llvm-project/lld/lib/ReaderWriter/PECOFF/WriterPECOFF.cpp

//===- lib/ReaderWriter/PECOFF/WriterPECOFF.cpp ---------------------------===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
///
/// \file
///
/// PE/COFF file consists of DOS Header, PE Header, COFF Header and Section
/// Tables followed by raw section data.
///
/// This writer is reponsible for writing Core Linker results to an Windows
/// executable file. Currently it can only output ".text" section; other
/// sections including the symbol table are silently ignored.
///
/// This writer currently supports 32 bit PE/COFF for x86 processor only.
///
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "WriterPECOFF"

#include <map>
#include <time.h>
#include <vector>

#include "Atoms.h"

#include "lld/Core/DefinedAtom.h"
#include "lld/Core/File.h"
#include "lld/ReaderWriter/AtomLayout.h"
#include "lld/ReaderWriter/PECOFFLinkingContext.h"
#include "lld/ReaderWriter/Writer.h"

#include "llvm/ADT/ArrayRef.h"
#include "llvm/Object/COFF.h"
#include "llvm/Support/COFF.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/ErrorOr.h"
#include "llvm/Support/FileOutputBuffer.h"
#include "llvm/Support/Format.h"

using llvm::support::ulittle16_t;
using llvm::support::ulittle32_t;

namespace lld {
namespace pecoff {

namespace {
class SectionChunk;

// Page size of x86 processor. Some data needs to be aligned at page boundary
// when loaded into memory.
const int PAGE_SIZE = 4096;

// Disk sector size. Some data needs to be aligned at disk sector boundary in
// file.
const int SECTOR_SIZE = 512;

// The address of the executable when loaded into memory.
const int32_t IMAGE_BASE = 0x400000;

/// A Chunk is an abstrace contiguous range in an output file.
class Chunk {
public:
  enum Kind {
    kindHeader,
    kindSection,
    kindDataDirectory
  };

  explicit Chunk(Kind kind) : _kind(kind), _size(0), _align(1) {}
  virtual ~Chunk() {};
  virtual void write(uint8_t *fileBuffer) = 0;

  virtual uint64_t fileOffset() const { return _fileOffset; }
  virtual uint64_t size() const { return _size; }
  virtual uint64_t align() const { return _align; }

  virtual void setFileOffset(uint64_t fileOffset) {
    _fileOffset = fileOffset;
  }

  Kind getKind() const { return _kind; }

protected:
  Kind _kind;
  uint64_t _size;
  uint64_t _fileOffset;
  uint64_t _align;
};

/// A HeaderChunk is an abstract class to represent a file header for
/// PE/COFF. The data in the header chunk is metadata about program and will
/// be consumed by the windows loader. HeaderChunks are not mapped to memory
/// when executed.
class HeaderChunk : public Chunk {
public:
  HeaderChunk() : Chunk(kindHeader) {}

  static bool classof(const Chunk *c) { return c->getKind() == kindHeader; }
};

/// A DOSStubChunk represents the DOS compatible header at the beginning
/// of PE/COFF files.
class DOSStubChunk : public HeaderChunk {
public:
  DOSStubChunk() : HeaderChunk() {
    // Make the DOS stub occupy the first 128 bytes of an exe. Technically
    // this can be as small as 64 bytes, but GNU binutil's objdump cannot
    // parse such irregular header.
    _size = 128;

    // A DOS stub is usually a small valid DOS program that prints out a message
    // "This program requires Microsoft Windows" to help user who accidentally
    // run a Windows executable on DOS. That's not a technical requirement, so
    // we don't bother to emit such code, at least for now. We simply fill the
    // DOS stub with null bytes.
    std::memset(&_dosHeader, 0, sizeof(_dosHeader));

    _dosHeader.Magic = 'M' | ('Z' << 8);
    _dosHeader.AddressOfNewExeHeader = _size;
  }

  virtual void write(uint8_t *fileBuffer) {
    std::memcpy(fileBuffer, &_dosHeader, sizeof(_dosHeader));
  }

private:
  llvm::object::dos_header _dosHeader;
};

/// A PEHeaderChunk represents PE header including COFF header.
class PEHeaderChunk : public HeaderChunk {
public:
  explicit PEHeaderChunk(const PECOFFLinkingContext &context) : HeaderChunk() {
    // Set the size of the chunk and initialize the header with null bytes.
    _size = sizeof(llvm::COFF::PEMagic) + sizeof(_coffHeader)
        + sizeof(_peHeader);
    std::memset(&_coffHeader, 0, sizeof(_coffHeader));
    std::memset(&_peHeader, 0, sizeof(_peHeader));

    _coffHeader.Machine = context.getMachineType();
    _coffHeader.TimeDateStamp = time(NULL);

    // The size of PE header including optional data directory is always 224.
    _coffHeader.SizeOfOptionalHeader = 224;

