lld/ELF/Symbols.h

//===- Symbols.h ------------------------------------------------*- C++ -*-===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// All symbols are handled as SymbolBodies regardless of their types.
// This file defines various types of SymbolBodies.
//
//===----------------------------------------------------------------------===//

#ifndef LLD_ELF_SYMBOLS_H
#define LLD_ELF_SYMBOLS_H

#include "InputSection.h"

#include "lld/Core/LLVM.h"
#include "llvm/Object/Archive.h"
#include "llvm/Object/ELF.h"

namespace lld {
namespace elf {

class ArchiveFile;
class InputFile;
class SymbolBody;
template <class ELFT> class ObjectFile;
template <class ELFT> class OutputSection;
template <class ELFT> class OutputSectionBase;
template <class ELFT> class SharedFile;

// Returns a demangled C++ symbol name. If Name is not a mangled
// name or the system does not provide __cxa_demangle function,
// it returns the unmodified string.
std::string demangle(StringRef Name);

// A real symbol object, SymbolBody, is usually accessed indirectly
// through a Symbol. There's always one Symbol for each symbol name.
// The resolver updates SymbolBody pointers as it resolves symbols.
// Symbol also holds computed properties of symbol names.
struct Symbol {
  SymbolBody *Body;

  // Symbol binding. This is on the Symbol to track changes during resolution.
  // In particular:
  // An undefined weak is still weak when it resolves to a shared library.
  // An undefined weak will not fetch archive members, but we have to remember
  // it is weak.
  uint8_t Binding;

  // Symbol visibility. This is the computed minimum visibility of all
  // observed non-DSO symbols.
  unsigned Visibility : 2;

  // True if the symbol was used for linking and thus need to be added to the
  // output file's symbol table. This is true for all symbols except for
  // unreferenced DSO symbols and bitcode symbols that are unreferenced except
  // by other bitcode objects.
  unsigned IsUsedInRegularObj : 1;

  // If this flag is true and the symbol has protected or default visibility, it
  // will appear in .dynsym. This flag is set by interposable DSO symbols in
  // executables, by most symbols in DSOs and executables built with
  // --export-dynamic, and by dynamic lists.
  unsigned ExportDynamic : 1;

  // This flag acts as an additional filter on the dynamic symbol list. It is
  // set if there is no version script, or if the symbol appears in the global
  // section of the version script.
  unsigned VersionScriptGlobal : 1;

  bool includeInDynsym() const;

  bool isWeak() const { return Binding == llvm::ELF::STB_WEAK; }
};

// The base class for real symbol classes.
class SymbolBody {
  void init();

public:
  enum Kind {
    DefinedFirst,
    DefinedRegularKind = DefinedFirst,
    SharedKind,
    DefinedCommonKind,
    DefinedBitcodeKind,
    DefinedSyntheticKind,
    DefinedLast = DefinedSyntheticKind,
    UndefinedKind,
    LazyArchiveKind,
    LazyObjectKind,
  };

  Kind kind() const { return static_cast<Kind>(SymbolKind); }

  bool isWeak() const { return Binding == llvm::ELF::STB_WEAK; }
  bool isUndefined() const { return SymbolKind == UndefinedKind; }
  bool isDefined() const { return SymbolKind <= DefinedLast; }
  bool isCommon() const { return SymbolKind == DefinedCommonKind; }
  bool isLazy() const {
    return SymbolKind == LazyArchiveKind || SymbolKind == LazyObjectKind;
  }
  bool isShared() const { return SymbolKind == SharedKind; }
  bool isLocal() const { return Binding == llvm::ELF::STB_LOCAL; }
  bool isPreemptible() const;

  // Returns the symbol name.
  StringRef getName() const {
    assert(!isLocal());
    return StringRef(Name.S, Name.Len);
  }
  uint32_t getNameOffset() const {
    assert(isLocal());
    return NameOffset;
  }

  uint8_t getVisibility() const { return StOther & 0x3; }

  unsigned DynsymIndex = 0;
  uint32_t GlobalDynIndex = -1;
  uint32_t GotIndex = -1;
  uint32_t GotPltIndex = -1;
  uint32_t PltIndex = -1;
  uint32_t ThunkIndex = -1;
  bool hasGlobalDynIndex() { return GlobalDynIndex != uint32_t(-1); }
  bool isInGot() const { return GotIndex != -1U; }
  bool isInPlt() const { return PltIndex != -1U; }
  bool hasThunk() const { return ThunkIndex != -1U; }

