llvm-project/lld/COFF/Writer.cpp

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

#include "Config.h"
#include "DLL.h"
#include "Error.h"
#include "InputFiles.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "Writer.h"
#include "lld/Core/Parallel.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/FileOutputBuffer.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cstdio>
#include <map>
#include <memory>
#include <utility>

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

static const int PageSize = 4096;
static const int SectorSize = 512;
static const int DOSStubSize = 64;
static const int NumberfOfDataDirectory = 16;

namespace {
// The writer writes a SymbolTable result to a file.
class Writer {
public:
  Writer(SymbolTable *T) : Symtab(T) {}
  void run();

private:
  void createSections();
  void createMiscChunks();
  void createImportTables();
  void createExportTable();
  void assignAddresses();
  void removeEmptySections();
  void createSymbolAndStringTable();
  void openFile(StringRef OutputPath);
  template <typename PEHeaderTy> void writeHeader();
  void fixSafeSEHSymbols();
  void writeSections();
  void sortExceptionTable();
  void applyRelocations();

  llvm::Optional<coff_symbol16> createSymbol(Defined *D);
  size_t addEntryToStringTable(StringRef Str);

  OutputSection *findSection(StringRef Name);
  OutputSection *createSection(StringRef Name);
  void addBaserels(OutputSection *Dest);
  void addBaserelBlocks(OutputSection *Dest, std::vector<Baserel> &V);

  uint32_t getSizeOfInitializedData();
  std::map<StringRef, std::vector<DefinedImportData *>> binImports();

  SymbolTable *Symtab;
  std::unique_ptr<llvm::FileOutputBuffer> Buffer;
  llvm::SpecificBumpPtrAllocator<OutputSection> CAlloc;
  llvm::SpecificBumpPtrAllocator<BaserelChunk> BAlloc;
  std::vector<OutputSection *> OutputSections;
  std::vector<char> Strtab;
  std::vector<llvm::object::coff_symbol16> OutputSymtab;
  IdataContents Idata;
  DelayLoadContents DelayIdata;
  EdataContents Edata;
  std::unique_ptr<SEHTableChunk> SEHTable;

  uint64_t FileSize;
  uint32_t PointerToSymbolTable = 0;
  uint64_t SizeOfImage;
  uint64_t SizeOfHeaders;

  std::vector<std::unique_ptr<Chunk>> Chunks;
};
} // anonymous namespace

namespace lld {
namespace coff {

void writeResult(SymbolTable *T) { Writer(T).run(); }

// OutputSection represents a section in an output file. It's a
// container of chunks. OutputSection and Chunk are 1:N relationship.
// Chunks cannot belong to more than one OutputSections. The writer
// creates multiple OutputSections and assign them unique,
// non-overlapping file offsets and RVAs.
class OutputSection {
public:
  OutputSection(StringRef N) : Name(N), Header({}) {}
  void setRVA(uint64_t);
  void setFileOffset(uint64_t);
  void addChunk(Chunk *C);
  StringRef getName() { return Name; }
  std::vector<Chunk *> &getChunks() { return Chunks; }
  void addPermissions(uint32_t C);
  uint32_t getPermissions() { return Header.Characteristics & PermMask; }
  uint32_t getCharacteristics() { return Header.Characteristics; }
  uint64_t getRVA() { return Header.VirtualAddress; }
  uint64_t getFileOff() { return Header.PointerToRawData; }
  void writeHeaderTo(uint8_t *Buf);

  // Returns the size of this section in an executable memory image.
  // This may be smaller than the raw size (the raw size is multiple
  // of disk sector size, so there may be padding at end), or may be
  // larger (if that's the case, the loader reserves spaces after end
  // of raw data).
  uint64_t getVirtualSize() { return Header.VirtualSize; }

  // Returns the size of the section in the output file.
  uint64_t getRawSize() { return Header.SizeOfRawData; }