    // Attributes of the executable.
    uint16_t characteristics = llvm::COFF::IMAGE_FILE_32BIT_MACHINE |
                               llvm::COFF::IMAGE_FILE_EXECUTABLE_IMAGE;
    if (context.getLargeAddressAware())
      characteristics |= llvm::COFF::IMAGE_FILE_LARGE_ADDRESS_AWARE;
    if (context.getSwapRunFromCD())
      characteristics |= llvm::COFF::IMAGE_FILE_REMOVABLE_RUN_FROM_SWAP;
    if (context.getSwapRunFromNet())
      characteristics |= llvm::COFF::IMAGE_FILE_NET_RUN_FROM_SWAP;
    if (!context.getBaseRelocationEnabled())
      characteristics |= llvm::COFF::IMAGE_FILE_RELOCS_STRIPPED;

    _coffHeader.Characteristics = characteristics;

    // 0x10b indicates a normal PE32 executable. For PE32+ it should be 0x20b.
    _peHeader.Magic = 0x10b;

    // The address of entry point relative to ImageBase. Windows executable
    // usually starts at address 0x401000.
    _peHeader.AddressOfEntryPoint = 0x1000;

    // The address of the executable when loaded into memory. The default for
    // DLLs is 0x10000000. The default for executables is 0x400000.
    _peHeader.ImageBase = context.getBaseAddress();

    // Sections should be page-aligned when loaded into memory, which is 4KB on
    // x86.
    _peHeader.SectionAlignment = context.getSectionAlignment();

    // Sections in an executable file on disk should be sector-aligned (512 byte).
    _peHeader.FileAlignment = SECTOR_SIZE;

    // The version number of the resultant executable/DLL. The number is purely
    // informative, and neither the linker nor the loader won't use it. User can
    // set the value using /version command line option. Default is 0.0.
    PECOFFLinkingContext::Version imageVersion = context.getImageVersion();
    _peHeader.MajorImageVersion = imageVersion.majorVersion;
    _peHeader.MinorImageVersion = imageVersion.minorVersion;

    // The required Windows version number. This is the internal version and
    // shouldn't be confused with product name. Windows 7 is version 6.1 and
    // Windows 8 is 6.2, for example.
    PECOFFLinkingContext::Version minOSVersion = context.getMinOSVersion();
    _peHeader.MajorOperatingSystemVersion = minOSVersion.majorVersion;
    _peHeader.MinorOperatingSystemVersion = minOSVersion.minorVersion;
    _peHeader.MajorSubsystemVersion = minOSVersion.majorVersion;
    _peHeader.MinorSubsystemVersion = minOSVersion.minorVersion;

    _peHeader.Subsystem = context.getSubsystem();

    // Despite its name, DLL characteristics field has meaning both for
    // executables and DLLs. We are not very sure if the following bits must
    // be set, but regular binaries seem to have these bits, so we follow
    // them.
    uint16_t dllCharacteristics =
        llvm::COFF::IMAGE_DLL_CHARACTERISTICS_NO_SEH;
    if (context.isTerminalServerAware())
      dllCharacteristics |=
          llvm::COFF::IMAGE_DLL_CHARACTERISTICS_TERMINAL_SERVER_AWARE;
    if (context.isNxCompat())
      dllCharacteristics |= llvm::COFF::IMAGE_DLL_CHARACTERISTICS_NX_COMPAT;
    if (context.getDynamicBaseEnabled())
      dllCharacteristics |= llvm::COFF::IMAGE_DLL_CHARACTERISTICS_DYNAMIC_BASE;
    if (!context.getAllowBind())
      dllCharacteristics |= llvm::COFF::IMAGE_DLL_CHARACTERISTICS_NO_BIND;
    if (!context.getAllowIsolation())
      dllCharacteristics |= llvm::COFF::IMAGE_DLL_CHARACTERISTICS_NO_ISOLATION;
    _peHeader.DLLCharacteristics = dllCharacteristics;

    _peHeader.SizeOfStackReserve = context.getStackReserve();
    _peHeader.SizeOfStackCommit = context.getStackCommit();
    _peHeader.SizeOfHeapReserve = context.getHeapReserve();
    _peHeader.SizeOfHeapCommit = context.getHeapCommit();

    // The number of data directory entries. We always have 16 entries.
    _peHeader.NumberOfRvaAndSize = 16;
  }

  virtual void write(uint8_t *fileBuffer) {
    fileBuffer += fileOffset();
    std::memcpy(fileBuffer, llvm::COFF::PEMagic, sizeof(llvm::COFF::PEMagic));
    fileBuffer += sizeof(llvm::COFF::PEMagic);
    std::memcpy(fileBuffer, &_coffHeader, sizeof(_coffHeader));
    fileBuffer += sizeof(_coffHeader);
    std::memcpy(fileBuffer, &_peHeader, sizeof(_peHeader));
  }

  virtual void setSizeOfHeaders(uint64_t size) {
    // Must be multiple of FileAlignment.
    _peHeader.SizeOfHeaders = llvm::RoundUpToAlignment(size, SECTOR_SIZE);
  }