  template <class ELFT>
  typename ELFT::uint getVA(typename ELFT::uint Addend = 0) const;

  template <class ELFT> typename ELFT::uint getGotOffset() const;
  template <class ELFT> typename ELFT::uint getGotVA() const;
  template <class ELFT> typename ELFT::uint getGotPltOffset() const;
  template <class ELFT> typename ELFT::uint getGotPltVA() const;
  template <class ELFT> typename ELFT::uint getPltVA() const;
  template <class ELFT> typename ELFT::uint getThunkVA() const;
  template <class ELFT> typename ELFT::uint getSize() const;

  // A SymbolBody has a backreference to a Symbol. Originally they are
  // doubly-linked. A backreference will never change. But the pointer
  // in the Symbol may be mutated by the resolver. If you have a
  // pointer P to a SymbolBody and are not sure whether the resolver
  // has chosen the object among other objects having the same name,
  // you can access P->Backref->Body to get the resolver's result.
  SymbolBody &repl() { return Backref ? *Backref->Body : *this; }

  // Decides which symbol should "win" in the symbol table, this or
  // the Other. Returns 1 if this wins, -1 if the Other wins, or 0 if
  // they are duplicate (conflicting) symbols.
  int compare(SymbolBody *Other);

protected:
  SymbolBody(Kind K, StringRef Name, uint8_t Binding, uint8_t StOther,
             uint8_t Type);

  SymbolBody(Kind K, uint32_t NameOffset, uint8_t StOther, uint8_t Type);

  const unsigned SymbolKind : 8;

public:
  // True if this symbol can be omitted from the symbol table if nothing else
  // requires it to be there. Right now this is only used for linkonce_odr in
  // LTO, but we could add the feature to ELF. It would be similar to
  // MachO's .weak_def_can_be_hidden.
  unsigned CanOmitFromDynSym : 1;

  // True if the linker has to generate a copy relocation for this shared
  // symbol or if the symbol should point to its plt entry.
  unsigned NeedsCopyOrPltAddr : 1;

  // True if the symbol is undefined and comes from a bitcode file. We need to
  // keep track of this because undefined symbols only prevent internalization
  // of bitcode symbols if they did not come from a bitcode file.
  unsigned IsUndefinedBitcode : 1;

  // The following fields have the same meaning as the ELF symbol attributes.
  uint8_t Type;    // symbol type
  uint8_t Binding; // symbol binding
  uint8_t StOther; // st_other field value

  bool isSection() const { return Type == llvm::ELF::STT_SECTION; }
  bool isTls() const { return Type == llvm::ELF::STT_TLS; }
  bool isFunc() const { return Type == llvm::ELF::STT_FUNC; }
  bool isGnuIFunc() const { return Type == llvm::ELF::STT_GNU_IFUNC; }
  bool isObject() const { return Type == llvm::ELF::STT_OBJECT; }
  bool isFile() const { return Type == llvm::ELF::STT_FILE; }

  Symbol *Backref = nullptr;

protected:
  struct Str {
    const char *S;
    size_t Len;
  };
  union {
    Str Name;
    uint32_t NameOffset;
  };
};

// The base class for any defined symbols.
class Defined : public SymbolBody {
public:
  Defined(Kind K, StringRef Name, uint8_t Binding, uint8_t StOther,
          uint8_t Type);
  Defined(Kind K, uint32_t NameOffset, uint8_t StOther, uint8_t Type);
  static bool classof(const SymbolBody *S) { return S->isDefined(); }
};

// The defined symbol in LLVM bitcode files.
class DefinedBitcode : public Defined {
public:
  DefinedBitcode(StringRef Name, bool IsWeak, uint8_t StOther);
  static bool classof(const SymbolBody *S);
};

class DefinedCommon : public Defined {
public:
  DefinedCommon(StringRef N, uint64_t Size, uint64_t Alignment, uint8_t Binding,
                uint8_t StOther, uint8_t Type);

  static bool classof(const SymbolBody *S) {
    return S->kind() == SymbolBody::DefinedCommonKind;
  }

  // The output offset of this common symbol in the output bss. Computed by the
  // writer.
  uint64_t OffsetInBss;

