root/courgette/disassembler_win32_x64.cc

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DEFINITIONS

This source file includes following definitions.
  1. number_of_data_directories_
  2. ParseHeader
  3. ParseRelocs
  4. RVAToSection
  5. RVAToFileOffset
  6. RVAToPointer
  7. SectionName
  8. ParseFile
  9. ParseAbs32Relocs
  10. ParseRel32RelocsFromSections
  11. ParseRel32RelocsFromSection
  12. ParseNonSectionFileRegion
  13. ParseFileRegion
  14. HistogramTargets
  15. DescribeRVA
  16. FindNextSection
  17. FileOffsetToRVA
  18. ReadDataDirectory

// Copyright 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "courgette/disassembler_win32_x64.h"

#include <algorithm>
#include <string>
#include <vector>

#include "base/basictypes.h"
#include "base/logging.h"

#include "courgette/assembly_program.h"
#include "courgette/courgette.h"
#include "courgette/encoded_program.h"

namespace courgette {

DisassemblerWin32X64::DisassemblerWin32X64(const void* start, size_t length)
  : Disassembler(start, length),
    incomplete_disassembly_(false),
    is_PE32_plus_(false),
    optional_header_(NULL),
    size_of_optional_header_(0),
    offset_of_data_directories_(0),
    machine_type_(0),
    number_of_sections_(0),
    sections_(NULL),
    has_text_section_(false),
    size_of_code_(0),
    size_of_initialized_data_(0),
    size_of_uninitialized_data_(0),
    base_of_code_(0),
    base_of_data_(0),
    image_base_(0),
    size_of_image_(0),
    number_of_data_directories_(0) {
}

// ParseHeader attempts to match up the buffer with the Windows data
// structures that exist within a Windows 'Portable Executable' format file.
// Returns 'true' if the buffer matches, and 'false' if the data looks
// suspicious.  Rather than try to 'map' the buffer to the numerous windows
// structures, we extract the information we need into the courgette::PEInfo
// structure.
//
bool DisassemblerWin32X64::ParseHeader() {
  if (length() < kOffsetOfFileAddressOfNewExeHeader + 4 /*size*/)
    return Bad("Too small");

  // Have 'MZ' magic for a DOS header?
  if (start()[0] != 'M' || start()[1] != 'Z')
    return Bad("Not MZ");

  // offset from DOS header to PE header is stored in DOS header.
  uint32 offset = ReadU32(start(),
                          kOffsetOfFileAddressOfNewExeHeader);

  if (offset >= length())
    return Bad("Bad offset to PE header");

  const uint8* const pe_header = OffsetToPointer(offset);
  const size_t kMinPEHeaderSize = 4 /*signature*/ + kSizeOfCoffHeader;
  if (pe_header <= start() ||
      pe_header >= end() - kMinPEHeaderSize)
    return Bad("Bad offset to PE header");

  if (offset % 8 != 0)
    return Bad("Misaligned PE header");

  // The 'PE' header is an IMAGE_NT_HEADERS structure as defined in WINNT.H.
  // See http://msdn.microsoft.com/en-us/library/ms680336(VS.85).aspx
  //
  // The first field of the IMAGE_NT_HEADERS is the signature.
  if (!(pe_header[0] == 'P' &&
        pe_header[1] == 'E' &&
        pe_header[2] == 0 &&
        pe_header[3] == 0))
    return Bad("no PE signature");

  // The second field of the IMAGE_NT_HEADERS is the COFF header.
  // The COFF header is also called an IMAGE_FILE_HEADER
  //   http://msdn.microsoft.com/en-us/library/ms680313(VS.85).aspx
  const uint8* const coff_header = pe_header + 4;
  machine_type_       = ReadU16(coff_header, 0);
  number_of_sections_ = ReadU16(coff_header, 2);
  size_of_optional_header_ = ReadU16(coff_header, 16);

  // The rest of the IMAGE_NT_HEADERS is the IMAGE_OPTIONAL_HEADER(32|64)
  const uint8* const optional_header = coff_header + kSizeOfCoffHeader;
  optional_header_ = optional_header;

  if (optional_header + size_of_optional_header_ >= end())
    return Bad("optional header past end of file");