  // Set offset into the string table storing this section name.
  // Used only when the name is longer than 8 bytes.
  void setStringTableOff(uint32_t V) { StringTableOff = V; }

  // N.B. The section index is one based.
  uint32_t SectionIndex = 0;

private:
  StringRef Name;
  coff_section Header;
  uint32_t StringTableOff = 0;
  std::vector<Chunk *> Chunks;
};

void OutputSection::setRVA(uint64_t RVA) {
  Header.VirtualAddress = RVA;
  for (Chunk *C : Chunks)
    C->setRVA(C->getRVA() + RVA);
}

void OutputSection::setFileOffset(uint64_t Off) {
  // If a section has no actual data (i.e. BSS section), we want to
  // set 0 to its PointerToRawData. Otherwise the output is rejected
  // by the loader.
  if (Header.SizeOfRawData == 0)
    return;
  Header.PointerToRawData = Off;
}

void OutputSection::addChunk(Chunk *C) {
  Chunks.push_back(C);
  C->setOutputSection(this);
  uint64_t Off = Header.VirtualSize;
  Off = alignTo(Off, C->getAlign());
  C->setRVA(Off);
  C->setOutputSectionOff(Off);
  Off += C->getSize();
  Header.VirtualSize = Off;
  if (C->hasData())
    Header.SizeOfRawData = alignTo(Off, SectorSize);
}

void OutputSection::addPermissions(uint32_t C) {
  Header.Characteristics |= C & PermMask;
}

// Write the section header to a given buffer.
void OutputSection::writeHeaderTo(uint8_t *Buf) {
  auto *Hdr = reinterpret_cast<coff_section *>(Buf);
  *Hdr = Header;
  if (StringTableOff) {
    // If name is too long, write offset into the string table as a name.
    sprintf(Hdr->Name, "/%d", StringTableOff);
  } else {
    assert(!Config->Debug || Name.size() <= COFF::NameSize);
    strncpy(Hdr->Name, Name.data(),
            std::min(Name.size(), (size_t)COFF::NameSize));
  }
}

uint64_t Defined::getSecrel() {
  if (auto *D = dyn_cast<DefinedRegular>(this))
    return getRVA() - D->getChunk()->getOutputSection()->getRVA();
  error("SECREL relocation points to a non-regular symbol");
}

uint64_t Defined::getSectionIndex() {
  if (auto *D = dyn_cast<DefinedRegular>(this))
    return D->getChunk()->getOutputSection()->SectionIndex;
  error("SECTION relocation points to a non-regular symbol");
}

bool Defined::isExecutable() {
  const auto X = IMAGE_SCN_MEM_EXECUTE;
  if (auto *D = dyn_cast<DefinedRegular>(this))
    return D->getChunk()->getOutputSection()->getPermissions() & X;
  return isa<DefinedImportThunk>(this);
}

} // namespace coff
} // namespace lld

// The main function of the writer.
void Writer::run() {
  createSections();
  createMiscChunks();
  createImportTables();
  createExportTable();
  if (Config->Relocatable)
    createSection(".reloc");
  assignAddresses();
  removeEmptySections();
  createSymbolAndStringTable();
  openFile(Config->OutputFile);
  if (Config->is64()) {
    writeHeader<pe32plus_header>();
  } else {
    writeHeader<pe32_header>();
  }
  fixSafeSEHSymbols();
  writeSections();
  sortExceptionTable();
  error(Buffer->commit(), "Failed to write the output file");
}

static StringRef getOutputSection(StringRef Name) {
  StringRef S = Name.split('$').first;
  auto It = Config->Merge.find(S);
  if (It == Config->Merge.end())
    return S;
  return It->second;
}

