text: #!/usr/bin/env python
  # Copyright 2018 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.
  
  """Wraps ml.exe or ml64.exe and postprocesses the output to be deterministic.
  Sets timestamp in .obj file to 0, hence incompatible with link.exe /incremental.
  
  Use by prefixing the ml(64).exe invocation with this script:
      python ml.py ml.exe [args...]"""
  
  import array
  import collections
  import struct
  import subprocess
  import sys
  
  
  class Struct(object):
    """A thin wrapper around the struct module that returns a namedtuple"""
    def __init__(self, name, *args):
      """Pass the name of the return type, and then an interleaved list of
      format strings as used by the struct module and of field names."""
      self.fmt = '<' + ''.join(args[0::2])
      self.type = collections.namedtuple(name, args[1::2])
  
    def pack_into(self, buffer, offset, data):
      return struct.pack_into(self.fmt, buffer, offset, *data)
  
    def unpack_from(self, buffer, offset=0):
      return self.type(*struct.unpack_from(self.fmt, buffer, offset))
  
    def size(self):
      return struct.calcsize(self.fmt)
  
  
  def Subtract(nt, **kwargs):
    """Subtract(nt, f=2) returns a new namedtuple with 2 subtracted from nt.f"""
    return nt._replace(**{k: getattr(nt, k) - v for k, v in kwargs.iteritems()})
  
  
  def MakeDeterministic(objdata):
    # Takes data produced by ml(64).exe (without any special flags) and
    # 1. Sets the timestamp to 0
    # 2. Strips the .debug$S section (which contains an unwanted absolute path)
  
    # This makes several assumptions about ml's output:
    # - Section data is in the same order as the corresponding section headers:
    #   section headers preceding the .debug$S section header have their data
    #   preceding the .debug$S section data; likewise for section headers
    #   following the .debug$S section.
    # - The .debug$S section contains only the absolute path to the obj file and
    #   nothing else, in particular there's only a single entry in the symbol
    #   table referring to the .debug$S section.
    # - There are no COFF line number entries.
    # - There's no IMAGE_SYM_CLASS_CLR_TOKEN symbol.
    # These seem to hold in practice; if they stop holding this script needs to
    # become smarter.
  
    objdata = array.array('c', objdata)  # Writable, e.g. via struct.pack_into.
  
    # Read coff header.
    COFFHEADER = Struct('COFFHEADER',
                        'H', 'Machine',
                        'H', 'NumberOfSections',
                        'I', 'TimeDateStamp',
                        'I', 'PointerToSymbolTable',
                        'I', 'NumberOfSymbols',
  
                        'H', 'SizeOfOptionalHeader',
                        'H', 'Characteristics')
    coff_header = COFFHEADER.unpack_from(objdata)
    assert coff_header.SizeOfOptionalHeader == 0  # Only set for binaries.
  
    # Read section headers following coff header.
    SECTIONHEADER = Struct('SECTIONHEADER',
                           '8s', 'Name',
                           'I', 'VirtualSize',
                           'I', 'VirtualAddress',
  
                           'I', 'SizeOfRawData',
                           'I', 'PointerToRawData',
                           'I', 'PointerToRelocations',
                           'I', 'PointerToLineNumbers',
  
                           'H', 'NumberOfRelocations',
                           'H', 'NumberOfLineNumbers',
                           'I', 'Characteristics')
    section_headers = []
    debug_section_index = -1
    for i in range(0, coff_header.NumberOfSections):
      section_header = SECTIONHEADER.unpack_from(
          objdata, offset=COFFHEADER.size() + i * SECTIONHEADER.size())
      assert not section_header[0].startswith('/')  # Support short names only.
      section_headers.append(section_header)
  
      if section_header.Name == '.debug$S':
        assert debug_section_index == -1
        debug_section_index = i
    assert debug_section_index != -1
  
    data_start = COFFHEADER.size() + len(section_headers) * SECTIONHEADER.size()
  