  // Check we can read the magic.
  if (size_of_optional_header_ < 2)
    return Bad("optional header no magic");

  uint16 magic = ReadU16(optional_header, 0);

  if (magic == kImageNtOptionalHdr32Magic) {
    is_PE32_plus_ = false;
    offset_of_data_directories_ =
      kOffsetOfDataDirectoryFromImageOptionalHeader32;
  } else if (magic == kImageNtOptionalHdr64Magic) {
    is_PE32_plus_ = true;
    offset_of_data_directories_ =
      kOffsetOfDataDirectoryFromImageOptionalHeader64;
  } else {
    return Bad("unrecognized magic");
  }

  // Check that we can read the rest of the the fixed fields.  Data directories
  // directly follow the fixed fields of the IMAGE_OPTIONAL_HEADER.
  if (size_of_optional_header_ < offset_of_data_directories_)
    return Bad("optional header too short");

  // The optional header is either an IMAGE_OPTIONAL_HEADER32 or
  // IMAGE_OPTIONAL_HEADER64
  // http://msdn.microsoft.com/en-us/library/ms680339(VS.85).aspx
  //
  // Copy the fields we care about.
  size_of_code_               = ReadU32(optional_header, 4);
  size_of_initialized_data_   = ReadU32(optional_header, 8);
  size_of_uninitialized_data_ = ReadU32(optional_header, 12);
  base_of_code_               = ReadU32(optional_header, 20);
  if (is_PE32_plus_) {
    base_of_data_ = 0;
    image_base_  = ReadU64(optional_header, 24);
  } else {
    base_of_data_ = ReadU32(optional_header, 24);
    image_base_   = ReadU32(optional_header, 28);
  }
  size_of_image_ = ReadU32(optional_header, 56);
  number_of_data_directories_ =
    ReadU32(optional_header, (is_PE32_plus_ ? 108 : 92));

  if (size_of_code_ >= length() ||
      size_of_initialized_data_ >= length() ||
      size_of_code_ + size_of_initialized_data_ >= length()) {
    // This validation fires on some perfectly fine executables.
    //  return Bad("code or initialized data too big");
  }

  // TODO(sra): we can probably get rid of most of the data directories.
  bool b = true;
  // 'b &= ...' could be short circuit 'b = b && ...' but it is not necessary
  // for correctness and it compiles smaller this way.
  b &= ReadDataDirectory(0, &export_table_);
  b &= ReadDataDirectory(1, &import_table_);
  b &= ReadDataDirectory(2, &resource_table_);
  b &= ReadDataDirectory(3, &exception_table_);
  b &= ReadDataDirectory(5, &base_relocation_table_);
  b &= ReadDataDirectory(11, &bound_import_table_);
  b &= ReadDataDirectory(12, &import_address_table_);
  b &= ReadDataDirectory(13, &delay_import_descriptor_);
  b &= ReadDataDirectory(14, &clr_runtime_header_);
  if (!b) {
    return Bad("malformed data directory");
  }

  // Sections follow the optional header.
  sections_ =
      reinterpret_cast<const Section*>(optional_header +
                                       size_of_optional_header_);
  size_t detected_length = 0;

  for (int i = 0;  i < number_of_sections_;  ++i) {
    const Section* section = &sections_[i];

    // TODO(sra): consider using the 'characteristics' field of the section
    // header to see if the section contains instructions.
    if (memcmp(section->name, ".text", 6) == 0)
      has_text_section_ = true;

    uint32 section_end =
        section->file_offset_of_raw_data + section->size_of_raw_data;
    if (section_end > detected_length)
      detected_length = section_end;
  }

  // Pretend our in-memory copy is only as long as our detected length.
  ReduceLength(detected_length);

  if (is_32bit()) {
    return Bad("32 bit executables are not supported by this disassembler");
  }

  if (!has_text_section()) {
    return Bad("Resource-only executables are not yet supported");
  }

  return Good();
}

bool DisassemblerWin32X64::Disassemble(AssemblyProgram* target) {
  if (!ok())
    return false;

  target->set_image_base(image_base());

  if (!ParseAbs32Relocs())
    return false;

  ParseRel32RelocsFromSections();

  if (!ParseFile(target))
    return false;

  target->DefaultAssignIndexes();

  return true;
}

////////////////////////////////////////////////////////////////////////////////

bool DisassemblerWin32X64::ParseRelocs(std::vector<RVA> *relocs) {
  relocs->clear();

  size_t relocs_size = base_relocation_table_.size_;
  if (relocs_size == 0)
    return true;