// Create output section objects and add them to OutputSections.
void Writer::createSections() {
  // First, bin chunks by name.
  std::map<StringRef, std::vector<Chunk *>> Map;
  for (Chunk *C : Symtab->getChunks()) {
    auto *SC = dyn_cast<SectionChunk>(C);
    if (SC && !SC->isLive()) {
      if (Config->Verbose)
        SC->printDiscardedMessage();
      continue;
    }
    Map[C->getSectionName()].push_back(C);
  }

  // Then create an OutputSection for each section.
  // '$' and all following characters in input section names are
  // discarded when determining output section. So, .text$foo
  // contributes to .text, for example. See PE/COFF spec 3.2.
  SmallDenseMap<StringRef, OutputSection *> Sections;
  for (auto Pair : Map) {
    StringRef Name = getOutputSection(Pair.first);
    OutputSection *&Sec = Sections[Name];
    if (!Sec) {
      Sec = new (CAlloc.Allocate()) OutputSection(Name);
      OutputSections.push_back(Sec);
    }
    std::vector<Chunk *> &Chunks = Pair.second;
    for (Chunk *C : Chunks) {
      Sec->addChunk(C);
      Sec->addPermissions(C->getPermissions());
    }
  }
}

void Writer::createMiscChunks() {
  // Create thunks for locally-dllimported symbols.
  if (!Symtab->LocalImportChunks.empty()) {
    OutputSection *Sec = createSection(".rdata");
    for (Chunk *C : Symtab->LocalImportChunks)
      Sec->addChunk(C);
  }

  // Create SEH table. x86-only.
  if (Config->Machine != I386)
    return;
  std::set<Defined *> Handlers;
  for (lld::coff::ObjectFile *File : Symtab->ObjectFiles) {
    if (!File->SEHCompat)
      return;
    for (SymbolBody *B : File->SEHandlers)
      Handlers.insert(cast<Defined>(B->repl()));
  }
  SEHTable.reset(new SEHTableChunk(Handlers));
  createSection(".rdata")->addChunk(SEHTable.get());
}

// Create .idata section for the DLL-imported symbol table.
// The format of this section is inherently Windows-specific.
// IdataContents class abstracted away the details for us,
// so we just let it create chunks and add them to the section.
void Writer::createImportTables() {
  if (Symtab->ImportFiles.empty())
    return;

  // Initialize DLLOrder so that import entries are ordered in
  // the same order as in the command line. (That affects DLL
  // initialization order, and this ordering is MSVC-compatible.)
  for (ImportFile *File : Symtab->ImportFiles) {
    std::string DLL = StringRef(File->DLLName).lower();
    if (Config->DLLOrder.count(DLL) == 0)
      Config->DLLOrder[DLL] = Config->DLLOrder.size();
  }

  OutputSection *Text = createSection(".text");
  for (ImportFile *File : Symtab->ImportFiles) {
    if (DefinedImportThunk *Thunk = File->ThunkSym)
      Text->addChunk(Thunk->getChunk());
    if (Config->DelayLoads.count(StringRef(File->DLLName).lower())) {
      DelayIdata.add(File->ImpSym);
    } else {
      Idata.add(File->ImpSym);
    }
  }
  if (!Idata.empty()) {
    OutputSection *Sec = createSection(".idata");
    for (Chunk *C : Idata.getChunks())
      Sec->addChunk(C);
  }
  if (!DelayIdata.empty()) {
    Defined *Helper = cast<Defined>(Config->DelayLoadHelper->repl());
    DelayIdata.create(Helper);
    OutputSection *Sec = createSection(".didat");
    for (Chunk *C : DelayIdata.getChunks())
      Sec->addChunk(C);
    Sec = createSection(".data");
    for (Chunk *C : DelayIdata.getDataChunks())
      Sec->addChunk(C);
    Sec = createSection(".text");
    for (std::unique_ptr<Chunk> &C : DelayIdata.getCodeChunks())
      Sec->addChunk(C.get());
  }
}

void Writer::createExportTable() {
  if (Config->Exports.empty())
    return;
  OutputSection *Sec = createSection(".edata");
  for (std::unique_ptr<Chunk> &C : Edata.Chunks)
    Sec->addChunk(C.get());
}

