courgette/disassembler_win32.cc - chromium/src - Git at Google

 // Copyright 2016 The Chromium Authors
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 #include "courgette/disassembler_win32.h"

 #include <algorithm>

 #include "base/bind.h"
 #include "base/logging.h"
 #include "courgette/assembly_program.h"
 #include "courgette/courgette.h"

 #if COURGETTE_HISTOGRAM_TARGETS
 #include <iostream>
 #endif

 namespace courgette {

 DisassemblerWin32::DisassemblerWin32(const uint8_t* start, size_t length)
     : Disassembler(start, length) {}

 RVA DisassemblerWin32::FileOffsetToRVA(FileOffset file_offset) const {
   for (int i = 0; i < number_of_sections_; ++i) {
     const Section* section = &sections_[i];
     if (file_offset >= section->file_offset_of_raw_data) {
       FileOffset offset_in_section =
           file_offset - section->file_offset_of_raw_data;
       if (offset_in_section < section->size_of_raw_data)
         return static_cast<RVA>(section->virtual_address + offset_in_section);
     }
   }

   NOTREACHED();
   return kNoRVA;
 }

 FileOffset DisassemblerWin32::RVAToFileOffset(RVA rva) const {
   const Section* section = RVAToSection(rva);
   if (section != nullptr) {
     FileOffset offset_in_section = rva - section->virtual_address;
     // Need this extra check, since an |rva| may be valid for a section, but is
     // non-existent in an image (e.g. uninit data).
     if (offset_in_section >= section->size_of_raw_data)
       return kNoFileOffset;

     return static_cast<FileOffset>(section->file_offset_of_raw_data +
                                    offset_in_section);
   }

   // 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 static_cast<FileOffset>(rva);

   NOTREACHED();
   return kNoFileOffset;
 }

 // 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 DisassemblerWin32::ParseHeader() {
   if (!IsRangeInBounds(kOffsetOfFileAddressOfNewExeHeader, 4))
     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.
   FileOffset pe_header_offset = static_cast<FileOffset>(
       ReadU32(start(), kOffsetOfFileAddressOfNewExeHeader));
   if (pe_header_offset % 8 != 0)
     return Bad("Misaligned PE header");
   if (pe_header_offset < kOffsetOfFileAddressOfNewExeHeader + 4)
     return Bad("PE header pathological overlap");
   if (!IsRangeInBounds(pe_header_offset, kMinPeHeaderSize))
     return Bad("PE header past end of file");

   const uint8_t* const pe_header = FileOffsetToPointer(pe_header_offset);

   // 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
   FileOffset coff_header_offset = pe_header_offset + 4;
   if (!IsRangeInBounds(coff_header_offset, kSizeOfCoffHeader))
     return Bad("COFF header past end of file");
   const uint8_t* const coff_header = start() + coff_header_offset;
   machine_type_ = ReadU16(coff_header, 0);
   number_of_sections_ = ReadU16(coff_header, 2);
   size_of_optional_header_ = ReadU16(coff_header, 16);
   // Check we can read the magic.
   if (size_of_optional_header_ < 2)
     return Bad("Optional header no 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_ < RelativeOffsetOfDataDirectories())
     return Bad("Optional header too short");

   // The rest of the IMAGE_NT_HEADERS is the IMAGE_OPTIONAL_HEADER(32|64)
   FileOffset optional_header_offset = pe_header_offset + kMinPeHeaderSize;
   if (!IsRangeInBounds(optional_header_offset, size_of_optional_header_))
     return Bad("Optional header past end of file");
   optional_header_ = start() + optional_header_offset;

   uint16_t magic = ReadU16(optional_header_, 0);
   switch (kind()) {
     case EXE_WIN_32_X86:
       if (magic != kImageNtOptionalHdr32Magic)
         return Bad("64 bit executables are not supported by this disassembler");
       break;

     case EXE_WIN_32_X64:
       if (magic != kImageNtOptionalHdr64Magic)
         return Bad("32 bit executables are not supported by this disassembler");
       break;

     default:
       return Bad("Unrecognized magic");
   }

   // 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);

   switch (kind()) {
     case EXE_WIN_32_X86:
       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_, 92);
       break;

     case EXE_WIN_32_X64:
       base_of_data_ = 0;
       image_base_ = ReadU64(optional_header_, 24);
       size_of_image_ = ReadU32(optional_header_, 56);
       number_of_data_directories_ = ReadU32(optional_header_, 108);
       break;

     default:
       NOTREACHED();
   }

   if (size_of_image_ >= 0x80000000U)
     return Bad("Invalid SizeOfImage");

   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.
   FileOffset sections_offset =
       optional_header_offset + size_of_optional_header_;
   if (!IsArrayInBounds(sections_offset, number_of_sections_, sizeof(Section)))
     return Bad("Sections past end of file");
   sections_ = reinterpret_cast<const Section*>(start() + sections_offset);
   if (!CheckSectionRanges())
     return Bad("Out of bound section");

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

   return Good();
 }

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

 bool DisassemblerWin32::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_t* relocs_start = RVAToPointer(base_relocation_table_.address_);
   if (relocs_start == nullptr || relocs_start < start() ||
       relocs_start >= end())
     return Bad(".relocs outside image");