  // The format of the base relocation table is a sequence of variable sized
  // IMAGE_BASE_RELOCATION blocks.  Search for
  //   "The format of the base relocation data is somewhat quirky"
  // at http://msdn.microsoft.com/en-us/library/ms809762.aspx

  const uint8* relocs_start = RVAToPointer(base_relocation_table_.address_);
  const uint8* relocs_end = relocs_start + relocs_size;

  // Make sure entire base relocation table is within the buffer.
  if (relocs_start < start() ||
      relocs_start >= end() ||
      relocs_end <= start() ||
      relocs_end > end()) {
    return Bad(".relocs outside image");
  }

  const uint8* block = relocs_start;

  // Walk the variable sized blocks.
  while (block + 8 < relocs_end) {
    RVA page_rva = ReadU32(block, 0);
    uint32 size = ReadU32(block, 4);
    if (size < 8 ||        // Size includes header ...
        size % 4  !=  0)   // ... and is word aligned.
      return Bad("unreasonable relocs block");

    const uint8* end_entries = block + size;

    if (end_entries <= block ||
        end_entries <= start() ||
        end_entries > end())
      return Bad(".relocs block outside image");

    // Walk through the two-byte entries.
    for (const uint8* p = block + 8;  p < end_entries;  p += 2) {
      uint16 entry = ReadU16(p, 0);
      int type = entry >> 12;
      int offset = entry & 0xFFF;

      RVA rva = page_rva + offset;
      if (type == 10) {         // IMAGE_REL_BASED_DIR64
        relocs->push_back(rva);
      } else if (type == 0) {  // IMAGE_REL_BASED_ABSOLUTE
        // Ignore, used as padding.
      } else {
        // Does not occur in Windows x64 executables.
        return Bad("unknown type of reloc");
      }
    }

    block += size;
  }

  std::sort(relocs->begin(), relocs->end());

  return true;
}

const Section* DisassemblerWin32X64::RVAToSection(RVA rva) const {
  for (int i = 0; i < number_of_sections_; i++) {
    const Section* section = &sections_[i];
    uint32 offset = rva - section->virtual_address;
    if (offset < section->virtual_size) {
      return section;
    }
  }
  return NULL;
}

int DisassemblerWin32X64::RVAToFileOffset(RVA rva) const {
  const Section* section = RVAToSection(rva);
  if (section) {
    uint32 offset = rva - section->virtual_address;
    if (offset < section->size_of_raw_data) {
      return section->file_offset_of_raw_data + offset;
    } else {
      return kNoOffset;  // In section but not in file (e.g. uninit data).
    }
  }

  // Small RVA values point into the file header in the loaded image.
  // RVA 0 is the module load address which Windows uses as the module handle.
  // RVA 2 sometimes occurs, I'm not sure what it is, but it would map into the
  // DOS header.
  if (rva == 0 || rva == 2)
    return rva;

  NOTREACHED();
  return kNoOffset;
}

const uint8* DisassemblerWin32X64::RVAToPointer(RVA rva) const {
  int file_offset = RVAToFileOffset(rva);
  if (file_offset == kNoOffset)
    return NULL;
  else
    return OffsetToPointer(file_offset);
}

std::string DisassemblerWin32X64::SectionName(const Section* section) {
  if (section == NULL)
    return "<none>";
  char name[9];
  memcpy(name, section->name, 8);
  name[8] = '\0';  // Ensure termination.
  return name;
}

CheckBool DisassemblerWin32X64::ParseFile(AssemblyProgram* program) {
  // Walk all the bytes in the file, whether or not in a section.
  uint32 file_offset = 0;
  while (file_offset < length()) {
    const Section* section = FindNextSection(file_offset);
    if (section == NULL) {
      // No more sections.  There should not be extra stuff following last
      // section.
      //   ParseNonSectionFileRegion(file_offset, pe_info().length(), program);
      break;
    }
    if (file_offset < section->file_offset_of_raw_data) {
      uint32 section_start_offset = section->file_offset_of_raw_data;
      if(!ParseNonSectionFileRegion(file_offset, section_start_offset,
                                    program))
        return false;

      file_offset = section_start_offset;
    }
    uint32 end = file_offset + section->size_of_raw_data;
    if (!ParseFileRegion(section, file_offset, end, program))
      return false;
    file_offset = end;
  }