// The Windows loader doesn't seem to like empty sections,
// so we remove them if any.
void Writer::removeEmptySections() {
  auto IsEmpty = [](OutputSection *S) { return S->getVirtualSize() == 0; };
  OutputSections.erase(
      std::remove_if(OutputSections.begin(), OutputSections.end(), IsEmpty),
      OutputSections.end());
  uint32_t Idx = 1;
  for (OutputSection *Sec : OutputSections)
    Sec->SectionIndex = Idx++;
}

size_t Writer::addEntryToStringTable(StringRef Str) {
  assert(Str.size() > COFF::NameSize);
  size_t OffsetOfEntry = Strtab.size() + 4; // +4 for the size field
  Strtab.insert(Strtab.end(), Str.begin(), Str.end());
  Strtab.push_back('\0');
  return OffsetOfEntry;
}

Optional<coff_symbol16> Writer::createSymbol(Defined *Def) {
  if (auto *D = dyn_cast<DefinedRegular>(Def))
    if (!D->getChunk()->isLive())
      return None;

  coff_symbol16 Sym;
  StringRef Name = Def->getName();
  if (Name.size() > COFF::NameSize) {
    Sym.Name.Offset.Zeroes = 0;
    Sym.Name.Offset.Offset = addEntryToStringTable(Name);
  } else {
    memset(Sym.Name.ShortName, 0, COFF::NameSize);
    memcpy(Sym.Name.ShortName, Name.data(), Name.size());
  }

  if (auto *D = dyn_cast<DefinedCOFF>(Def)) {
    COFFSymbolRef Ref = D->getCOFFSymbol();
    Sym.Type = Ref.getType();
    Sym.StorageClass = Ref.getStorageClass();
  } else {
    Sym.Type = IMAGE_SYM_TYPE_NULL;
    Sym.StorageClass = IMAGE_SYM_CLASS_EXTERNAL;
  }
  Sym.NumberOfAuxSymbols = 0;

  switch (Def->kind()) {
  case SymbolBody::DefinedAbsoluteKind:
  case SymbolBody::DefinedRelativeKind:
    Sym.Value = Def->getRVA();
    Sym.SectionNumber = IMAGE_SYM_ABSOLUTE;
    break;
  default: {
    uint64_t RVA = Def->getRVA();
    OutputSection *Sec = nullptr;
    for (OutputSection *S : OutputSections) {
      if (S->getRVA() > RVA)
        break;
      Sec = S;
    }
    Sym.Value = RVA - Sec->getRVA();
    Sym.SectionNumber = Sec->SectionIndex;
    break;
  }
  }
  return Sym;
}

void Writer::createSymbolAndStringTable() {
  if (!Config->Debug || !Config->WriteSymtab)
    return;

  // Name field in the section table is 8 byte long. Longer names need
  // to be written to the string table. First, construct string table.
  for (OutputSection *Sec : OutputSections) {
    StringRef Name = Sec->getName();
    if (Name.size() <= COFF::NameSize)
      continue;
    Sec->setStringTableOff(addEntryToStringTable(Name));
  }

  for (lld::coff::ObjectFile *File : Symtab->ObjectFiles)
    for (SymbolBody *B : File->getSymbols())
      if (auto *D = dyn_cast<Defined>(B))
        if (Optional<coff_symbol16> Sym = createSymbol(D))
          OutputSymtab.push_back(*Sym);

  for (ImportFile *File : Symtab->ImportFiles)
    for (SymbolBody *B : File->getSymbols())
      if (Optional<coff_symbol16> Sym = createSymbol(cast<Defined>(B)))
        OutputSymtab.push_back(*Sym);

