lld/COFF/Chunks.cpp

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

#include "Chunks.h"
#include "InputFiles.h"
#include "Writer.h"
#include "llvm/ADT/Hashing.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Object/COFF.h"
#include "llvm/Support/COFF.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>

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

namespace lld {
namespace coff {

SectionChunk::SectionChunk(ObjectFile *F, const coff_section *H)
    : File(F), Ptr(this), Header(H),
      Relocs(File->getCOFFObj()->getRelocations(Header)),
      NumRelocs(std::distance(Relocs.begin(), Relocs.end())) {
  // Initialize SectionName.
  File->getCOFFObj()->getSectionName(Header, SectionName);

  // Bit [20:24] contains section alignment. Both 0 and 1 mean alignment 1.
  unsigned Shift = (Header->Characteristics >> 20) & 0xF;
  if (Shift > 0)
    Align = uint32_t(1) << (Shift - 1);

  // When a new chunk is created, we don't if if it's going to make it
  // to the final output. Initially all sections are unmarked in terms
  // of garbage collection. The writer will call markLive() to mark
  // all reachable section chunks.
  Live = false;

  // COMDAT sections are not GC root. Non-text sections are not
  // subject of garbage collection (thus they are root).
  if (!isCOMDAT() && !(Header->Characteristics & IMAGE_SCN_CNT_CODE))
    Root = true;
}

static void add16(uint8_t *P, int16_t V) { write16le(P, read16le(P) + V); }
static void add32(uint8_t *P, int32_t V) { write32le(P, read32le(P) + V); }
static void add64(uint8_t *P, int64_t V) { write64le(P, read64le(P) + V); }

void SectionChunk::writeTo(uint8_t *Buf) {
  if (!hasData())
    return;
  // Copy section contents from source object file to output file.
  ArrayRef<uint8_t> Data;
  File->getCOFFObj()->getSectionContents(Header, Data);
  memcpy(Buf + FileOff, Data.data(), Data.size());

  // Apply relocations.
  for (const coff_relocation &Rel : Relocs) {
    uint8_t *Off = Buf + FileOff + Rel.VirtualAddress;
    SymbolBody *Body = File->getSymbolBody(Rel.SymbolTableIndex);
    uint64_t S = cast<Defined>(Body)->getRVA();
    uint64_t P = RVA + Rel.VirtualAddress;
    switch (Rel.Type) {
    case IMAGE_REL_AMD64_ADDR32:   add32(Off, S + Config->ImageBase); break;
    case IMAGE_REL_AMD64_ADDR64:   add64(Off, S + Config->ImageBase); break;
    case IMAGE_REL_AMD64_ADDR32NB: add32(Off, S); break;
    case IMAGE_REL_AMD64_REL32:    add32(Off, S - P - 4); break;
    case IMAGE_REL_AMD64_REL32_1:  add32(Off, S - P - 5); break;
    case IMAGE_REL_AMD64_REL32_2:  add32(Off, S - P - 6); break;
    case IMAGE_REL_AMD64_REL32_3:  add32(Off, S - P - 7); break;
    case IMAGE_REL_AMD64_REL32_4:  add32(Off, S - P - 8); break;
    case IMAGE_REL_AMD64_REL32_5:  add32(Off, S - P - 9); break;
    case IMAGE_REL_AMD64_SECTION:  add16(Off, Out->getSectionIndex()); break;
    case IMAGE_REL_AMD64_SECREL:   add32(Off, S - Out->getRVA()); break;
    default:
      llvm::report_fatal_error("Unsupported relocation type");
    }
  }
}

void SectionChunk::mark() {
  assert(!Live);
  Live = true;

  // Mark all symbols listed in the relocation table for this section.
  for (const coff_relocation &Rel : Relocs) {
    SymbolBody *B = File->getSymbolBody(Rel.SymbolTableIndex);
    if (auto *Def = dyn_cast<Defined>(B))
      Def->markLive();
  }

  // Mark associative sections if any.
  for (Chunk *C : AssocChildren)
    C->markLive();
}

void SectionChunk::addAssociative(SectionChunk *Child) {
  AssocChildren.push_back(Child);
  // Associative sections are live if their parent COMDATs are live,
  // and vice versa, so they are not considered live by themselves.
  Child->Root = false;
}

// Windows-specific.
// Collect all locations that contain absolute 64-bit addresses,
// which need to be fixed by the loader if load-time relocation is needed.
// Only called when base relocation is enabled.
void SectionChunk::getBaserels(std::vector<uint32_t> *Res, Defined *ImageBase) {
  for (const coff_relocation &Rel : Relocs) {
    // ADDR64 relocations contain absolute addresses.
    // Symbol __ImageBase is special -- it's an absolute symbol, but its
    // address never changes even if image is relocated.
    if (Rel.Type != IMAGE_REL_AMD64_ADDR64)
      continue;
    SymbolBody *Body = File->getSymbolBody(Rel.SymbolTableIndex);
    if (Body == ImageBase)
      continue;
    Res->push_back(RVA + Rel.VirtualAddress);
  }
}

bool SectionChunk::hasData() const {
  return !(Header->Characteristics & IMAGE_SCN_CNT_UNINITIALIZED_DATA);
}

uint32_t SectionChunk::getPermissions() const {
  return Header->Characteristics & PermMask;
}

bool SectionChunk::isCOMDAT() const {
  return Header->Characteristics & IMAGE_SCN_LNK_COMDAT;
}

void SectionChunk::printDiscardedMessage() {
  if (this == Ptr) {
    // Removed by dead-stripping.
    llvm::dbgs() << "Discarded " << Sym->getName() << "\n";
  } else {
    // Removed by ICF.
    llvm::dbgs() << "Replaced " << Sym->getName() << "\n";
  }
}

