llvm-project/lld/ELF/Symbols.h

420 lines
14 KiB
C++

//===- Symbols.h ------------------------------------------------*- C++ -*-===//
//
// The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines various types of Symbols.
//
//===----------------------------------------------------------------------===//
#ifndef LLD_ELF_SYMBOLS_H
#define LLD_ELF_SYMBOLS_H
#include "InputSection.h"
#include "lld/Common/LLVM.h"
#include "lld/Common/Strings.h"
#include "llvm/Object/Archive.h"
#include "llvm/Object/ELF.h"
namespace lld {
namespace elf {
class Symbol;
class InputFile;
} // namespace elf
std::string toString(const elf::Symbol &);
std::string toString(const elf::InputFile *);
namespace elf {
class ArchiveFile;
class BitcodeFile;
class BssSection;
class InputFile;
class LazyObjFile;
template <class ELFT> class ObjFile;
class OutputSection;
template <class ELFT> class SharedFile;
// This is a StringRef-like container that doesn't run strlen().
//
// ELF string tables contain a lot of null-terminated strings. Most of them
// are not necessary for the linker because they are names of local symbols,
// and the linker doesn't use local symbol names for name resolution. So, we
// use this class to represents strings read from string tables.
struct StringRefZ {
StringRefZ(const char *S) : Data(S), Size(-1) {}
StringRefZ(StringRef S) : Data(S.data()), Size(S.size()) {}
const char *Data;
const uint32_t Size;
};
// The base class for real symbol classes.
class Symbol {
public:
enum Kind {
PlaceholderKind,
DefinedKind,
SharedKind,
UndefinedKind,
LazyArchiveKind,
LazyObjectKind,
};
Kind kind() const { return static_cast<Kind>(SymbolKind); }
// The file from which this symbol was created.
InputFile *File;
protected:
const char *NameData;
mutable uint32_t NameSize;
public:
uint32_t DynsymIndex = 0;
uint32_t GotIndex = -1;
uint32_t PltIndex = -1;
uint32_t GlobalDynIndex = -1;
// This field is a index to the symbol's version definition.
uint32_t VerdefIndex = -1;
// Version definition index.
uint16_t VersionId;
// An index into the .branch_lt section on PPC64.
uint16_t PPC64BranchltIndex = -1;
// Symbol binding. This is not overwritten by replaceSymbol 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;
// The following fields have the same meaning as the ELF symbol attributes.
uint8_t Type; // symbol type
uint8_t StOther; // st_other field value
uint8_t SymbolKind;
// 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;
// False if LTO shouldn't inline whatever this symbol points to. If a symbol
// is overwritten after LTO, LTO shouldn't inline the symbol because it
// doesn't know the final contents of the symbol.
unsigned CanInline : 1;
// True if this symbol is specified by --trace-symbol option.
unsigned Traced : 1;
bool includeInDynsym() const;
uint8_t computeBinding() const;
bool isWeak() const { return Binding == llvm::ELF::STB_WEAK; }
bool isUndefined() const { return SymbolKind == UndefinedKind; }
bool isDefined() const { return SymbolKind == DefinedKind; }
bool isShared() const { return SymbolKind == SharedKind; }
bool isLocal() const { return Binding == llvm::ELF::STB_LOCAL; }
bool isLazy() const {
return SymbolKind == LazyArchiveKind || SymbolKind == LazyObjectKind;
}
// True if this is an undefined weak symbol. This only works once
// all input files have been added.
bool isUndefWeak() const {
// See comment on lazy symbols for details.
return isWeak() && (isUndefined() || isLazy());
}
StringRef getName() const {
if (NameSize == (uint32_t)-1)
NameSize = strlen(NameData);
return {NameData, NameSize};
}
void setName(StringRef S) {
NameData = S.data();
NameSize = S.size();
}
void parseSymbolVersion();
bool isInGot() const { return GotIndex != -1U; }
bool isInPlt() const { return PltIndex != -1U; }
bool isInPPC64Branchlt() const { return PPC64BranchltIndex != 0xffff; }
uint64_t getVA(int64_t Addend = 0) const;
uint64_t getGotOffset() const;
uint64_t getGotVA() const;
uint64_t getGotPltOffset() const;
uint64_t getGotPltVA() const;
uint64_t getPltVA() const;
uint64_t getPPC64LongBranchTableVA() const;
uint64_t getPPC64LongBranchOffset() const;
uint64_t getSize() const;
OutputSection *getOutputSection() const;
protected:
Symbol(Kind K, InputFile *File, StringRefZ Name, uint8_t Binding,
uint8_t StOther, uint8_t Type)
: File(File), NameData(Name.Data), NameSize(Name.Size), Binding(Binding),
Type(Type), StOther(StOther), SymbolKind(K), NeedsPltAddr(false),
IsInIplt(false), IsInIgot(false), IsPreemptible(false),
Used(!Config->GcSections), NeedsTocRestore(false),
ScriptDefined(false) {}
public:
// True the symbol should point to its PLT entry.