  OutputSection *LastSection = OutputSections.back();
  // We position the symbol table to be adjacent to the end of the last section.
  uint64_t FileOff = LastSection->getFileOff() +
                     alignTo(LastSection->getRawSize(), SectorSize);
  if (!OutputSymtab.empty()) {
    PointerToSymbolTable = FileOff;
    FileOff += OutputSymtab.size() * sizeof(coff_symbol16);
  }
  if (!Strtab.empty())
    FileOff += Strtab.size() + 4;
  FileSize = alignTo(FileOff, SectorSize);
}

// Visits all sections to assign incremental, non-overlapping RVAs and
// file offsets.
void Writer::assignAddresses() {
  SizeOfHeaders = DOSStubSize + sizeof(PEMagic) + sizeof(coff_file_header) +
                  sizeof(data_directory) * NumberfOfDataDirectory +
                  sizeof(coff_section) * OutputSections.size();
  SizeOfHeaders +=
      Config->is64() ? sizeof(pe32plus_header) : sizeof(pe32_header);
  SizeOfHeaders = alignTo(SizeOfHeaders, SectorSize);
  uint64_t RVA = 0x1000; // The first page is kept unmapped.
  FileSize = SizeOfHeaders;
  // Move DISCARDABLE (or non-memory-mapped) sections to the end of file because
  // the loader cannot handle holes.
  std::stable_partition(
      OutputSections.begin(), OutputSections.end(), [](OutputSection *S) {
        return (S->getPermissions() & IMAGE_SCN_MEM_DISCARDABLE) == 0;
      });
  for (OutputSection *Sec : OutputSections) {
    if (Sec->getName() == ".reloc")
      addBaserels(Sec);
    Sec->setRVA(RVA);
    Sec->setFileOffset(FileSize);
    RVA += alignTo(Sec->getVirtualSize(), PageSize);
    FileSize += alignTo(Sec->getRawSize(), SectorSize);
  }
  SizeOfImage = SizeOfHeaders + alignTo(RVA - 0x1000, PageSize);
}

template <typename PEHeaderTy> void Writer::writeHeader() {
  // Write DOS stub
  uint8_t *Buf = Buffer->getBufferStart();
  auto *DOS = reinterpret_cast<dos_header *>(Buf);
  Buf += DOSStubSize;
  DOS->Magic[0] = 'M';
  DOS->Magic[1] = 'Z';
  DOS->AddressOfRelocationTable = sizeof(dos_header);
  DOS->AddressOfNewExeHeader = DOSStubSize;

  // Write PE magic
  memcpy(Buf, PEMagic, sizeof(PEMagic));
  Buf += sizeof(PEMagic);

  // Write COFF header
  auto *COFF = reinterpret_cast<coff_file_header *>(Buf);
  Buf += sizeof(*COFF);
  COFF->Machine = Config->Machine;
  COFF->NumberOfSections = OutputSections.size();
  COFF->Characteristics = IMAGE_FILE_EXECUTABLE_IMAGE;
  if (Config->LargeAddressAware)
    COFF->Characteristics |= IMAGE_FILE_LARGE_ADDRESS_AWARE;
  if (!Config->is64())
    COFF->Characteristics |= IMAGE_FILE_32BIT_MACHINE;
  if (Config->DLL)
    COFF->Characteristics |= IMAGE_FILE_DLL;
  if (!Config->Relocatable)
    COFF->Characteristics |= IMAGE_FILE_RELOCS_STRIPPED;
  COFF->SizeOfOptionalHeader =
      sizeof(PEHeaderTy) + sizeof(data_directory) * NumberfOfDataDirectory;