  virtual void setSizeOfCode(uint64_t size) {
    _peHeader.SizeOfCode = size;
  }

  virtual void setSizeOfInitializedData(uint64_t size) {
    _peHeader.SizeOfInitializedData = size;
  }

  virtual void setSizeOfUninitializedData(uint64_t size) {
    _peHeader.SizeOfUninitializedData = size;
  }

  virtual void setNumberOfSections(uint32_t num) {
    _coffHeader.NumberOfSections = num;
  }

  virtual void setBaseOfCode(uint32_t rva) { _peHeader.BaseOfCode = rva; }

  virtual void setBaseOfData(uint32_t rva) { _peHeader.BaseOfData = rva; }

  virtual void setSizeOfImage(uint32_t size) { _peHeader.SizeOfImage = size; }

  virtual void setAddressOfEntryPoint(uint32_t address) {
    _peHeader.AddressOfEntryPoint = address;
  }

private:
  llvm::object::coff_file_header _coffHeader;
  llvm::object::pe32_header _peHeader;
};

/// A SectionHeaderTableChunk represents Section Table Header of PE/COFF
/// format, which is a list of section headers.
class SectionHeaderTableChunk : public HeaderChunk {
public:
  SectionHeaderTableChunk() : HeaderChunk() {}
  void addSection(SectionChunk *chunk);
  virtual uint64_t size() const;
  virtual void write(uint8_t *fileBuffer);

private:
  std::vector<SectionChunk *> _sections;
};

/// An AtomChunk represents a section containing atoms.
class AtomChunk : public Chunk {
public:
  virtual void write(uint8_t *fileBuffer) {
    for (const auto *layout : _atomLayouts) {
      const DefinedAtom *atom = cast<DefinedAtom>(layout->_atom);
      ArrayRef<uint8_t> rawContent = atom->rawContent();
      std::memcpy(fileBuffer + layout->_fileOffset, rawContent.data(),
                  rawContent.size());
    }
  }

  /// Add all atoms to the given map. This data will be used to do relocation.
  void buildAtomToVirtualAddr(std::map<const Atom *, uint64_t> &atomRva) {
    for (const auto *layout : _atomLayouts)
      atomRva[layout->_atom] = layout->_virtualAddr;
  }

  void applyRelocations(uint8_t *fileBuffer,
                        std::map<const Atom *, uint64_t> &atomRva,
                        uint64_t imageBaseAddress) {
    for (const auto *layout : _atomLayouts) {
      const DefinedAtom *atom = cast<DefinedAtom>(layout->_atom);
      for (const Reference *ref : *atom) {
        auto relocSite = reinterpret_cast<ulittle32_t *>(
            fileBuffer + layout->_fileOffset + ref->offsetInAtom());
        uint64_t targetAddr = atomRva[ref->target()];
        // Also account for whatever offset is already stored at the relocation site.
        targetAddr += *relocSite;

        // Skip if this reference is not for relocation.
        if (ref->kind() < lld::Reference::kindTargetLow)
          continue;

        switch (ref->kind()) {
        case llvm::COFF::IMAGE_REL_I386_ABSOLUTE:
          // This relocation is no-op.
          break;
        case llvm::COFF::IMAGE_REL_I386_DIR32:
          // Set target's 32-bit VA.
          *relocSite = targetAddr + imageBaseAddress;
          break;
        case llvm::COFF::IMAGE_REL_I386_DIR32NB:
          // Set target's 32-bit RVA.
          *relocSite = targetAddr;
          break;
        case llvm::COFF::IMAGE_REL_I386_REL32: {
          // Set 32-bit relative address of the target. This relocation is
          // usually used for relative branch or call instruction.
          uint32_t disp = atomRva[atom] + ref->offsetInAtom() + 4;
          *relocSite = targetAddr - disp;
          break;
        }
        default:
          llvm_unreachable("Unsupported relocation kind");
        }
      }
    }
  }

  /// Print atom VAs. Used only for debugging.
  void printAtomAddresses(uint64_t baseAddr) {
    for (const auto *layout : _atomLayouts) {
      const DefinedAtom *atom = cast<DefinedAtom>(layout->_atom);
      uint64_t addr = layout->_virtualAddr;
      llvm::dbgs() << llvm::format("0x%08llx: ", addr + baseAddr)
                   << (atom->name().empty() ? "(anonymous)" : atom->name())
                   << "\n";
    }
  }

  /// List all virtual addresses (and not relative virtual addresses) that need
  /// to be fixed up if image base is relocated. The only relocation type that
  /// needs to be fixed is DIR32 on i386. REL32 is not (and should not be)
  /// fixed up because it's PC-relative.
  void addBaseRelocations(std::vector<uint64_t> &relocSites) {
    // TODO: llvm-objdump doesn't support parsing the base relocation table, so
    // we can't really test this at the moment. As a temporary solution, we
    // should output debug messages with atom names and addresses so that we
    // can inspect relocations, and fix the tests (base-reloc.test, maybe
    // others) to use those messages.
    for (const auto *layout : _atomLayouts) {
      const DefinedAtom *atom = cast<DefinedAtom>(layout->_atom);
      for (const Reference *ref : *atom)
        if (ref->kind() == llvm::COFF::IMAGE_REL_I386_DIR32)
          relocSites.push_back(layout->_virtualAddr + ref->offsetInAtom());
    }
  }