#if COURGETTE_HISTOGRAM_TARGETS
  HistogramTargets("abs32 relocs", abs32_target_rvas_);
  HistogramTargets("rel32 relocs", rel32_target_rvas_);
#endif

  return true;
}

bool DisassemblerWin32X64::ParseAbs32Relocs() {
  abs32_locations_.clear();
  if (!ParseRelocs(&abs32_locations_))
    return false;

  std::sort(abs32_locations_.begin(), abs32_locations_.end());

#if COURGETTE_HISTOGRAM_TARGETS
  for (size_t i = 0;  i < abs32_locations_.size(); ++i) {
    RVA rva = abs32_locations_[i];
    // The 4 bytes at the relocation are a reference to some address.
    uint32 target_address = Read32LittleEndian(RVAToPointer(rva));
    ++abs32_target_rvas_[target_address - image_base()];
  }
#endif
  return true;
}

void DisassemblerWin32X64::ParseRel32RelocsFromSections() {
  uint32 file_offset = 0;
  while (file_offset < length()) {
    const Section* section = FindNextSection(file_offset);
    if (section == NULL)
      break;
    if (file_offset < section->file_offset_of_raw_data)
      file_offset = section->file_offset_of_raw_data;
    ParseRel32RelocsFromSection(section);
    file_offset += section->size_of_raw_data;
  }
  std::sort(rel32_locations_.begin(), rel32_locations_.end());

#if COURGETTE_HISTOGRAM_TARGETS
  VLOG(1) << "abs32_locations_ " << abs32_locations_.size()
          << "\nrel32_locations_ " << rel32_locations_.size()
          << "\nabs32_target_rvas_ " << abs32_target_rvas_.size()
          << "\nrel32_target_rvas_ " << rel32_target_rvas_.size();

  int common = 0;
  std::map<RVA, int>::iterator abs32_iter = abs32_target_rvas_.begin();
  std::map<RVA, int>::iterator rel32_iter = rel32_target_rvas_.begin();
  while (abs32_iter != abs32_target_rvas_.end() &&
         rel32_iter != rel32_target_rvas_.end()) {
    if (abs32_iter->first < rel32_iter->first)
      ++abs32_iter;
    else if (rel32_iter->first < abs32_iter->first)
      ++rel32_iter;
    else {
      ++common;
      ++abs32_iter;
      ++rel32_iter;
    }
  }
  VLOG(1) << "common " << common;
#endif
}

void DisassemblerWin32X64::ParseRel32RelocsFromSection(const Section* section) {
  // TODO(sra): use characteristic.
  bool isCode = strcmp(section->name, ".text") == 0;
  if (!isCode)
    return;

  uint32 start_file_offset = section->file_offset_of_raw_data;
  uint32 end_file_offset = start_file_offset + section->size_of_raw_data;
  RVA relocs_start_rva = base_relocation_table().address_;

  const uint8* start_pointer = OffsetToPointer(start_file_offset);
  const uint8* end_pointer = OffsetToPointer(end_file_offset);

  RVA start_rva = FileOffsetToRVA(start_file_offset);
  RVA end_rva = start_rva + section->virtual_size;

  // Quick way to convert from Pointer to RVA within a single Section is to
  // subtract 'pointer_to_rva'.
  const uint8* const adjust_pointer_to_rva = start_pointer - start_rva;

  std::vector<RVA>::iterator abs32_pos = abs32_locations_.begin();

  // Find the rel32 relocations.
  const uint8* p = start_pointer;
  while (p < end_pointer) {
    RVA current_rva = static_cast<RVA>(p - adjust_pointer_to_rva);
    if (current_rva == relocs_start_rva) {
      uint32 relocs_size = base_relocation_table().size_;
      if (relocs_size) {
        p += relocs_size;
        continue;
      }
    }

    //while (abs32_pos != abs32_locations_.end() && *abs32_pos < current_rva)
    //  ++abs32_pos;

    // Heuristic discovery of rel32 locations in instruction stream: are the
    // next few bytes the start of an instruction containing a rel32
    // addressing mode?
    const uint8* rel32 = NULL;

    if (p + 5 <= end_pointer) {
      if (*p == 0xE8 || *p == 0xE9) {  // jmp rel32 and call rel32
        rel32 = p + 1;
      }
    }
    if (p + 6 <= end_pointer) {
      if (*p == 0x0F  &&  (*(p+1) & 0xF0) == 0x80) {  // Jcc long form
        if (p[1] != 0x8A && p[1] != 0x8B)  // JPE/JPO unlikely
          rel32 = p + 2;
      }
    }
    if (rel32) {
      RVA rel32_rva = static_cast<RVA>(rel32 - adjust_pointer_to_rva);