  // Write PE header
  auto *PE = reinterpret_cast<PEHeaderTy *>(Buf);
  Buf += sizeof(*PE);
  PE->Magic = Config->is64() ? PE32Header::PE32_PLUS : PE32Header::PE32;
  PE->ImageBase = Config->ImageBase;
  PE->SectionAlignment = PageSize;
  PE->FileAlignment = SectorSize;
  PE->MajorImageVersion = Config->MajorImageVersion;
  PE->MinorImageVersion = Config->MinorImageVersion;
  PE->MajorOperatingSystemVersion = Config->MajorOSVersion;
  PE->MinorOperatingSystemVersion = Config->MinorOSVersion;
  PE->MajorSubsystemVersion = Config->MajorOSVersion;
  PE->MinorSubsystemVersion = Config->MinorOSVersion;
  PE->Subsystem = Config->Subsystem;
  PE->SizeOfImage = SizeOfImage;
  PE->SizeOfHeaders = SizeOfHeaders;
  if (!Config->NoEntry) {
    Defined *Entry = cast<Defined>(Config->Entry->repl());
    PE->AddressOfEntryPoint = Entry->getRVA();
    // Pointer to thumb code must have the LSB set, so adjust it.
    if (Config->Machine == ARMNT)
      PE->AddressOfEntryPoint |= 1;
  }
  PE->SizeOfStackReserve = Config->StackReserve;
  PE->SizeOfStackCommit = Config->StackCommit;
  PE->SizeOfHeapReserve = Config->HeapReserve;
  PE->SizeOfHeapCommit = Config->HeapCommit;
  if (Config->DynamicBase)
    PE->DLLCharacteristics |= IMAGE_DLL_CHARACTERISTICS_DYNAMIC_BASE;
  if (Config->HighEntropyVA)
    PE->DLLCharacteristics |= IMAGE_DLL_CHARACTERISTICS_HIGH_ENTROPY_VA;
  if (!Config->AllowBind)
    PE->DLLCharacteristics |= IMAGE_DLL_CHARACTERISTICS_NO_BIND;
  if (Config->NxCompat)
    PE->DLLCharacteristics |= IMAGE_DLL_CHARACTERISTICS_NX_COMPAT;
  if (!Config->AllowIsolation)
    PE->DLLCharacteristics |= IMAGE_DLL_CHARACTERISTICS_NO_ISOLATION;
  if (Config->TerminalServerAware)
    PE->DLLCharacteristics |= IMAGE_DLL_CHARACTERISTICS_TERMINAL_SERVER_AWARE;
  PE->NumberOfRvaAndSize = NumberfOfDataDirectory;
  if (OutputSection *Text = findSection(".text")) {
    PE->BaseOfCode = Text->getRVA();
    PE->SizeOfCode = Text->getRawSize();
  }
  PE->SizeOfInitializedData = getSizeOfInitializedData();

  // Write data directory
  auto *Dir = reinterpret_cast<data_directory *>(Buf);
  Buf += sizeof(*Dir) * NumberfOfDataDirectory;
  if (OutputSection *Sec = findSection(".edata")) {
    Dir[EXPORT_TABLE].RelativeVirtualAddress = Sec->getRVA();
    Dir[EXPORT_TABLE].Size = Sec->getVirtualSize();
  }
  if (!Idata.empty()) {
    Dir[IMPORT_TABLE].RelativeVirtualAddress = Idata.getDirRVA();
    Dir[IMPORT_TABLE].Size = Idata.getDirSize();
    Dir[IAT].RelativeVirtualAddress = Idata.getIATRVA();
    Dir[IAT].Size = Idata.getIATSize();
  }
  if (!DelayIdata.empty()) {
    Dir[DELAY_IMPORT_DESCRIPTOR].RelativeVirtualAddress =
        DelayIdata.getDirRVA();
    Dir[DELAY_IMPORT_DESCRIPTOR].Size = DelayIdata.getDirSize();
  }
  if (OutputSection *Sec = findSection(".rsrc")) {
    Dir[RESOURCE_TABLE].RelativeVirtualAddress = Sec->getRVA();
    Dir[RESOURCE_TABLE].Size = Sec->getVirtualSize();
  }
  if (OutputSection *Sec = findSection(".reloc")) {
    Dir[BASE_RELOCATION_TABLE].RelativeVirtualAddress = Sec->getRVA();
    Dir[BASE_RELOCATION_TABLE].Size = Sec->getVirtualSize();
  }
  if (OutputSection *Sec = findSection(".pdata")) {
    Dir[EXCEPTION_TABLE].RelativeVirtualAddress = Sec->getRVA();
    Dir[EXCEPTION_TABLE].Size = Sec->getVirtualSize();
  }
  if (Symbol *Sym = Symtab->findUnderscore("_tls_used")) {
    if (Defined *B = dyn_cast<Defined>(Sym->Body)) {
      Dir[TLS_TABLE].RelativeVirtualAddress = B->getRVA();
      Dir[TLS_TABLE].Size = 40;
    }
  }
  if (Symbol *Sym = Symtab->findUnderscore("_load_config_used")) {
    if (Defined *B = dyn_cast<Defined>(Sym->Body)) {
      Dir[LOAD_CONFIG_TABLE].RelativeVirtualAddress = B->getRVA();
      Dir[LOAD_CONFIG_TABLE].Size = Config->is64() ? 112 : 64;
    }
  }

