native_client_sdk/src/tools/lib/elf.py

   1 # Copyright 2014 The Chromium Authors. All rights reserved.
   2 # Use of this source code is governed by a BSD-style license that can be
   3 # found in the LICENSE file.
   4
   5 """Helper script for extracting information from ELF files"""
   6
   7 import struct
   8
   9
  10 class Error(Exception):
  11   '''Local Error class for this file.'''
  12   pass
  13
  14
  15 def ParseElfHeader(path):
  16   """Determine properties of a nexe by parsing elf header.
  17   Return tuple of architecture and boolean signalling whether
  18   the executable is dynamic (has INTERP header) or static.
  19   """
  20   # From elf.h:
  21   # typedef struct
  22   # {
  23   #   unsigned char e_ident[EI_NIDENT]; /* Magic number and other info */
  24   #   Elf64_Half e_type; /* Object file type */
  25   #   Elf64_Half e_machine; /* Architecture */
  26   #   ...
  27   # } Elf32_Ehdr;
  28   elf_header_format = '16s2H'
  29   elf_header_size = struct.calcsize(elf_header_format)
  30
  31   with open(path, 'rb') as f:
  32     header = f.read(elf_header_size)
  33
  34   try:
  35     header = struct.unpack(elf_header_format, header)
  36   except struct.error:
  37     raise Error("error parsing elf header: %s" % path)
  38   e_ident, _, e_machine = header[:3]
  39
  40   elf_magic = '\x7fELF'
  41   if e_ident[:4] != elf_magic:
  42     raise Error('Not a valid NaCl executable: %s' % path)
  43
  44   e_machine_mapping = {
  45     3 : 'x86-32',
  46     8 : 'mips32',
  47     40 : 'arm',
  48     62 : 'x86-64'
  49   }
  50   if e_machine not in e_machine_mapping:
  51     raise Error('Unknown machine type: %s' % e_machine)
  52
  53   # Set arch based on the machine type in the elf header
  54   arch = e_machine_mapping[e_machine]
  55
  56   # Now read the full header in either 64bit or 32bit mode
  57   dynamic = IsDynamicElf(path, arch == 'x86-64')
  58   return arch, dynamic
  59
  60
  61 def IsDynamicElf(path, is64bit):
  62   """Examine an elf file to determine if it is dynamically
  63   linked or not.
  64   This is determined by searching the program headers for
  65   a header of type PT_INTERP.
  66   """
  67   if is64bit:
  68     elf_header_format = '16s2HI3QI3H'
  69   else:
  70     elf_header_format = '16s2HI3II3H'
  71
  72   elf_header_size = struct.calcsize(elf_header_format)
  73
  74   with open(path, 'rb') as f:
  75     header = f.read(elf_header_size)
  76     header = struct.unpack(elf_header_format, header)
  77     p_header_offset = header[5]
  78     p_header_entry_size = header[9]
  79     num_p_header = header[10]
  80     f.seek(p_header_offset)
  81     p_headers = f.read(p_header_entry_size*num_p_header)
  82
  83   # Read the first word of each Phdr to find out its type.
  84   #
  85   # typedef struct
  86   # {
  87   #   Elf32_Word  p_type;     /* Segment type */
  88   #   ...
  89   # } Elf32_Phdr;
  90   elf_phdr_format = 'I'
  91   PT_INTERP = 3
  92
  93   while p_headers:
  94     p_header = p_headers[:p_header_entry_size]
  95     p_headers = p_headers[p_header_entry_size:]
  96     phdr_type = struct.unpack(elf_phdr_format, p_header[:4])[0]
  97     if phdr_type == PT_INTERP:
  98       return True
  99
 100   return False