      // Is there an abs32 reloc overlapping the candidate?
      while (abs32_pos != abs32_locations_.end() && *abs32_pos < rel32_rva - 3)
        ++abs32_pos;
      // Now: (*abs32_pos > rel32_rva - 4) i.e. the lowest addressed 4-byte
      // region that could overlap rel32_rva.
      if (abs32_pos != abs32_locations_.end()) {
        if (*abs32_pos < rel32_rva + 4) {
          // Beginning of abs32 reloc is before end of rel32 reloc so they
          // overlap.  Skip four bytes past the abs32 reloc.
          p += (*abs32_pos + 4) - current_rva;
          continue;
        }
      }

      RVA target_rva = rel32_rva + 4 + Read32LittleEndian(rel32);
      // To be valid, rel32 target must be within image, and within this
      // section.
      if (IsValidRVA(target_rva) &&
          start_rva <= target_rva && target_rva < end_rva) {
        rel32_locations_.push_back(rel32_rva);
#if COURGETTE_HISTOGRAM_TARGETS
        ++rel32_target_rvas_[target_rva];
#endif
        p = rel32 + 4;
        continue;
      }
    }
    p += 1;
  }
}

CheckBool DisassemblerWin32X64::ParseNonSectionFileRegion(
    uint32 start_file_offset,
    uint32 end_file_offset,
    AssemblyProgram* program) {
  if (incomplete_disassembly_)
    return true;

  const uint8* start = OffsetToPointer(start_file_offset);
  const uint8* end = OffsetToPointer(end_file_offset);

  const uint8* p = start;

  while (p < end) {
    if (!program->EmitByteInstruction(*p))
      return false;
    ++p;
  }

  return true;
}

CheckBool DisassemblerWin32X64::ParseFileRegion(
    const Section* section,
    uint32 start_file_offset, uint32 end_file_offset,
    AssemblyProgram* program) {
  RVA relocs_start_rva = base_relocation_table().address_;

  const uint8* start_pointer = OffsetToPointer(start_file_offset);
  const uint8* end_pointer = OffsetToPointer(end_file_offset);

  RVA start_rva = FileOffsetToRVA(start_file_offset);
  RVA end_rva = start_rva + section->virtual_size;

  // Quick way to convert from Pointer to RVA within a single Section is to
  // subtract 'pointer_to_rva'.
  const uint8* const adjust_pointer_to_rva = start_pointer - start_rva;

  std::vector<RVA>::iterator rel32_pos = rel32_locations_.begin();
  std::vector<RVA>::iterator abs32_pos = abs32_locations_.begin();

  if (!program->EmitOriginInstruction(start_rva))
    return false;

  const uint8* p = start_pointer;

  while (p < end_pointer) {
    RVA current_rva = static_cast<RVA>(p - adjust_pointer_to_rva);

    // The base relocation table is usually in the .relocs section, but it could
    // actually be anywhere.  Make sure we skip it because we will regenerate it
    // during assembly.
    if (current_rva == relocs_start_rva) {
      if (!program->EmitPeRelocsInstruction())
        return false;
      uint32 relocs_size = base_relocation_table().size_;
      if (relocs_size) {
        p += relocs_size;
        continue;
      }
    }

    while (abs32_pos != abs32_locations_.end() && *abs32_pos < current_rva)
      ++abs32_pos;

    if (abs32_pos != abs32_locations_.end() && *abs32_pos == current_rva) {
      uint32 target_address = Read32LittleEndian(p);
      RVA target_rva = target_address - image_base();
      // TODO(sra): target could be Label+offset.  It is not clear how to guess
      // which it might be.  We assume offset==0.
      if (!program->EmitAbs32(program->FindOrMakeAbs32Label(target_rva)))
        return false;
      p += 4;
      continue;
    }

    while (rel32_pos != rel32_locations_.end() && *rel32_pos < current_rva)
      ++rel32_pos;

    if (rel32_pos != rel32_locations_.end() && *rel32_pos == current_rva) {
      RVA target_rva = current_rva + 4 + Read32LittleEndian(p);
      if (!program->EmitRel32(program->FindOrMakeRel32Label(target_rva)))
        return false;
      p += 4;
      continue;
    }

