Relocations.cpp

//===- Relocations.cpp ----------------------------------------------------===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains platform-independent functions to process relocations.
// I'll describe the overview of this file here.
//
// Simple relocations are easy to handle for the linker. For example,
// for R_X86_64_PC64 relocs, the linker just has to fix up locations
// with the relative offsets to the target symbols. It would just be
// reading records from relocation sections and applying them to output.
//
// But not all relocations are that easy to handle. For example, for
// R_386_GOTOFF relocs, the linker has to create new GOT entries for
// symbols if they don't exist, and fix up locations with GOT entry
// offsets from the beginning of GOT section. So there is more than
// fixing addresses in relocation processing.
//
// ELF defines a large number of complex relocations.
//
// The functions in this file analyze relocations and do whatever needs
// to be done. It includes, but not limited to, the following.
//
//  - create GOT/PLT entries
//  - create new relocations in .dynsym to let the dynamic linker resolve
//    them at runtime (since ELF supports dynamic linking, not all
//    relocations can be resolved at link-time)
//  - create COPY relocs and reserve space in .bss
//  - replace expensive relocs (in terms of runtime cost) with cheap ones
//  - error out infeasible combinations such as PIC and non-relative relocs
//
// Note that the functions in this file don't actually apply relocations
// because it doesn't know about the output file nor the output file buffer.
// It instead stores Relocation objects to InputSection's Relocations
// vector to let it apply later in InputSection::writeTo.
//
//===----------------------------------------------------------------------===//

#include "Relocations.h"
#include "Config.h"
#include "LinkerScript.h"
#include "Memory.h"
#include "OutputSections.h"
#include "Strings.h"
#include "SymbolTable.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "Thunks.h"

#include "llvm/Support/Endian.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>

using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace llvm::support::endian;

using namespace lld;
using namespace lld::elf;

// Construct a message in the following format.
//
// >>> defined in /home/alice/src/foo.o
// >>> referenced by bar.c:12 (/home/alice/src/bar.c:12)
// >>>               /home/alice/src/bar.o:(.text+0x1)
template <class ELFT>
static std::string getLocation(InputSectionBase &S, const SymbolBody &Sym,
                               uint64_t Off) {
  std::string Msg =
      "\n>>> defined in " + toString(Sym.getFile()) + "\n>>> referenced by ";
  std::string Src = S.getSrcMsg<ELFT>(Off);
  if (!Src.empty())
    Msg += Src + "\n>>>               ";
  return Msg + S.getObjMsg<ELFT>(Off);
}

static bool isPreemptible(const SymbolBody &Body, uint32_t Type) {
  // In case of MIPS GP-relative relocations always resolve to a definition
  // in a regular input file, ignoring the one-definition rule. So we,
  // for example, should not attempt to create a dynamic relocation even
  // if the target symbol is preemptible. There are two two MIPS GP-relative
  // relocations R_MIPS_GPREL16 and R_MIPS_GPREL32. But only R_MIPS_GPREL16
  // can be against a preemptible symbol.
  // To get MIPS relocation type we apply 0xff mask. In case of O32 ABI all
  // relocation types occupy eight bit. In case of N64 ABI we extract first
  // relocation from 3-in-1 packet because only the first relocation can
  // be against a real symbol.
  if (Config->EMachine == EM_MIPS && (Type & 0xff) == R_MIPS_GPREL16)
    return false;
  return Body.isPreemptible();
}

// This function is similar to the `handleTlsRelocation`. MIPS does not
// support any relaxations for TLS relocations so by factoring out MIPS
// handling in to the separate function we can simplify the code and do not
// pollute other `handleTlsRelocation` by MIPS `ifs` statements.
// Mips has a custom MipsGotSection that handles the writing of GOT entries
// without dynamic relocations.
template <class ELFT>
static unsigned handleMipsTlsRelocation(uint32_t Type, SymbolBody &Body,
                                        InputSectionBase &C, uint64_t Offset,
                                        int64_t Addend, RelExpr Expr) {
  if (Expr == R_MIPS_TLSLD) {
    if (InX::MipsGot->addTlsIndex() && Config->Pic)
      In<ELFT>::RelaDyn->addReloc({Target->TlsModuleIndexRel, InX::MipsGot,
                                   InX::MipsGot->getTlsIndexOff(), false,
                                   nullptr, 0});
    C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
    return 1;
  }