  // Write section table
  for (OutputSection *Sec : OutputSections) {
    Sec->writeHeaderTo(Buf);
    Buf += sizeof(coff_section);
  }

  if (OutputSymtab.empty())
    return;

  COFF->PointerToSymbolTable = PointerToSymbolTable;
  uint32_t NumberOfSymbols = OutputSymtab.size();
  COFF->NumberOfSymbols = NumberOfSymbols;
  auto *SymbolTable = reinterpret_cast<coff_symbol16 *>(
      Buffer->getBufferStart() + COFF->PointerToSymbolTable);
  for (size_t I = 0; I != NumberOfSymbols; ++I)
    SymbolTable[I] = OutputSymtab[I];
  // Create the string table, it follows immediately after the symbol table.
  // The first 4 bytes is length including itself.
  Buf = reinterpret_cast<uint8_t *>(&SymbolTable[NumberOfSymbols]);
  write32le(Buf, Strtab.size() + 4);
  memcpy(Buf + 4, Strtab.data(), Strtab.size());
}

void Writer::openFile(StringRef Path) {
  ErrorOr<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
      FileOutputBuffer::create(Path, FileSize, FileOutputBuffer::F_executable);
  error(BufferOrErr, Twine("failed to open ") + Path);
  Buffer = std::move(*BufferOrErr);
}

void Writer::fixSafeSEHSymbols() {
  if (!SEHTable)
    return;
  Config->SEHTable->setRVA(SEHTable->getRVA());
  Config->SEHCount->setVA(SEHTable->getSize() / 4);
}

// Write section contents to a mmap'ed file.
void Writer::writeSections() {
  uint8_t *Buf = Buffer->getBufferStart();
  for (OutputSection *Sec : OutputSections) {
    uint8_t *SecBuf = Buf + Sec->getFileOff();
    // Fill gaps between functions in .text with INT3 instructions
    // instead of leaving as NUL bytes (which can be interpreted as
    // ADD instructions).
    if (Sec->getPermissions() & IMAGE_SCN_CNT_CODE)
      memset(SecBuf, 0xCC, Sec->getRawSize());
    parallel_for_each(Sec->getChunks().begin(), Sec->getChunks().end(),
                      [&](Chunk *C) { C->writeTo(SecBuf); });
  }
}