  // Set the file offset of the beginning of this section.
  virtual void setFileOffset(uint64_t fileOffset) {
    Chunk::setFileOffset(fileOffset);
    for (AtomLayout *layout : _atomLayouts)
      layout->_fileOffset += fileOffset;
  }

  uint64_t getSectionRva() {
    assert(_atomLayouts.size() > 0);
    return _atomLayouts[0]->_virtualAddr;
  }

  virtual void setVirtualAddress(uint32_t rva) {
    for (AtomLayout *layout : _atomLayouts)
      layout->_virtualAddr += rva;
  }

  uint64_t getAtomVirtualAddress(StringRef name) {
    for (auto atomLayout : _atomLayouts)
      if (atomLayout->_atom->name() == name)
        return atomLayout->_virtualAddr;
    return 0;
  }

  static bool classof(const Chunk *c) {
    Kind kind = c->getKind();
    return kind == kindSection || kind == kindDataDirectory;
  }

protected:
  AtomChunk(Kind kind) : Chunk(kind) {}
  std::vector<AtomLayout *> _atomLayouts;
};

/// A DataDirectoryChunk represents data directory entries that follows the PE
/// header in the output file. An entry consists of an 8 byte field that
/// indicates a relative virtual address (the starting address of the entry data
/// in memory) and 8 byte entry data size.
class DataDirectoryChunk : public AtomChunk {
public:
  DataDirectoryChunk(const File &linkedFile)
      : AtomChunk(kindDataDirectory), _file(linkedFile) {
    // Extract atoms from the linked file and append them to this section.
    for (const DefinedAtom *atom : linkedFile.defined()) {
      if (atom->contentType() == DefinedAtom::typeDataDirectoryEntry) {
        uint64_t offset = atom->ordinal() * sizeof(llvm::object::data_directory);
        _atomLayouts.push_back(new (_alloc) AtomLayout(atom, offset, offset));
      }
    }
  }

  virtual uint64_t size() const {
    return sizeof(llvm::object::data_directory) * 16;
  }

  void setBaseRelocField(uint32_t addr, uint32_t size) {
    auto *atom = new (_alloc) coff::COFFDataDirectoryAtom(
        _file, llvm::COFF::DataDirectoryIndex::BASE_RELOCATION_TABLE, size,
        addr);
    uint64_t offset = atom->ordinal() * sizeof(llvm::object::data_directory);
    _atomLayouts.push_back(new (_alloc) AtomLayout(atom, offset, offset));
  }

  virtual void write(uint8_t *fileBuffer) {
    for (const AtomLayout *layout : _atomLayouts) {
      if (!layout)
        continue;
      ArrayRef<uint8_t> content = static_cast<const DefinedAtom *>(layout->_atom)->rawContent();
      std::memcpy(fileBuffer + layout->_fileOffset, content.data(), content.size());
    }
  }

private:
  const File &_file;
  mutable llvm::BumpPtrAllocator _alloc;
};

/// A SectionChunk represents a section containing atoms. It consists of a
/// section header that to be written to PECOFF header and atoms which to be
/// written to the raw data section.
class SectionChunk : public AtomChunk {
public:
  /// Returns the size of the section on disk. The returned value is multiple
  /// of disk sector, so the size may include the null padding at the end of
  /// section.
  virtual uint64_t size() const {
    return llvm::RoundUpToAlignment(_size, _align);
  }

  virtual uint64_t rawSize() const {
    return _size;
  }

  // Set the file offset of the beginning of this section.
  virtual void setFileOffset(uint64_t fileOffset) {
    AtomChunk::setFileOffset(fileOffset);
    _sectionHeader.PointerToRawData = fileOffset;
  }

  virtual void setVirtualAddress(uint32_t rva) {
    _sectionHeader.VirtualAddress = rva;
    AtomChunk::setVirtualAddress(rva);
  }

  virtual uint32_t getVirtualAddress() { return _sectionHeader.VirtualAddress; }

  virtual llvm::object::coff_section &getSectionHeader() {
    // Fix up section size before returning it. VirtualSize should be the size
    // of the actual content, and SizeOfRawData should be aligned to the section
    // alignment.
    _sectionHeader.VirtualSize = _size;
    _sectionHeader.SizeOfRawData = size();
    return _sectionHeader;
  }

  ulittle32_t getSectionCharacteristics() {
    return _sectionHeader.Characteristics;
  }

  void appendAtom(const DefinedAtom *atom) {
    // Atom may have to be at a proper alignment boundary. If so, move the
    // pointer to make a room after the last atom before adding new one.
    _size = llvm::RoundUpToAlignment(_size, 1 << atom->alignment().powerOf2);

