>(Name);
    break;
  default:
    llvm_unreachable("Invalid kind");
  }
}

template <class ELFT> void SymbolTable::addUndefinedSym(StringRef Name) {
  resolve<ELFT>(new (Alloc) Undefined<ELFT>(Name, Undefined<ELFT>::Optional));
}

template <class ELFT>
void SymbolTable::addSyntheticSym(StringRef Name, OutputSection<ELFT> &Section,
                                  typename ELFFile<ELFT>::uintX_t Value) {
  typedef typename DefinedSynthetic<ELFT>::Elf_Sym Elf_Sym;
  auto ESym = new (Alloc) Elf_Sym;
  memset(ESym, 0, sizeof(Elf_Sym));
  ESym->st_value = Value;
  auto Sym = new (Alloc) DefinedSynthetic<ELFT>(Name, *ESym, Section);
  resolve<ELFT>(Sym);
}

template <class ELFT> void SymbolTable::addIgnoredSym(StringRef Name) {
  auto Sym = new (Alloc)
      DefinedAbsolute<ELFT>(Name, DefinedAbsolute<ELFT>::IgnoreUndef);
  resolve<ELFT>(Sym);
}

template <class ELFT> void SymbolTable::init(uint16_t EMachine) {
  Target.reset(createTarget(EMachine));
  if (Config->Shared)
    return;
  EntrySym = new (Alloc) Undefined<ELFT>(
      Config->Entry.empty() ? Target->getDefaultEntry() : Config->Entry,
      Undefined<ELFT>::Required);
  resolve<ELFT>(EntrySym);

  // 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.
  addIgnoredSym<ELFT>("_GLOBAL_OFFSET_TABLE_");
}

template <class ELFT> void SymbolTable::addELFFile(ELFFileBase *File) {
  const ELFFileBase *Old = getFirstELF();
  if (auto *O = dyn_cast<ObjectFileBase>(File))
    ObjectFiles.emplace_back(O);
  else if (auto *S = dyn_cast<SharedFile<ELFT>>(File))
    SharedFiles.emplace_back(S);

  if (!Old)
    init<ELFT>(File->getEMachine());

  if (auto *O = dyn_cast<ObjectFileBase>(File)) {
    for (SymbolBody *Body : O->getSymbols())
      resolve<ELFT>(Body);
  }

  if (auto *S = dyn_cast<SharedFile<ELFT>>(File)) {
    for (SharedSymbol<ELFT> &Body : S->getSharedSymbols())
      resolve<ELFT>(&Body);
  }
}

void SymbolTable::addELFFile(ELFFileBase *File) {
  switch (File->getELFKind()) {
  case ELF32LEKind:
    addELFFile<ELF32LE>(File);
    break;
  case ELF32BEKind:
    addELFFile<ELF32BE>(File);
    break;
  case ELF64LEKind:
    addELFFile<ELF64LE>(File);
    break;
  case ELF64BEKind:
    addELFFile<ELF64BE>(File);
    break;
  default:
    llvm_unreachable("Invalid kind");
  }
}

template <class ELFT>
void SymbolTable::reportConflict(const Twine &Message, const SymbolBody &Old,
                                 const SymbolBody &New, bool Warning) {
  typedef typename ELFFile<ELFT>::Elf_Sym Elf_Sym;
  typedef typename ELFFile<ELFT>::Elf_Sym_Range Elf_Sym_Range;

  const Elf_Sym &OldE = cast<ELFSymbolBody<ELFT>>(Old).Sym;
  const Elf_Sym &NewE = cast<ELFSymbolBody<ELFT>>(New).Sym;
  ELFFileBase *OldFile = nullptr;
  ELFFileBase *NewFile = nullptr;

  for (const std::unique_ptr<ObjectFileBase> &F : ObjectFiles) {
    const auto &File = cast<ObjectFile<ELFT>>(*F);
    Elf_Sym_Range Syms = File.getObj().symbols(File.getSymbolTable());
    if (&OldE > Syms.begin() && &OldE < Syms.end())
      OldFile = F.get();
    if (&NewE > Syms.begin() && &NewE < Syms.end())
      NewFile = F.get();
  }

  std::string Msg = (Message + ": " + Old.getName() + " in " +
                     OldFile->getName() + " and " + NewFile->getName())
                        .str();
  if (Warning)
    warning(Msg);
  else
    error(Msg);
}