  // The maximum alignment we have seen for this symbol.
  uint64_t Alignment;

  uint64_t Size;
};

// Regular defined symbols read from object file symbol tables.
template <class ELFT> class DefinedRegular : public Defined {
  typedef typename ELFT::Sym Elf_Sym;
  typedef typename ELFT::uint uintX_t;

public:
  DefinedRegular(StringRef Name, const Elf_Sym &Sym,
                 InputSectionBase<ELFT> *Section)
      : Defined(SymbolBody::DefinedRegularKind, Name, Sym.getBinding(),
                Sym.st_other, Sym.getType()),
        Value(Sym.st_value), Size(Sym.st_size),
        Section(Section ? Section->Repl : NullInputSection) {}

  DefinedRegular(const Elf_Sym &Sym, InputSectionBase<ELFT> *Section)
      : Defined(SymbolBody::DefinedRegularKind, Sym.st_name, Sym.st_other,
                Sym.getType()),
        Value(Sym.st_value), Size(Sym.st_size),
        Section(Section ? Section->Repl : NullInputSection) {
    assert(isLocal());
  }

  DefinedRegular(StringRef Name, uint8_t Binding, uint8_t StOther)
      : Defined(SymbolBody::DefinedRegularKind, Name, Binding, StOther,
                llvm::ELF::STT_NOTYPE),
        Value(0), Size(0), Section(NullInputSection) {}

  static bool classof(const SymbolBody *S) {
    return S->kind() == SymbolBody::DefinedRegularKind;
  }

  uintX_t Value;
  uintX_t Size;

  // The input section this symbol belongs to. Notice that this is
  // a reference to a pointer. We are using two levels of indirections
  // because of ICF. If ICF decides two sections need to be merged, it
  // manipulates this Section pointers so that they point to the same
  // section. This is a bit tricky, so be careful to not be confused.
  // If this is null, the symbol is an absolute symbol.
  InputSectionBase<ELFT> *&Section;

private:
  static InputSectionBase<ELFT> *NullInputSection;
};

template <class ELFT>
InputSectionBase<ELFT> *DefinedRegular<ELFT>::NullInputSection;

// DefinedSynthetic is a class to represent linker-generated ELF symbols.
// The difference from the regular symbol is that DefinedSynthetic symbols
// don't belong to any input files or sections. Thus, its constructor
// takes an output section to calculate output VA, etc.
template <class ELFT> class DefinedSynthetic : public Defined {
public:
  typedef typename ELFT::uint uintX_t;
  DefinedSynthetic(StringRef N, uintX_t Value,
                   OutputSectionBase<ELFT> &Section);

  static bool classof(const SymbolBody *S) {
    return S->kind() == SymbolBody::DefinedSyntheticKind;
  }

  // Special value designates that the symbol 'points'
  // to the end of the section.
  static const uintX_t SectionEnd = uintX_t(-1);

  uintX_t Value;
  const OutputSectionBase<ELFT> &Section;
};

class Undefined : public SymbolBody {
public:
  Undefined(StringRef Name, uint8_t Binding, uint8_t StOther, uint8_t Type,
            uint64_t Size, bool IsBitcode);
  Undefined(uint32_t NameOffset, uint8_t StOther, uint8_t Type, uint64_t Size);

  uint64_t Size;

  static bool classof(const SymbolBody *S) {
    return S->kind() == UndefinedKind;
  }
};

template <class ELFT> class SharedSymbol : public Defined {
  typedef typename ELFT::Sym Elf_Sym;
  typedef typename ELFT::Verdef Elf_Verdef;
  typedef typename ELFT::uint uintX_t;

public:
  static bool classof(const SymbolBody *S) {
    return S->kind() == SymbolBody::SharedKind;
  }

  SharedSymbol(SharedFile<ELFT> *F, StringRef Name, const Elf_Sym &Sym,
               const Elf_Verdef *Verdef)
      : Defined(SymbolBody::SharedKind, Name, Sym.getBinding(), Sym.st_other,
                Sym.getType()),
        File(F), Sym(Sym), Verdef(Verdef) {
    // IFuncs defined in DSOs are treated as functions by the static linker.
    if (isGnuIFunc())
      Type = llvm::ELF::STT_FUNC;
  }

