# Copyright 2014 The Chromium Authors
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.
"""Helper script for extracting information from ELF files"""
import struct
class Error(Exception):
'''Local Error class for this file.'''
pass
def ParseElfHeader(path):
"""Determine properties of a nexe by parsing elf header.
Return tuple of architecture and boolean signalling whether
the executable is dynamic (has INTERP header) or static.
"""
# From elf.h:
# typedef struct
# {
# unsigned char e_ident[EI_NIDENT]; /* Magic number and other info */
# Elf64_Half e_type; /* Object file type */
# Elf64_Half e_machine; /* Architecture */
# ...
# } Elf32_Ehdr;
elf_header_format = '16s2H'
elf_header_size = struct.calcsize(elf_header_format)
with open(path, 'rb') as f:
header = f.read(elf_header_size)
try:
header = struct.unpack(elf_header_format, header)
except struct.error:
raise Error("error parsing elf header: %s" % path)
e_ident, _, e_machine = header[:3]
elf_magic = b'\x7fELF'
if e_ident[:4] != elf_magic:
raise Error('Not a valid NaCl executable: %s' % path)
e_machine_mapping = {
3 : 'x86-32',
8 : 'mips32',
40 : 'arm',
62 : 'x86-64'
}
if e_machine not in e_machine_mapping:
raise Error('Unknown machine type: %s' % e_machine)
# Set arch based on the machine type in the elf header
arch = e_machine_mapping[e_machine]
# Now read the full header in either 64bit or 32bit mode
dynamic = IsDynamicElf(path, arch == 'x86-64')
return arch, dynamic
def IsDynamicElf(path, is64bit):
"""Examine an elf file to determine if it is dynamically
linked or not.
This is determined by searching the program headers for
a header of type PT_INTERP.
"""
if is64bit:
elf_header_format = '16s2HI3QI3H'
else:
elf_header_format = '16s2HI3II3H'
elf_header_size = struct.calcsize(elf_header_format)
with open(path, 'rb') as f:
header = f.read(elf_header_size)
header = struct.unpack(elf_header_format, header)
p_header_offset = header[5]
p_header_entry_size = header[9]
num_p_header = header[10]
f.seek(p_header_offset)
p_headers = f.read(p_header_entry_size*num_p_header)
# Read the first word of each Phdr to find out its type.
#
# typedef struct
# {
# Elf32_Word p_type; /* Segment type */
# ...
# } Elf32_Phdr;
elf_phdr_format = 'I'
PT_INTERP = 3
while p_headers:
p_header = p_headers[:p_header_entry_size]
p_headers = p_headers[p_header_entry_size:]
phdr_type = struct.unpack(elf_phdr_format, p_header[:4])[0]
if phdr_type == PT_INTERP:
return True
return False
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