// For SharedSymbol only.
unsigned NeedsPltAddr : 1;
// True if this symbol is in the Iplt sub-section of the Plt.
unsigned IsInIplt : 1;
// True if this symbol is in the Igot sub-section of the .got.plt or .got.
unsigned IsInIgot : 1;
// True if this symbol is preemptible at load time.
unsigned IsPreemptible : 1;
// True if an undefined or shared symbol is used from a live section.
unsigned Used : 1;
// True if a call to this symbol needs to be followed by a restore of the
// PPC64 toc pointer.
unsigned NeedsTocRestore : 1;
// True if this symbol is defined by a linker script.
unsigned ScriptDefined : 1;
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; }
};
// Represents a symbol that is defined in the current output file.
class Defined : public Symbol {
public:
Defined(InputFile *File, StringRefZ Name, uint8_t Binding, uint8_t StOther,
uint8_t Type, uint64_t Value, uint64_t Size, SectionBase *Section)
: Symbol(DefinedKind, File, Name, Binding, StOther, Type), Value(Value),
Size(Size), Section(Section) {}
static bool classof(const Symbol *S) { return S->isDefined(); }
uint64_t Value;
uint64_t Size;
SectionBase *Section;
};
class Undefined : public Symbol {
public:
Undefined(InputFile *File, StringRefZ Name, uint8_t Binding, uint8_t StOther,
uint8_t Type)
: Symbol(UndefinedKind, File, Name, Binding, StOther, Type) {}
static bool classof(const Symbol *S) { return S->kind() == UndefinedKind; }
};
class SharedSymbol : public Symbol {
public:
static bool classof(const Symbol *S) { return S->kind() == SharedKind; }
SharedSymbol(InputFile &File, StringRef Name, uint8_t Binding,
uint8_t StOther, uint8_t Type, uint64_t Value, uint64_t Size,
uint32_t Alignment, uint32_t VerdefIndex)
: Symbol(SharedKind, &File, Name, Binding, StOther, Type),
Alignment(Alignment), Value(Value), Size(Size) {
this->VerdefIndex = VerdefIndex;
// GNU ifunc is a mechanism to allow user-supplied functions to
// resolve PLT slot values at load-time. This is contrary to the
// regular symbol resolution scheme in which symbols are resolved just
// by name. Using this hook, you can program how symbols are solved
// for you program. For example, you can make "memcpy" to be resolved
// to a SSE-enabled version of memcpy only when a machine running the
// program supports the SSE instruction set.
//
// Naturally, such symbols should always be called through their PLT
// slots. What GNU ifunc symbols point to are resolver functions, and
// calling them directly doesn't make sense (unless you are writing a
// loader).
//
// For DSO symbols, we always call them through PLT slots anyway.
// So there's no difference between GNU ifunc and regular function
// symbols if they are in DSOs. So we can handle GNU_IFUNC as FUNC.
if (this->Type == llvm::ELF::STT_GNU_IFUNC)
this->Type = llvm::ELF::STT_FUNC;
}
template <class ELFT> SharedFile<ELFT> &getFile() const {
return *cast<SharedFile<ELFT>>(File);
}
uint32_t Alignment;
uint64_t Value; // st_value
uint64_t Size; // st_size
};
// LazyArchive and LazyObject represent a symbols that is not yet in the link,
// but we know where to find it if needed. If the resolver finds both Undefined
// and Lazy for the same name, it will ask the Lazy to load a file.
//
// A special complication is the handling of weak undefined symbols. They should
// not load a file, but we have to remember we have seen both the weak undefined
// and the lazy. We represent that with a lazy symbol with a weak binding. This
// means that code looking for undefined symbols normally also has to take lazy
// symbols into consideration.