    // Create an AtomLayout and move the current pointer.
    auto *layout = new (_alloc) AtomLayout(atom, _size, _size);
    _atomLayouts.push_back(layout);
    _size += atom->size();
  }

  static bool classof(const Chunk *c) { return c->getKind() == kindSection; }

protected:
  SectionChunk(StringRef sectionName, uint32_t characteristics)
      : AtomChunk(kindSection),
        _sectionHeader(createSectionHeader(sectionName, characteristics)) {
    // The section should be aligned to disk sector.
    _align = SECTOR_SIZE;
  }

  void buildContents(const File &linkedFile,
                     bool (*isEligible)(const DefinedAtom *)) {
    // Extract atoms from the linked file and append them to this section.
    for (const DefinedAtom *atom : linkedFile.defined()) {
      assert(atom->sectionChoice() == DefinedAtom::sectionBasedOnContent);
      if (isEligible(atom))
        appendAtom(atom);
    }

    // Now that we have a list of atoms that to be written in this section,
    // and we know the size of the section. Let's write them to the section
    // header. VirtualSize should be the size of the actual content, and
    // SizeOfRawData should be aligned to the section alignment.
    _sectionHeader.VirtualSize = _size;
    _sectionHeader.SizeOfRawData = size();
  }

private:
  llvm::object::coff_section
  createSectionHeader(StringRef sectionName, uint32_t characteristics) const {
    llvm::object::coff_section header;

    // Section name equal to or shorter than 8 byte fits in the section
    // header. Longer names should be stored to string table, which is not
    // implemented yet.
    if (sizeof(header.Name) < sectionName.size())
      llvm_unreachable("Cannot handle section name longer than 8 byte");

    // Name field must be NUL-padded. If the name is exactly 8 byte long,
    // there's no terminating NUL.
    std::memset(header.Name, 0, sizeof(header.Name));
    std::strncpy(header.Name, sectionName.data(), sizeof(header.Name));

    header.VirtualSize = 0;
    header.VirtualAddress = 0;
    header.SizeOfRawData = 0;
    header.PointerToRawData = 0;
    header.PointerToRelocations = 0;
    header.PointerToLinenumbers = 0;
    header.NumberOfRelocations = 0;
    header.NumberOfLinenumbers = 0;
    header.Characteristics = characteristics;
    return header;
  }

  llvm::object::coff_section _sectionHeader;
  mutable llvm::BumpPtrAllocator _alloc;
};

void SectionHeaderTableChunk::addSection(SectionChunk *chunk) {
  _sections.push_back(chunk);
}

uint64_t SectionHeaderTableChunk::size() const {
  return _sections.size() * sizeof(llvm::object::coff_section);
}

void SectionHeaderTableChunk::write(uint8_t *fileBuffer) {
  uint64_t offset = 0;
  fileBuffer += fileOffset();
  for (const auto &chunk : _sections) {
    const llvm::object::coff_section &header = chunk->getSectionHeader();
    std::memcpy(fileBuffer + offset, &header, sizeof(header));
    offset += sizeof(header);
  }
}

// \brief A TextSectionChunk represents a .text section.
class TextSectionChunk : public SectionChunk {
public:
  TextSectionChunk(const File &linkedFile)
      : SectionChunk(".text", characteristics) {
    buildContents(linkedFile, [](const DefinedAtom *atom) {
      return atom->contentType() == DefinedAtom::typeCode;
    });
  }

private:
  // When loaded into memory, text section should be readable and executable.
  static const uint32_t characteristics =
      llvm::COFF::IMAGE_SCN_CNT_CODE | llvm::COFF::IMAGE_SCN_MEM_EXECUTE |
      llvm::COFF::IMAGE_SCN_MEM_READ;
};

// \brief A RDataSectionChunk represents a .rdata section.
class RDataSectionChunk : public SectionChunk {
public:
  RDataSectionChunk(const File &linkedFile)
      : SectionChunk(".rdata", characteristics) {
    buildContents(linkedFile, [](const DefinedAtom *atom) {
      return (atom->contentType() == DefinedAtom::typeData &&
              atom->permissions() == DefinedAtom::permR__);
    });
  }

private:
  // When loaded into memory, rdata section should be readable.
  static const uint32_t characteristics =
      llvm::COFF::IMAGE_SCN_MEM_READ |
      llvm::COFF::IMAGE_SCN_CNT_INITIALIZED_DATA;
};

// \brief A DataSectionChunk represents a .data section.
class DataSectionChunk : public SectionChunk {
public:
  DataSectionChunk(const File &linkedFile)
      : SectionChunk(".data", characteristics) {
    buildContents(linkedFile, [](const DefinedAtom *atom) {
      return (atom->contentType() == DefinedAtom::typeData &&
              atom->permissions() == DefinedAtom::permRW_);
    });
  }

private:
  // When loaded into memory, data section should be readable and writable.
  static const uint32_t characteristics =
      llvm::COFF::IMAGE_SCN_MEM_READ |
      llvm::COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
      llvm::COFF::IMAGE_SCN_MEM_WRITE;
};