StringRef SectionChunk::getDebugName() {
  return Sym->getName();
}

uint64_t SectionChunk::getHash() const {
  ArrayRef<uint8_t> A;
  File->getCOFFObj()->getSectionContents(Header, A);
  return hash_combine(getPermissions(),
                      llvm::hash_value(SectionName),
                      NumRelocs,
                      uint32_t(Header->SizeOfRawData),
                      std::distance(Relocs.end(), Relocs.begin()),
                      hash_combine_range(A.data(), A.data() + A.size()));
}

// Returns true if this and a given chunk are identical COMDAT sections.
bool SectionChunk::equals(const SectionChunk *X) const {
  // Compare headers
  if (getPermissions() != X->getPermissions())
    return false;
  if (SectionName != X->SectionName)
    return false;
  if (Header->SizeOfRawData != X->Header->SizeOfRawData)
    return false;
  if (NumRelocs != X->NumRelocs)
    return false;

  // Compare data
  ArrayRef<uint8_t> A, B;
  File->getCOFFObj()->getSectionContents(Header, A);
  X->File->getCOFFObj()->getSectionContents(X->Header, B);
  assert(A.size() == B.size());
  if (memcmp(A.data(), B.data(), A.size()))
    return false;

  // Compare relocations
  auto Eq = [&](const coff_relocation &R1, const coff_relocation &R2) {
    if (R1.Type != R2.Type)
      return false;
    if (R1.VirtualAddress != R2.VirtualAddress)
      return false;
    SymbolBody *B1 = File->getSymbolBody(R1.SymbolTableIndex);
    SymbolBody *B2 = X->File->getSymbolBody(R2.SymbolTableIndex);
    if (auto *C1 = dyn_cast<DefinedCOMDAT>(B1))
      if (auto *C2 = dyn_cast<DefinedCOMDAT>(B2))
        if (C1->getChunk() == C2->getChunk())
          return true;
    return B1 == B2;
  };
  return std::equal(Relocs.begin(), Relocs.end(), X->Relocs.begin(), Eq);
}

// Returns a pointer to this chunk or its replacement.
SectionChunk *SectionChunk::repl() {
  while (Ptr != Ptr->Ptr)
    Ptr = Ptr->Ptr;
  return Ptr;
}

void SectionChunk::replaceWith(SectionChunk *Other) {
  Ptr = Other;
  Live = false;
}

CommonChunk::CommonChunk(const COFFSymbolRef S) : Sym(S) {
  // Common symbols are aligned on natural boundaries up to 32 bytes.
  // This is what MSVC link.exe does.
  Align = std::min(uint64_t(32), NextPowerOf2(Sym.getValue()));
}

uint32_t CommonChunk::getPermissions() const {
  return IMAGE_SCN_CNT_UNINITIALIZED_DATA | IMAGE_SCN_MEM_READ |
         IMAGE_SCN_MEM_WRITE;
}

void StringChunk::writeTo(uint8_t *Buf) {
  memcpy(Buf + FileOff, Str.data(), Str.size());
}

void ImportThunkChunk::writeTo(uint8_t *Buf) {
  memcpy(Buf + FileOff, ImportThunkData, sizeof(ImportThunkData));
  // The first two bytes is a JMP instruction. Fill its operand.
  uint32_t Operand = ImpSymbol->getRVA() - RVA - getSize();
  write32le(Buf + FileOff + 2, Operand);
}

// Windows-specific.
// This class represents a block in .reloc section.
BaserelChunk::BaserelChunk(uint32_t Page, uint32_t *Begin, uint32_t *End) {
  // Block header consists of 4 byte page RVA and 4 byte block size.
  // Each entry is 2 byte. Last entry may be padding.
  Data.resize(RoundUpToAlignment((End - Begin) * 2 + 8, 4));
  uint8_t *P = Data.data();
  write32le(P, Page);
  write32le(P + 4, Data.size());
  P += 8;
  for (uint32_t *I = Begin; I != End; ++I) {
    write16le(P, (IMAGE_REL_BASED_DIR64 << 12) | (*I - Page));
    P += 2;
  }
}

void LocalImportChunk::writeTo(uint8_t *Buf) {
  write32le(Buf + FileOff, Sym->getRVA());
}

void BaserelChunk::writeTo(uint8_t *Buf) {
  memcpy(Buf + FileOff, Data.data(), Data.size());
}

} // namespace coff
} // namespace lld