// 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.
class LazyArchive : public Symbol {
public:
LazyArchive(InputFile &File, uint8_t Type,
const llvm::object::Archive::Symbol S)
: Symbol(LazyArchiveKind, &File, S.getName(), llvm::ELF::STB_GLOBAL,
llvm::ELF::STV_DEFAULT, Type),
Sym(S) {}
static bool classof(const Symbol *S) { return S->kind() == LazyArchiveKind; }
InputFile *fetch();
MemoryBufferRef getMemberBuffer();
private:
const llvm::object::Archive::Symbol Sym;
};
// LazyObject symbols represents symbols in object files between
// --start-lib and --end-lib options.
class LazyObject : public Symbol {
public:
LazyObject(InputFile &File, uint8_t Type, StringRef Name)
: Symbol(LazyObjectKind, &File, Name, llvm::ELF::STB_GLOBAL,
llvm::ELF::STV_DEFAULT, Type) {}
static bool classof(const Symbol *S) { return S->kind() == LazyObjectKind; }
};
// Some linker-generated symbols need to be created as
// Defined symbols.
struct ElfSym {
// __bss_start
static Defined *Bss;
// etext and _etext
static Defined *Etext1;
static Defined *Etext2;
// edata and _edata
static Defined *Edata1;
static Defined *Edata2;
// end and _end
static Defined *End1;
static Defined *End2;
// The _GLOBAL_OFFSET_TABLE_ symbol is defined by target convention to
// be at some offset from the base of the .got section, usually 0 or
// the end of the .got.
static Defined *GlobalOffsetTable;
// _gp, _gp_disp and __gnu_local_gp symbols. Only for MIPS.
static Defined *MipsGp;
static Defined *MipsGpDisp;
static Defined *MipsLocalGp;
// __rel{,a}_iplt_{start,end} symbols.
static Defined *RelaIpltStart;
static Defined *RelaIpltEnd;
};
// A buffer class that is large enough to hold any Symbol-derived
// object. We allocate memory using this class and instantiate a symbol
// using the placement new.
union SymbolUnion {
alignas(Defined) char A[sizeof(Defined)];
alignas(Undefined) char C[sizeof(Undefined)];
alignas(SharedSymbol) char D[sizeof(SharedSymbol)];
alignas(LazyArchive) char E[sizeof(LazyArchive)];
alignas(LazyObject) char F[sizeof(LazyObject)];
};
void printTraceSymbol(Symbol *Sym);
template <typename T, typename... ArgT>
void replaceSymbol(Symbol *S, ArgT &&... Arg) {
using llvm::ELF::STT_TLS;
static_assert(std::is_trivially_destructible<T>(),
"Symbol types must be trivially destructible");
static_assert(sizeof(T) <= sizeof(SymbolUnion), "SymbolUnion too small");
static_assert(alignof(T) <= alignof(SymbolUnion),
"SymbolUnion not aligned enough");
assert(static_cast<Symbol *>(static_cast<T *>(nullptr)) == nullptr &&
"Not a Symbol");
Symbol Sym = *S;
new (S) T(std::forward<ArgT>(Arg)...);
S->VersionId = Sym.VersionId;
S->Visibility = Sym.Visibility;
S->IsUsedInRegularObj = Sym.IsUsedInRegularObj;
S->ExportDynamic = Sym.ExportDynamic;
S->CanInline = Sym.CanInline;
S->Traced = Sym.Traced;
S->ScriptDefined = Sym.ScriptDefined;
// Symbols representing thread-local variables must be referenced by
// TLS-aware relocations, and non-TLS symbols must be reference by
// non-TLS relocations, so there's a clear distinction between TLS
// and non-TLS symbols. It is an error if the same symbol is defined
// as a TLS symbol in one file and as a non-TLS symbol in other file.
bool TlsMismatch = (Sym.Type == STT_TLS && S->Type != STT_TLS) ||
(Sym.Type != STT_TLS && S->Type == STT_TLS);
if (Sym.SymbolKind != Symbol::PlaceholderKind && TlsMismatch && !Sym.isLazy())
error("TLS attribute mismatch: " + toString(Sym) + "\n>>> defined in " +
toString(Sym.File) + "\n>>> defined in " + toString(S->File));
// Print out a log message if --trace-symbol was specified.
// This is for debugging.
if (S->Traced)
printTraceSymbol(S);
}
void maybeWarnUnorderableSymbol(const Symbol *Sym);
} // namespace elf
} // namespace lld
#endif