// \brief A BSSSectionChunk represents a .bss section.
//
// Seems link.exe does not emit .bss section but instead merges it with .data
// section. In COFF, if the size of the section in the header is greater than
// the size of the actual data on disk, the section on memory is zero-padded.
// That's why .bss can be merge with .data just by appending it at the end of
// the section.
//
// The executable with .bss is also valid and easier to understand. So we chose
// to create .bss in LLD.
class BssSectionChunk : public SectionChunk {
public:
  // BSS section does not have contents, so write should be no-op.
  virtual void write(uint8_t *fileBuffer) {}

  virtual llvm::object::coff_section &getSectionHeader() {
    llvm::object::coff_section &sectionHeader =
        SectionChunk::getSectionHeader();
    sectionHeader.VirtualSize = 0;
    sectionHeader.PointerToRawData = 0;
    return sectionHeader;
  }

  BssSectionChunk(const File &linkedFile)
      : SectionChunk(".bss", characteristics) {
    buildContents(linkedFile, [](const DefinedAtom *atom) {
      return atom->contentType() == DefinedAtom::typeZeroFill;
    });
  }

private:
  // When loaded into memory, bss section should be readable and writable.
  static const uint32_t characteristics =
      llvm::COFF::IMAGE_SCN_MEM_READ |
      llvm::COFF::IMAGE_SCN_CNT_UNINITIALIZED_DATA |
      llvm::COFF::IMAGE_SCN_MEM_WRITE;
};

/// A BaseRelocAtom represents a base relocation block in ".reloc" section.
class BaseRelocAtom : public coff::COFFLinkerInternalAtom {
public:
  BaseRelocAtom(const File &file, std::vector<uint8_t> data)
      : COFFLinkerInternalAtom(file, std::move(data)) {}

  virtual ContentType contentType() const { return typeData; }
  virtual Alignment alignment() const { return Alignment(2); }
};

/// A BaseRelocChunk represents ".reloc" section.
///
/// .reloc section contains a list of addresses. If the PE/COFF loader decides
/// to load the binary at a memory address different from its preferred base
/// address, which is specified by ImageBase field in the COFF header, the
/// loader needs to relocate the binary, so that all the addresses in the binary
/// point to new locations. The loader will do that by fixing up the addresses
/// specified by .reloc section.
///
/// The executable is almost always loaded at the preferred base address because
/// it's loaded into an empty address space. The DLL is however an subject of
/// load-time relocation because it may conflict with other DLLs or the
/// executable.
class BaseRelocChunk : public SectionChunk {
  typedef std::vector<std::unique_ptr<Chunk>> ChunkVectorT;
  typedef std::map<uint64_t, std::vector<uint16_t>> PageOffsetT;

public:
  BaseRelocChunk(const File &linkedFile)
      : SectionChunk(".reloc", characteristics), _file(linkedFile) {}

  /// Creates .reloc section content from the other sections. The content of
  /// .reloc is basically a list of relocation sites. The relocation sites are
  /// divided into blocks. Each block represents the base relocation for a 4K
  /// page.
  ///
  /// By dividing 32 bit RVAs into blocks, COFF saves disk and memory space for
  /// the base relocation. A block consists of a 32 bit page RVA and 16 bit
  /// relocation entries which represent offsets in the page. That is a more
  /// compact representation than a simple vector of 32 bit RVAs.
  void setContents(ChunkVectorT &chunks) {
    std::vector<uint64_t> relocSites = listRelocSites(chunks);
    PageOffsetT blocks = groupByPage(relocSites);
    for (auto &i : blocks) {
      uint64_t pageAddr = i.first;
      const std::vector<uint16_t> &offsetsInPage = i.second;
      appendAtom(createBaseRelocBlock(_file, pageAddr, offsetsInPage));
    }
  }

private:
  // When loaded into memory, reloc section should be readable and writable.
  static const uint32_t characteristics =
      llvm::COFF::IMAGE_SCN_MEM_READ |
      llvm::COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
      llvm::COFF::IMAGE_SCN_MEM_DISCARDABLE;

  // Returns a list of RVAs that needs to be relocated if the binary is loaded
  // at an address different from its preferred one.
  std::vector<uint64_t> listRelocSites(ChunkVectorT &chunks) {
    std::vector<uint64_t> ret;
    for (auto &cp : chunks)
      if (SectionChunk *chunk = dyn_cast<SectionChunk>(&*cp))
        chunk->addBaseRelocations(ret);
    return std::move(ret);
  }

  // Divide the given RVAs into blocks.
  PageOffsetT groupByPage(std::vector<uint64_t> relocSites) {
    PageOffsetT blocks;
    uint64_t mask = static_cast<uint64_t>(PAGE_SIZE) - 1;
    for (uint64_t addr : relocSites)
      blocks[addr & ~mask].push_back(addr & mask);
    return std::move(blocks);
  }