    if (incomplete_disassembly_) {
      if ((abs32_pos == abs32_locations_.end() || end_rva <= *abs32_pos) &&
          (rel32_pos == rel32_locations_.end() || end_rva <= *rel32_pos) &&
          (end_rva <= relocs_start_rva || current_rva >= relocs_start_rva)) {
        // No more relocs in this section, don't bother encoding bytes.
        break;
      }
    }

    if (!program->EmitByteInstruction(*p))
      return false;
    p += 1;
  }

  return true;
}

#if COURGETTE_HISTOGRAM_TARGETS
// Histogram is printed to std::cout.  It is purely for debugging the algorithm
// and is only enabled manually in 'exploration' builds.  I don't want to add
// command-line configuration for this feature because this code has to be
// small, which means compiled-out.
void DisassemblerWin32X64::HistogramTargets(const char* kind,
                                            const std::map<RVA, int>& map) {
  int total = 0;
  std::map<int, std::vector<RVA> > h;
  for (std::map<RVA, int>::const_iterator p = map.begin();
       p != map.end();
       ++p) {
    h[p->second].push_back(p->first);
    total += p->second;
  }

  std::cout << total << " " << kind << " to "
            << map.size() << " unique targets" << std::endl;

  std::cout << "indegree: #targets-with-indegree (example)" << std::endl;
  const int kFirstN = 15;
  bool someSkipped = false;
  int index = 0;
  for (std::map<int, std::vector<RVA> >::reverse_iterator p = h.rbegin();
       p != h.rend();
       ++p) {
    ++index;
    if (index <= kFirstN || p->first <= 3) {
      if (someSkipped) {
        std::cout << "..." << std::endl;
      }
      size_t count = p->second.size();
      std::cout << std::dec << p->first << ": " << count;
      if (count <= 2) {
        for (size_t i = 0;  i < count;  ++i)
          std::cout << "  " << DescribeRVA(p->second[i]);
      }
      std::cout << std::endl;
      someSkipped = false;
    } else {
      someSkipped = true;
    }
  }
}
#endif  // COURGETTE_HISTOGRAM_TARGETS


// DescribeRVA is for debugging only.  I would put it under #ifdef DEBUG except
// that during development I'm finding I need to call it when compiled in
// Release mode.  Hence:
// TODO(sra): make this compile only for debug mode.
std::string DisassemblerWin32X64::DescribeRVA(RVA rva) const {
  const Section* section = RVAToSection(rva);
  std::ostringstream s;
  s << std::hex << rva;
  if (section) {
    s << " (";
    s << SectionName(section) << "+"
      << std::hex << (rva - section->virtual_address)
      << ")";
  }
  return s.str();
}

const Section* DisassemblerWin32X64::FindNextSection(uint32 fileOffset) const {
  const Section* best = 0;
  for (int i = 0; i < number_of_sections_; i++) {
    const Section* section = &sections_[i];
    if (section->size_of_raw_data > 0) {  // i.e. has data in file.
      if (fileOffset <= section->file_offset_of_raw_data) {
        if (best == 0 ||
            section->file_offset_of_raw_data < best->file_offset_of_raw_data) {
          best = section;
        }
      }
    }
  }
  return best;
}

RVA DisassemblerWin32X64::FileOffsetToRVA(uint32 file_offset) const {
  for (int i = 0; i < number_of_sections_; i++) {
    const Section* section = &sections_[i];
    uint32 offset = file_offset - section->file_offset_of_raw_data;
    if (offset < section->size_of_raw_data) {
      return section->virtual_address + offset;
    }
  }
  return 0;
}

bool DisassemblerWin32X64::ReadDataDirectory(
    int index,
    ImageDataDirectory* directory) {

  if (index < number_of_data_directories_) {
    size_t offset = index * 8 + offset_of_data_directories_;
    if (offset >= size_of_optional_header_)
      return Bad("number of data directories inconsistent");
    const uint8* data_directory = optional_header_ + offset;
    if (data_directory < start() ||
        data_directory + 8 >= end())
      return Bad("data directory outside image");
    RVA rva = ReadU32(data_directory, 0);
    size_t size  = ReadU32(data_directory, 4);
    if (size > size_of_image_)
      return Bad("data directory size too big");

    // TODO(sra): validate RVA.
    directory->address_ = rva;
    directory->size_ = static_cast<uint32>(size);
    return true;
  } else {
    directory->address_ = 0;
    directory->size_ = 0;
    return true;
  }
}

}  // namespace courgette

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