// This function resolves conflicts if there's an existing symbol with
// the same name. Decisions are made based on symbol type.
template <class ELFT> void SymbolTable::resolve(SymbolBody *New) {
  Symbol *Sym = insert(New);
  if (Sym->Body == New)
    return;

  SymbolBody *Existing = Sym->Body;

  if (Lazy *L = dyn_cast<Lazy>(Existing)) {
    if (New->isUndefined()) {
      if (New->isWeak()) {
        // See the explanation in SymbolTable::addLazy
        L->setUsedInRegularObj();
        L->setWeak();
        return;
      }
      addMemberFile(L);
      return;
    }

    // Found a definition for something also in an archive. Ignore the archive
    // definition.
    Sym->Body = New;
    return;
  }

  if (New->isTLS() != Existing->isTLS())
    reportConflict<ELFT>("TLS attribute mismatch for symbol", *Existing, *New,
                         false);

  // compare() returns -1, 0, or 1 if the lhs symbol is less preferable,
  // equivalent (conflicting), or more preferable, respectively.
  int comp = Existing->compare<ELFT>(New);
  if (comp < 0)
    Sym->Body = New;
  else if (comp == 0)
    reportConflict<ELFT>("duplicate symbol", *Existing, *New,
                         Config->AllowMultipleDefinition);
}

Symbol *SymbolTable::insert(SymbolBody *New) {
  // Find an existing Symbol or create and insert a new one.
  StringRef Name = New->getName();
  Symbol *&Sym = Symtab[Name];
  if (!Sym) {
    Sym = new (Alloc) Symbol(New);
    New->setBackref(Sym);
    return Sym;
  }
  New->setBackref(Sym);
  return Sym;
}

void SymbolTable::addLazy(Lazy *New) {
  Symbol *Sym = insert(New);
  if (Sym->Body == New)
    return;
  SymbolBody *Existing = Sym->Body;
  if (Existing->isDefined() || Existing->isLazy())
    return;
  Sym->Body = New;
  assert(Existing->isUndefined() && "Unexpected symbol kind.");

  // Weak undefined symbols should not fetch members from archives.
  // If we were to keep old symbol we would not know that an archive member was
  // available if a strong undefined symbol shows up afterwards in the link.
  // If a strong undefined symbol never shows up, this lazy symbol will
  // get to the end of the link and must be treated as the weak undefined one.
  // We set UsedInRegularObj in a similar way to what is done with shared
  // symbols and mark it as weak to reduce how many special cases are needed.
  if (Existing->isWeak()) {
    New->setUsedInRegularObj();
    New->setWeak();
    return;
  }
  addMemberFile(New);
}

void SymbolTable::addMemberFile(Lazy *Body) {
  std::unique_ptr<InputFile> File = Body->getMember();

  // getMember returns nullptr if the member was already read from the library.
  if (!File)
    return;

  addFile(std::move(File));
}

namespace lld {
namespace elf2 {
template void SymbolTable::addSyntheticSym(StringRef, OutputSection<ELF32LE> &,
                                           ELFFile<ELF32LE>::uintX_t);
template void SymbolTable::addSyntheticSym(StringRef, OutputSection<ELF32BE> &,
                                           ELFFile<ELF32BE>::uintX_t);
template void SymbolTable::addSyntheticSym(StringRef, OutputSection<ELF64LE> &,
                                           ELFFile<ELF64LE>::uintX_t);
template void SymbolTable::addSyntheticSym(StringRef, OutputSection<ELF64BE> &,
                                           ELFFile<ELF64BE>::uintX_t);

template void SymbolTable::addIgnoredSym<ELF32LE>(StringRef);
template void SymbolTable::addIgnoredSym<ELF32BE>(StringRef);
template void SymbolTable::addIgnoredSym<ELF64LE>(StringRef);
template void SymbolTable::addIgnoredSym<ELF64BE>(StringRef);
}
}