  if (Expr == R_MIPS_TLSGD) {
    if (InX::MipsGot->addDynTlsEntry(Body) && Body.isPreemptible()) {
      uint64_t Off = InX::MipsGot->getGlobalDynOffset(Body);
      In<ELFT>::RelaDyn->addReloc(
          {Target->TlsModuleIndexRel, InX::MipsGot, Off, false, &Body, 0});
      if (Body.isPreemptible())
        In<ELFT>::RelaDyn->addReloc({Target->TlsOffsetRel, InX::MipsGot,
                                     Off + Config->Wordsize, false, &Body, 0});
    }
    C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
    return 1;
  }
  return 0;
}

// This function is similar to the `handleMipsTlsRelocation`. ARM also does not
// support any relaxations for TLS relocations. ARM is logically similar to Mips
// in how it handles TLS, but Mips uses its own custom GOT which handles some
// of the cases that ARM uses GOT relocations for.
//
// We look for TLS global dynamic and local dynamic relocations, these may
// require the generation of a pair of GOT entries that have associated
// dynamic relocations. When the results of the dynamic relocations can be
// resolved at static link time we do so. This is necessary for static linking
// as there will be no dynamic loader to resolve them at load-time.
//
// The pair of GOT entries created are of the form
// GOT[e0] Module Index (Used to find pointer to TLS block at run-time)
// GOT[e1] Offset of symbol in TLS block
template <class ELFT>
static unsigned handleARMTlsRelocation(uint32_t Type, SymbolBody &Body,
                                       InputSectionBase &C, uint64_t Offset,
                                       int64_t Addend, RelExpr Expr) {
  // The Dynamic TLS Module Index Relocation for a symbol defined in an
  // executable is always 1. If the target Symbol is not preemptible then
  // we know the offset into the TLS block at static link time.
  bool NeedDynId = Body.isPreemptible() || Config->Shared;
  bool NeedDynOff = Body.isPreemptible();

  auto AddTlsReloc = [&](uint64_t Off, uint32_t Type, SymbolBody *Dest,
                         bool Dyn) {
    if (Dyn)
      In<ELFT>::RelaDyn->addReloc({Type, InX::Got, Off, false, Dest, 0});
    else
      InX::Got->Relocations.push_back({R_ABS, Type, Off, 0, Dest});
  };

  // Local Dynamic is for access to module local TLS variables, while still
  // being suitable for being dynamically loaded via dlopen.
  // GOT[e0] is the module index, with a special value of 0 for the current
  // module. GOT[e1] is unused. There only needs to be one module index entry.
  if (Expr == R_TLSLD_PC && InX::Got->addTlsIndex()) {
    AddTlsReloc(InX::Got->getTlsIndexOff(), Target->TlsModuleIndexRel,
                NeedDynId ? nullptr : &Body, NeedDynId);
    C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
    return 1;
  }

  // Global Dynamic is the most general purpose access model. When we know
  // the module index and offset of symbol in TLS block we can fill these in
  // using static GOT relocations.
  if (Expr == R_TLSGD_PC) {
    if (InX::Got->addDynTlsEntry(Body)) {
      uint64_t Off = InX::Got->getGlobalDynOffset(Body);
      AddTlsReloc(Off, Target->TlsModuleIndexRel, &Body, NeedDynId);
      AddTlsReloc(Off + Config->Wordsize, Target->TlsOffsetRel, &Body,
                  NeedDynOff);
    }
    C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
    return 1;
  }
  return 0;
}

// Returns the number of relocations processed.
template <class ELFT>
static unsigned
handleTlsRelocation(uint32_t Type, SymbolBody &Body, InputSectionBase &C,
                    typename ELFT::uint Offset, int64_t Addend, RelExpr Expr) {
  if (!(C.Flags & SHF_ALLOC))
    return 0;

  if (!Body.isTls())