  // Create the content of a relocation block.
  DefinedAtom *createBaseRelocBlock(const File &file, uint64_t pageAddr,
                                    const std::vector<uint16_t> &offsets) {
    // Relocation blocks should be padded with IMAGE_REL_I386_ABSOLUTE to be
    // aligned to a DWORD size boundary.
    uint32_t size = llvm::RoundUpToAlignment(sizeof(ulittle32_t) * 2
        + sizeof(ulittle16_t) * offsets.size(), sizeof(ulittle32_t));
    std::vector<uint8_t> contents(size);
    uint8_t *ptr = &contents[0];

    // The first four bytes is the page RVA.
    *reinterpret_cast<ulittle32_t *>(ptr) = pageAddr;
    ptr += sizeof(ulittle32_t);

    // The second four bytes is the size of the block, including the the page
    // RVA and this size field.
    *reinterpret_cast<ulittle32_t *>(ptr) = size;
    ptr += sizeof(ulittle32_t);

    // The rest of the block consists of offsets in the page.
    for (uint16_t offset : offsets) {
      assert(offset < PAGE_SIZE);
      uint16_t val = (llvm::COFF::IMAGE_REL_BASED_HIGHLOW << 12) | offset;
      *reinterpret_cast<ulittle16_t *>(ptr) = val;
      ptr += sizeof(ulittle16_t);
    }
    return new (_alloc) BaseRelocAtom(file, std::move(contents));
  }

  mutable llvm::BumpPtrAllocator _alloc;
  const File &_file;
};

}  // end anonymous namespace

class ExecutableWriter : public Writer {
public:
  explicit ExecutableWriter(const PECOFFLinkingContext &context)
      : _PECOFFLinkingContext(context), _numSections(0),
        _imageSizeInMemory(PAGE_SIZE), _imageSizeOnDisk(0) {}

  // Create all chunks that consist of the output file.
  void build(const File &linkedFile) {
    // Create file chunks and add them to the list.
    auto *dosStub = new DOSStubChunk();
    auto *peHeader = new PEHeaderChunk(_PECOFFLinkingContext);
    auto *dataDirectory = new DataDirectoryChunk(linkedFile);
    auto *sectionTable = new SectionHeaderTableChunk();
    auto *text = new TextSectionChunk(linkedFile);
    auto *rdata = new RDataSectionChunk(linkedFile);
    auto *data = new DataSectionChunk(linkedFile);
    auto *bss = new BssSectionChunk(linkedFile);
    BaseRelocChunk *baseReloc = nullptr;
    if (_PECOFFLinkingContext.getBaseRelocationEnabled())
      baseReloc = new BaseRelocChunk(linkedFile);

    addChunk(dosStub);
    addChunk(peHeader);
    addChunk(dataDirectory);
    addChunk(sectionTable);

    // Do not add the empty section. Windows loader does not like a section of
    // size zero and rejects such executable.
    if (text->size())
      addSectionChunk(text, sectionTable);
    if (rdata->size())
      addSectionChunk(rdata, sectionTable);
    if (data->size())
      addSectionChunk(data, sectionTable);
    if (bss->size())
      addSectionChunk(bss, sectionTable);

    // Now that we know the addresses of all defined atoms that needs to be
    // relocated. So we can create the ".reloc" section which contains all the
    // relocation sites.
    if (baseReloc) {
      baseReloc->setContents(_chunks);
      if (baseReloc->size()) {
        addSectionChunk(baseReloc, sectionTable);
        dataDirectory->setBaseRelocField(baseReloc->getSectionRva(),
                                         baseReloc->rawSize());
      }
    }

    setImageSizeOnDisk();

    // Now that we know the size and file offset of sections. Set the file
    // header accordingly.
    peHeader->setSizeOfCode(calcSizeOfCode());
    if (text->size()) {
      peHeader->setBaseOfCode(text->getVirtualAddress());
    }
    if (rdata->size()) {
      peHeader->setBaseOfData(rdata->getVirtualAddress());
    } else if (data->size()) {
      peHeader->setBaseOfData(data->getVirtualAddress());
    }
    peHeader->setSizeOfInitializedData(calcSizeOfInitializedData());
    peHeader->setSizeOfUninitializedData(calcSizeOfUninitializedData());
    peHeader->setNumberOfSections(_numSections);
    peHeader->setSizeOfImage(_imageSizeInMemory);