    # Verify the .debug$S section looks like we expect.
    assert section_headers[debug_section_index].Name == '.debug$S'
    assert section_headers[debug_section_index].VirtualSize == 0
    assert section_headers[debug_section_index].VirtualAddress == 0
    debug_size = section_headers[debug_section_index].SizeOfRawData
    debug_offset = section_headers[debug_section_index].PointerToRawData
    assert section_headers[debug_section_index].PointerToRelocations == 0
    assert section_headers[debug_section_index].PointerToLineNumbers == 0
    assert section_headers[debug_section_index].NumberOfRelocations == 0
    assert section_headers[debug_section_index].NumberOfLineNumbers == 0
  
    # Make sure sections in front of .debug$S have their data preceding it.
    for header in section_headers[:debug_section_index]:
      assert header.PointerToRawData < debug_offset
      assert header.PointerToRelocations < debug_offset
      assert header.PointerToLineNumbers < debug_offset
  
    # Make sure sections after of .debug$S have their data following it.
    for header in section_headers[debug_section_index + 1:]:
      # Make sure the .debug$S data is at the very end of section data:
      assert header.PointerToRawData > debug_offset
      assert header.PointerToRelocations == 0
      assert header.PointerToLineNumbers == 0
  
    # Make sure the first non-empty section's data starts right after the section
    # headers.
    for section_header in section_headers:
      if section_header.PointerToRawData == 0:
        assert section_header.PointerToRelocations == 0
        assert section_header.PointerToLineNumbers == 0
        continue
      assert section_header.PointerToRawData == data_start
      break
  
    # Make sure the symbol table (and hence, string table) appear after the last
    # section:
    assert (coff_header.PointerToSymbolTable >=
        section_headers[-1].PointerToRawData + section_headers[-1].SizeOfRawData)
  
    # The symbol table contains a symbol for the no-longer-present .debug$S
    # section. If we leave it there, lld-link will complain:
    #
    #    lld-link: error: .debug$S should not refer to non-existent section 5
    #
    # so we need to remove that symbol table entry as well. This shifts symbol
    # entries around and we need to update symbol table indices in:
    # - relocations
    # - line number records (never present)
    # - one aux symbol entry (IMAGE_SYM_CLASS_CLR_TOKEN; not present in ml output)
    SYM = Struct('SYM',
                 '8s', 'Name',
                 'I', 'Value',
                 'h', 'SectionNumber',  # Note: Signed!
                 'H', 'Type',
  
                 'B', 'StorageClass',
                 'B', 'NumberOfAuxSymbols')
    i = 0
    debug_sym = -1
    while i < coff_header.NumberOfSymbols:
      sym_offset = coff_header.PointerToSymbolTable + i * SYM.size()
      sym = SYM.unpack_from(objdata, sym_offset)
  
      # 107 is IMAGE_SYM_CLASS_CLR_TOKEN, which has aux entry "CLR Token
      # Definition", which contains a symbol index. Check it's never present.
      assert sym.StorageClass != 107
  
      # Note: sym.SectionNumber is 1-based, debug_section_index is 0-based.
      if sym.SectionNumber - 1 == debug_section_index:
        assert debug_sym == -1, 'more than one .debug$S symbol found'
        debug_sym = i
        # Make sure the .debug$S symbol looks like we expect.
        # In particular, it should have exactly one aux symbol.
        assert sym.Name == '.debug$S'
        assert sym.Value == 0
        assert sym.Type == 0
        assert sym.StorageClass == 3
        assert sym.NumberOfAuxSymbols == 1
      elif sym.SectionNumber > debug_section_index:
        sym = Subtract(sym, SectionNumber=1)
        SYM.pack_into(objdata, sym_offset, sym)
      i += 1 + sym.NumberOfAuxSymbols
    assert debug_sym != -1, '.debug$S symbol not found'
  
    # Note: Usually the .debug$S section is the last, but for files saying
    # `includelib foo.lib`, like safe_terminate_process.asm in 32-bit builds,
    # this isn't true: .drectve is after .debug$S.
  