// Sort .pdata section contents according to PE/COFF spec 5.5.
void Writer::sortExceptionTable() {
  OutputSection *Sec = findSection(".pdata");
  if (!Sec)
    return;
  // We assume .pdata contains function table entries only.
  uint8_t *Begin = Buffer->getBufferStart() + Sec->getFileOff();
  uint8_t *End = Begin + Sec->getVirtualSize();
  if (Config->Machine == AMD64) {
    struct Entry { ulittle32_t Begin, End, Unwind; };
    parallel_sort(
        (Entry *)Begin, (Entry *)End,
        [](const Entry &A, const Entry &B) { return A.Begin < B.Begin; });
    return;
  }
  if (Config->Machine == ARMNT) {
    struct Entry { ulittle32_t Begin, Unwind; };
    parallel_sort(
        (Entry *)Begin, (Entry *)End,
        [](const Entry &A, const Entry &B) { return A.Begin < B.Begin; });
    return;
  }
  errs() << "warning: don't know how to handle .pdata.\n";
}

OutputSection *Writer::findSection(StringRef Name) {
  for (OutputSection *Sec : OutputSections)
    if (Sec->getName() == Name)
      return Sec;
  return nullptr;
}

uint32_t Writer::getSizeOfInitializedData() {
  uint32_t Res = 0;
  for (OutputSection *S : OutputSections)
    if (S->getPermissions() & IMAGE_SCN_CNT_INITIALIZED_DATA)
      Res += S->getRawSize();
  return Res;
}

// Returns an existing section or create a new one if not found.
OutputSection *Writer::createSection(StringRef Name) {
  if (auto *Sec = findSection(Name))
    return Sec;
  const auto DATA = IMAGE_SCN_CNT_INITIALIZED_DATA;
  const auto BSS = IMAGE_SCN_CNT_UNINITIALIZED_DATA;
  const auto CODE = IMAGE_SCN_CNT_CODE;
  const auto DISCARDABLE = IMAGE_SCN_MEM_DISCARDABLE;
  const auto R = IMAGE_SCN_MEM_READ;
  const auto W = IMAGE_SCN_MEM_WRITE;
  const auto X = IMAGE_SCN_MEM_EXECUTE;
  uint32_t Perms = StringSwitch<uint32_t>(Name)
                       .Case(".bss", BSS | R | W)
                       .Case(".data", DATA | R | W)
                       .Case(".didat", DATA | R)
                       .Case(".edata", DATA | R)
                       .Case(".idata", DATA | R)
                       .Case(".rdata", DATA | R)
                       .Case(".reloc", DATA | DISCARDABLE | R)
                       .Case(".text", CODE | R | X)
                       .Default(0);
  if (!Perms)
    llvm_unreachable("unknown section name");
  auto Sec = new (CAlloc.Allocate()) OutputSection(Name);
  Sec->addPermissions(Perms);
  OutputSections.push_back(Sec);
  return Sec;
}

// Dest is .reloc section. Add contents to that section.
void Writer::addBaserels(OutputSection *Dest) {
  std::vector<Baserel> V;
  for (OutputSection *Sec : OutputSections) {
    if (Sec == Dest)
      continue;
    // Collect all locations for base relocations.
    for (Chunk *C : Sec->getChunks())
      C->getBaserels(&V);
    // Add the addresses to .reloc section.
    if (!V.empty())
      addBaserelBlocks(Dest, V);
    V.clear();
  }
}

// Add addresses to .reloc section. Note that addresses are grouped by page.
void Writer::addBaserelBlocks(OutputSection *Dest, std::vector<Baserel> &V) {
  const uint32_t Mask = ~uint32_t(PageSize - 1);
  uint32_t Page = V[0].RVA & Mask;
  size_t I = 0, J = 1;
  for (size_t E = V.size(); J < E; ++J) {
    uint32_t P = V[J].RVA & Mask;
    if (P == Page)
      continue;
    BaserelChunk *Buf = BAlloc.Allocate();
    Dest->addChunk(new (Buf) BaserelChunk(Page, &V[I], &V[0] + J));
    I = J;
    Page = P;
  }
  if (I == J)
    return;
  BaserelChunk *Buf = BAlloc.Allocate();
  Dest->addChunk(new (Buf) BaserelChunk(Page, &V[I], &V[0] + J));
}