    // The combined size of the DOS, PE and section headers including garbage
    // between the end of the header and the beginning of the first section.
    peHeader->setSizeOfHeaders(dosStub->size() + peHeader->size() +
        sectionTable->size() + dataDirectory->size());

    setAddressOfEntryPoint(text, peHeader);
  }

  virtual error_code writeFile(const File &linkedFile, StringRef path) {
    this->build(linkedFile);

    uint64_t totalSize = _chunks.back()->fileOffset() + _chunks.back()->size();
    OwningPtr<llvm::FileOutputBuffer> buffer;
    error_code ec = llvm::FileOutputBuffer::create(
        path, totalSize, buffer, llvm::FileOutputBuffer::F_executable);
    if (ec)
      return ec;

    for (const auto &chunk : _chunks)
      chunk->write(buffer->getBufferStart());
    applyAllRelocations(buffer->getBufferStart());
    DEBUG(printAllAtomAddresses());
    return buffer->commit();
  }

private:
  /// Apply relocations to the output file buffer. This two pass. In the first
  /// pass, we visit all atoms to create a map from atom to its virtual
  /// address. In the second pass, we visit all relocation references to fix
  /// up addresses in the buffer.
  void applyAllRelocations(uint8_t *bufferStart) {
    for (auto &cp : _chunks)
      if (AtomChunk *chunk = dyn_cast<AtomChunk>(&*cp))
        chunk->applyRelocations(bufferStart, atomRva,
                                _PECOFFLinkingContext.getBaseAddress());
  }

  /// Print atom VAs. Used only for debugging.
  void printAllAtomAddresses() {
    for (auto &cp : _chunks)
      if (AtomChunk *chunk = dyn_cast<AtomChunk>(&*cp))
        chunk->printAtomAddresses(_PECOFFLinkingContext.getBaseAddress());
  }

  void addChunk(Chunk *chunk) {
    _chunks.push_back(std::unique_ptr<Chunk>(chunk));
  }

  void addSectionChunk(SectionChunk *chunk,
                       SectionHeaderTableChunk *table) {
    _chunks.push_back(std::unique_ptr<Chunk>(chunk));
    table->addSection(chunk);
    _numSections++;

    // Compute and set the starting address of sections when loaded in
    // memory. They are different from positions on disk because sections need
    // to be sector-aligned on disk but page-aligned in memory.
    chunk->setVirtualAddress(_imageSizeInMemory);
    chunk->buildAtomToVirtualAddr(atomRva);
    _imageSizeInMemory = llvm::RoundUpToAlignment(
        _imageSizeInMemory + chunk->size(), PAGE_SIZE);
  }

  void setImageSizeOnDisk() {
    for (auto &chunk : _chunks) {
      // Compute and set the offset of the chunk in the output file.
      _imageSizeOnDisk = llvm::RoundUpToAlignment(_imageSizeOnDisk,
                                                  chunk->align());
      chunk->setFileOffset(_imageSizeOnDisk);
      _imageSizeOnDisk += chunk->size();
    }
  }

  void setAddressOfEntryPoint(TextSectionChunk *text, PEHeaderChunk *peHeader) {
    // Find the virtual address of the entry point symbol if any.
    // PECOFF spec says that entry point for dll images is optional, in which
    // case it must be set to 0.
    if (_PECOFFLinkingContext.entrySymbolName().empty() &&
        _PECOFFLinkingContext.getImageType()
                              == PECOFFLinkingContext::IMAGE_DLL) {
      peHeader->setAddressOfEntryPoint(0);
    } else {
      uint64_t entryPointAddress = text->getAtomVirtualAddress(
        _PECOFFLinkingContext.entrySymbolName());
      if (entryPointAddress != 0)
        peHeader->setAddressOfEntryPoint(entryPointAddress);
    }
  }

  uint64_t calcSectionSize(llvm::COFF::SectionCharacteristics sectionType) {
    uint64_t ret = 0;
    for (auto &cp : _chunks)
      if (SectionChunk *chunk = dyn_cast<SectionChunk>(&*cp))
        if (chunk->getSectionCharacteristics() & sectionType)
          ret += chunk->size();
    return ret;
  }

  uint64_t calcSizeOfInitializedData() {
    return calcSectionSize(llvm::COFF::IMAGE_SCN_CNT_INITIALIZED_DATA);
  }

  uint64_t calcSizeOfUninitializedData() {
    return calcSectionSize(llvm::COFF::IMAGE_SCN_CNT_UNINITIALIZED_DATA);
  }

  uint64_t calcSizeOfCode() {
    return calcSectionSize(llvm::COFF::IMAGE_SCN_CNT_CODE);
  }

  std::vector<std::unique_ptr<Chunk>> _chunks;
  const PECOFFLinkingContext &_PECOFFLinkingContext;
  uint32_t _numSections;

  // The size of the image in memory. This is initialized with PAGE_SIZE, as the
  // first page starting at ImageBase is usually left unmapped. IIUC there's no
  // technical reason to do so, but we'll follow that convention so that we
  // don't produce odd-looking binary.
  uint32_t _imageSizeInMemory;

  // The size of the image on disk. This is basically the sum of all chunks in
  // the output file with paddings between them.
  uint32_t _imageSizeOnDisk;

  // The map from defined atoms to its RVAs. Will be used for relocation.
  std::map<const Atom *, uint64_t> atomRva;
};

} // end namespace pecoff

std::unique_ptr<Writer> createWriterPECOFF(const PECOFFLinkingContext &info) {
  return std::unique_ptr<Writer>(new pecoff::ExecutableWriter(info));
}

} // end namespace lld