    # Update symbol table indices in relocations.
    # There are a few processor types that have one or two relocation types
    # where SymbolTableIndex has a different meaning, but not for x86.
    REL = Struct('REL',
                 'I', 'VirtualAddress',
                 'I', 'SymbolTableIndex',
                 'H', 'Type')
    for header in section_headers[0:debug_section_index]:
      for j in range(0, header.NumberOfRelocations):
        rel_offset = header.PointerToRelocations + j * REL.size()
        rel = REL.unpack_from(objdata, rel_offset)
        assert rel.SymbolTableIndex != debug_sym
        if rel.SymbolTableIndex > debug_sym:
          rel = Subtract(rel, SymbolTableIndex=2)
          REL.pack_into(objdata, rel_offset, rel)
  
    # Update symbol table indices in line numbers -- just check they don't exist.
    for header in section_headers:
      assert header.NumberOfLineNumbers == 0
  
    # Now that all indices are updated, remove the symbol table entry referring to
    # .debug$S and its aux entry.
    del objdata[coff_header.PointerToSymbolTable + debug_sym * SYM.size():
                coff_header.PointerToSymbolTable + (debug_sym + 2) * SYM.size()]
  
    # Now we know that it's safe to write out the input data, with just the
    # timestamp overwritten to 0, the last section header cut out (and the
    # offsets of all other section headers decremented by the size of that
    # one section header), and the last section's data cut out. The symbol
    # table offset needs to be reduced by one section header and the size of
    # the missing section.
    # (The COFF spec only requires on-disk sections to be aligned in image files,
    # for obj files it's not required. If that wasn't the case, deleting slices
    # if data would not generally be safe.)
  
    # Update section offsets and remove .debug$S section data.
    for i in range(0, debug_section_index):
      header = section_headers[i]
      if header.SizeOfRawData:
        header = Subtract(header, PointerToRawData=SECTIONHEADER.size())
      if header.NumberOfRelocations:
        header = Subtract(header, PointerToRelocations=SECTIONHEADER.size())
      if header.NumberOfLineNumbers:
        header = Subtract(header, PointerToLineNumbers=SECTIONHEADER.size())
      SECTIONHEADER.pack_into(
          objdata, COFFHEADER.size() + i * SECTIONHEADER.size(), header)
    for i in range(debug_section_index + 1, len(section_headers)):
      header = section_headers[i]
      shift = SECTIONHEADER.size() + debug_size
      if header.SizeOfRawData:
        header = Subtract(header, PointerToRawData=shift)
      if header.NumberOfRelocations:
        header = Subtract(header, PointerToRelocations=shift)
      if header.NumberOfLineNumbers:
        header = Subtract(header, PointerToLineNumbers=shift)
      SECTIONHEADER.pack_into(
          objdata, COFFHEADER.size() + i * SECTIONHEADER.size(), header)
  
    del objdata[debug_offset:debug_offset + debug_size]
  
    # Finally, remove .debug$S section header and update coff header.
    coff_header = coff_header._replace(TimeDateStamp=0)
    coff_header = Subtract(coff_header,
                           NumberOfSections=1,
                           PointerToSymbolTable=SECTIONHEADER.size() + debug_size,
                           NumberOfSymbols=2)
    COFFHEADER.pack_into(objdata, 0, coff_header)
  
    del objdata[
        COFFHEADER.size() + debug_section_index * SECTIONHEADER.size():
        COFFHEADER.size() + (debug_section_index + 1) * SECTIONHEADER.size()]
  
    # All done!
    return objdata.tostring()
  
  
  def main():
    ml_result = subprocess.call(sys.argv[1:])
    if ml_result != 0:
      return ml_result
  
    objfile = None
    for i in range(1, len(sys.argv)):
      if sys.argv[i].startswith('/Fo'):
        objfile = sys.argv[i][len('/Fo'):]
    assert objfile, 'failed to find ml output'
  
    with open(objfile, 'rb') as f:
      objdata = f.read()
    objdata = MakeDeterministic(objdata)
    with open(objfile, 'wb') as f:
      f.write(objdata)
  
  
  if __name__ == '__main__':
    sys.exit(main())
  