   // Make sure entire base relocation table is within the buffer.
   if (relocs_size > static_cast<size_t>(end() - relocs_start))
     return Bad(".relocs outside image");
   const uint8_t* relocs_end = relocs_start + relocs_size;

   const uint8_t* block = relocs_start;

   // Walk the variable sized blocks.
   while (block + 8 < relocs_end) {
     RVA page_rva = ReadU32(block, 0);
     uint32_t size = ReadU32(block, 4);
     if (size < 8 ||     // Size includes header ...
         size % 4 != 0)  // ... and is word aligned.
       return Bad("Unreasonable relocs block");

     const uint8_t* 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_t* p = block + 8; p < end_entries; p += 2) {
       uint16_t entry = ReadU16(p, 0);
       int type = entry >> 12;
       int offset = entry & 0xFFF;

       RVA rva = page_rva + offset;
       // Skip the relocs that live outside of the image. It might be the case
       // if a reloc is relative to a register, e.g.:
       //     mov    ecx,dword ptr [eax+044D5888h]
       RVA target_rva = PointerToTargetRVA(RVAToPointer(rva));
       if (target_rva == kNoRVA) {
         continue;
       }

       if (SupportsRelTableType(type)) {
         relocs->push_back(rva);
       } else if (type == 0) {  // IMAGE_REL_BASED_ABSOLUTE
         // Ignore, used as padding.
       } else {
         // Does not occur in Windows x86/x64 executables.
         return Bad("Unknown type of reloc");
       }
     }

     block += size;
   }

   std::sort(relocs->begin(), relocs->end());
   DCHECK(relocs->empty() || relocs->back() != kUnassignedRVA);

   return true;
 }

 const Section* DisassemblerWin32::RVAToSection(RVA rva) const {
   for (int i = 0; i < number_of_sections_; ++i) {
     const Section* section = &sections_[i];
     if (rva >= section->virtual_address) {
       FileOffset offset_in_section = rva - section->virtual_address;
       if (offset_in_section < section->virtual_size)
         return section;
     }
   }
   return nullptr;
 }

 std::string DisassemblerWin32::SectionName(const Section* section) {
   if (section == nullptr)
     return "<none>";
   char name[9];
   memcpy(name, section->name, 8);
   name[8] = '\0';  // Ensure termination.
   return name;
 }

 // static
 bool DisassemblerWin32::QuickDetect(const uint8_t* start,
                                     size_t length,
                                     uint16_t magic) {
   if (length < kOffsetOfFileAddressOfNewExeHeader + 4)
     return false;

   // Have 'MZ' magic for a DOS header?
   if (start[0] != 'M' || start[1] != 'Z')
     return false;

   FileOffset pe_header_offset = static_cast<FileOffset>(
       ReadU32(start, kOffsetOfFileAddressOfNewExeHeader));
   if (pe_header_offset % 8 != 0 ||
       pe_header_offset < kOffsetOfFileAddressOfNewExeHeader + 4 ||
       pe_header_offset >= length ||
       length - pe_header_offset < kMinPeHeaderSize) {
     return false;
   }
   const uint8_t* pe_header = start + pe_header_offset;
   if (!(pe_header[0] == 'P' && pe_header[1] == 'E' && pe_header[2] == 0 &&
         pe_header[3] == 0)) {
     return false;
   }

   FileOffset optional_header_offset = pe_header_offset + kMinPeHeaderSize;
   if (optional_header_offset >= length || length - optional_header_offset < 2)
     return false;
   const uint8_t* optional_header = start + optional_header_offset;
   return magic == ReadU16(optional_header, 0);
 }

 bool DisassemblerWin32::IsRvaRangeInBounds(size_t start, size_t length) {
   return start < size_of_image_ && length <= size_of_image_ - start;
 }

 bool DisassemblerWin32::CheckSectionRanges() {
   for (int i = 0; i < number_of_sections_; ++i) {
     const Section* section = &sections_[i];
     if (!IsRangeInBounds(section->file_offset_of_raw_data,
                          section->size_of_raw_data) ||
         !IsRvaRangeInBounds(section->virtual_address, section->virtual_size)) {
       return false;
     }
   }
   return true;
 }

 bool DisassemblerWin32::ExtractAbs32Locations() {
   abs32_locations_.clear();
   if (!ParseRelocs(&abs32_locations_))
     return false;

 #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.
     ++abs32_target_rvas_[PointerToTargetRVA(RVAToPointer(rva))];
   }
 #endif
   return true;
 }

 bool DisassemblerWin32::ExtractRel32Locations() {
   FileOffset file_offset = 0;
   while (file_offset < length()) {
     const Section* section = FindNextSection(file_offset);
     if (section == nullptr)
       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());
   DCHECK(rel32_locations_.empty() || rel32_locations_.back() != kUnassignedRVA);

 #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
   return true;
 }

 RvaVisitor* DisassemblerWin32::CreateAbs32TargetRvaVisitor() {
   return new RvaVisitor_Abs32(abs32_locations_, *this);
 }