  SharedFile<ELFT> *File;
  const Elf_Sym &Sym;

  // This field is initially a pointer to the symbol's version definition. As
  // symbols are added to the version table, this field is replaced with the
  // version identifier to be stored in .gnu.version in the output file.
  union {
    const Elf_Verdef *Verdef;
    uint16_t VersionId;
  };

  // OffsetInBss is significant only when needsCopy() is true.
  uintX_t OffsetInBss = 0;

  bool needsCopy() const { return this->NeedsCopyOrPltAddr && !this->isFunc(); }
};

// This class represents a symbol defined in an archive file. It is
// created from an archive file header, and it knows how to load an
// object file from an archive to replace itself with a defined
// symbol. If the resolver finds both Undefined and Lazy for
// the same name, it will ask the Lazy to load a file.
class Lazy : public SymbolBody {
public:
  Lazy(SymbolBody::Kind K, StringRef Name)
      : SymbolBody(K, Name, llvm::ELF::STB_GLOBAL, llvm::ELF::STV_DEFAULT,
                   /* Type */ 0) {}

  static bool classof(const SymbolBody *S) { return S->isLazy(); }

  // Returns an object file for this symbol, or a nullptr if the file
  // was already returned.
  std::unique_ptr<InputFile> getFile();
};

// LazyArchive symbols represents symbols in archive files.
class LazyArchive : public Lazy {
public:
  LazyArchive(ArchiveFile *F, const llvm::object::Archive::Symbol S)
      : Lazy(LazyArchiveKind, S.getName()), File(F), Sym(S) {}

  static bool classof(const SymbolBody *S) {
    return S->kind() == LazyArchiveKind;
  }

  std::unique_ptr<InputFile> getFile();

private:
  ArchiveFile *File;
  const llvm::object::Archive::Symbol Sym;
};

// LazyObject symbols represents symbols in object files between
// --start-lib and --end-lib options.
class LazyObject : public Lazy {
public:
  LazyObject(StringRef Name, MemoryBufferRef M)
      : Lazy(LazyObjectKind, Name), MBRef(M) {}

  static bool classof(const SymbolBody *S) {
    return S->kind() == LazyObjectKind;
  }

  std::unique_ptr<InputFile> getFile();

private:
  MemoryBufferRef MBRef;
};

// Some linker-generated symbols need to be created as
// DefinedRegular symbols.
template <class ELFT> struct ElfSym {
  // The content for _etext and etext symbols.
  static DefinedRegular<ELFT> *Etext;
  static DefinedRegular<ELFT> *Etext2;

  // The content for _edata and edata symbols.
  static DefinedRegular<ELFT> *Edata;
  static DefinedRegular<ELFT> *Edata2;

  // The content for _end and end symbols.
  static DefinedRegular<ELFT> *End;
  static DefinedRegular<ELFT> *End2;

  // The content for _gp symbol for MIPS target.
  static SymbolBody *MipsGp;

  static SymbolBody *MipsLocalGp;
  static SymbolBody *MipsGpDisp;

  // __rel_iplt_start/__rel_iplt_end for signaling
  // where R_[*]_IRELATIVE relocations do live.
  static SymbolBody *RelaIpltStart;
  static SymbolBody *RelaIpltEnd;
};

template <class ELFT> DefinedRegular<ELFT> *ElfSym<ELFT>::Etext;
template <class ELFT> DefinedRegular<ELFT> *ElfSym<ELFT>::Etext2;
template <class ELFT> DefinedRegular<ELFT> *ElfSym<ELFT>::Edata;
template <class ELFT> DefinedRegular<ELFT> *ElfSym<ELFT>::Edata2;
template <class ELFT> DefinedRegular<ELFT> *ElfSym<ELFT>::End;
template <class ELFT> DefinedRegular<ELFT> *ElfSym<ELFT>::End2;
template <class ELFT> SymbolBody *ElfSym<ELFT>::MipsGp;
template <class ELFT> SymbolBody *ElfSym<ELFT>::MipsLocalGp;
template <class ELFT> SymbolBody *ElfSym<ELFT>::MipsGpDisp;
template <class ELFT> SymbolBody *ElfSym<ELFT>::RelaIpltStart;
template <class ELFT> SymbolBody *ElfSym<ELFT>::RelaIpltEnd;

} // namespace elf
} // namespace lld

#endif