 RvaVisitor* DisassemblerWin32::CreateRel32TargetRvaVisitor() {
   return new RvaVisitor_Rel32(rel32_locations_, *this);
 }

 void DisassemblerWin32::RemoveUnusedRel32Locations(
     AssemblyProgram* program) {
   auto cond = [this, program](RVA rva) -> bool {
     // + 4 since offset is relative to start of next instruction.
     RVA target_rva = rva + 4 + Read32LittleEndian(RVAToPointer(rva));
     return program->FindRel32Label(target_rva) == nullptr;
   };
   rel32_locations_.erase(
       std::remove_if(rel32_locations_.begin(), rel32_locations_.end(), cond),
       rel32_locations_.end());
 }

 InstructionGenerator DisassemblerWin32::GetInstructionGenerator(
     AssemblyProgram* program) {
   return base::BindRepeating(&DisassemblerWin32::ParseFile,
                              base::Unretained(this), program);
 }

 CheckBool DisassemblerWin32::ParseFile(AssemblyProgram* program,
                                        InstructionReceptor* receptor) const {
   // Walk all the bytes in the file, whether or not in a section.
   FileOffset file_offset = 0;
   while (file_offset < length()) {
     const Section* section = FindNextSection(file_offset);
     if (section == nullptr) {
       // No more sections. There should not be extra stuff following last
       // section.
       //   ParseNonSectionFileRegion(file_offset, pe_info().length(), receptor);
       break;
     }
     if (file_offset < section->file_offset_of_raw_data) {
       FileOffset section_start_offset = section->file_offset_of_raw_data;
       if (!ParseNonSectionFileRegion(file_offset, section_start_offset,
                                      receptor)) {
         return false;
       }

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

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

   return true;
 }

 CheckBool DisassemblerWin32::ParseNonSectionFileRegion(
     FileOffset start_file_offset,
     FileOffset end_file_offset,
     InstructionReceptor* receptor) const {
   if (incomplete_disassembly_)
     return true;

   if (end_file_offset > start_file_offset) {
     if (!receptor->EmitMultipleBytes(FileOffsetToPointer(start_file_offset),
                                      end_file_offset - start_file_offset)) {
       return false;
     }
   }

   return true;
 }

 CheckBool DisassemblerWin32::ParseFileRegion(
     const Section* section,
     FileOffset start_file_offset,
     FileOffset end_file_offset,
     AssemblyProgram* program,
     InstructionReceptor* receptor) const {
   RVA relocs_start_rva = base_relocation_table().address_;

   const uint8_t* start_pointer = FileOffsetToPointer(start_file_offset);
   const uint8_t* end_pointer = FileOffsetToPointer(end_file_offset);

   RVA start_rva = FileOffsetToRVA(start_file_offset);
   RVA end_rva = start_rva + section->virtual_size;
   const int kVAWidth = AbsVAWidth();

   // Quick way to convert from Pointer to RVA within a single Section is to
   // subtract 'pointer_to_rva'.
   const uint8_t* const adjust_pointer_to_rva = start_pointer - start_rva;

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

   if (!receptor->EmitOrigin(start_rva))
     return false;

   const uint8_t* 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 (!receptor->EmitPeRelocs())
         return false;
       uint32_t 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) {
       RVA target_rva = PointerToTargetRVA(p);
       DCHECK_NE(kNoRVA, target_rva);
       // TODO(sra): target could be Label+offset.  It is not clear how to guess
       // which it might be.  We assume offset==0.
       Label* label = program->FindAbs32Label(target_rva);
       DCHECK(label);
       if (!EmitAbs(label, receptor))
         return false;
       p += kVAWidth;
       continue;
     }

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

     if (rel32_pos != rel32_locations_.end() && *rel32_pos == current_rva) {
       // + 4 since offset is relative to start of next instruction.
       RVA target_rva = current_rva + 4 + Read32LittleEndian(p);
       Label* label = program->FindRel32Label(target_rva);
       DCHECK(label);
       if (!receptor->EmitRel32(label))
         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 (!receptor->EmitSingleByte(*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 DisassemblerWin32::HistogramTargets(const char* kind,
                                          const std::map<RVA, int>& map) const {
   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 DisassemblerWin32::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* DisassemblerWin32::FindNextSection(
     FileOffset file_offset) const {
   const Section* best = nullptr;
   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 (file_offset <= section->file_offset_of_raw_data) {
         if (best == nullptr ||
             section->file_offset_of_raw_data < best->file_offset_of_raw_data) {
           best = section;
         }
       }
     }
   }
   return best;
 }

 bool DisassemblerWin32::ReadDataDirectory(int index,
                                           ImageDataDirectory* directory) {
   if (index < number_of_data_directories_) {
     FileOffset file_offset = index * 8 + RelativeOffsetOfDataDirectories();
     if (file_offset >= size_of_optional_header_)
       return Bad("Number of data directories inconsistent");
     const uint8_t* data_directory = optional_header_ + file_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_t>(size);
     return true;
   } else {
     directory->address_ = 0;
     directory->size_ = 0;
     return true;
   }
 }

 }  // namespace courgette