#!/usr/bin/env python3
# Copyright 2019 The Chromium Authors
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.
"""Extracts an LLD partition from an ELF file."""
import argparse
import hashlib
import os
import struct
import subprocess
import sys
import tempfile
def _ComputeNewBuildId(old_build_id, file_path):
"""
Computes the new build-id from old build-id and file_path.
Args:
old_build_id: Original build-id in bytearray.
file_path: Path to output ELF file.
Returns:
New build id with the same length as |old_build_id|.
"""
m = hashlib.sha256()
m.update(old_build_id)
m.update(os.path.basename(file_path).encode('utf-8'))
hash_bytes = m.digest()
# In case build_id is longer than hash computed, repeat the hash
# to the desired length first.
id_size = len(old_build_id)
hash_size = len(hash_bytes)
return (hash_bytes * (id_size // hash_size + 1))[:id_size]
def _ExtractPartition(objcopy, input_elf, output_elf, partition):
"""
Extracts a partition from an ELF file.
For partitions other than main partition, we need to rewrite
the .note.gnu.build-id section so that the build-id remains
unique.
Note:
- `objcopy` does not modify build-id when partitioning the
combined ELF file by default.
- The new build-id is calculated as hash of original build-id
and partitioned ELF file name.
Args:
objcopy: Path to objcopy binary.
input_elf: Path to input ELF file.
output_elf: Path to output ELF file.
partition: Partition to extract from combined ELF file. None when
extracting main partition.
"""
if not partition: # main partition
# We do not overwrite build-id on main partition to allow the expected
# partition build ids to be synthesized given a libchrome.so binary,
# if necessary.
subprocess.check_call(
[objcopy, '--extract-main-partition', input_elf, output_elf])
return
# partitioned libs
build_id_section = '.note.gnu.build-id'
with tempfile.TemporaryDirectory() as tempdir:
temp_elf = os.path.join(tempdir, 'obj_without_id.so')
old_build_id_file = os.path.join(tempdir, 'old_build_id')
new_build_id_file = os.path.join(tempdir, 'new_build_id')
# Dump out build-id section.
subprocess.check_call([
objcopy,
'--extract-partition',
partition,
'--dump-section',
'{}={}'.format(build_id_section, old_build_id_file),
input_elf,
temp_elf,
])
with open(old_build_id_file, 'rb') as f:
note_content = f.read()
# .note section has following format according to <elf/external.h>
# typedef struct {
# unsigned char namesz[4]; /* Size of entry's owner string */
# unsigned char descsz[4]; /* Size of the note descriptor */
# unsigned char type[4]; /* Interpretation of the descriptor */
# char name[1]; /* Start of the name+desc data */
# } Elf_External_Note;
# `build-id` rewrite is only required on Android platform,
# where we have partitioned lib.
# Android platform uses little-endian.
# <: little-endian
# 4x: Skip 4 bytes
# L: unsigned long, 4 bytes
descsz, = struct.Struct('<4xL').unpack_from(note_content)
prefix = note_content[:-descsz]
build_id = note_content[-descsz:]
with open(new_build_id_file, 'wb') as f:
f.write(prefix + _ComputeNewBuildId(build_id, output_elf))
# Update the build-id section.
subprocess.check_call([
objcopy,
'--update-section',
'{}={}'.format(build_id_section, new_build_id_file),
temp_elf,
output_elf,
])
def main():
parser = argparse.ArgumentParser(description=__doc__)
parser.add_argument(
'--partition',
help='Name of partition if not the main partition',
metavar='PART')
parser.add_argument(
'--objcopy',
required=True,
help='Path to llvm-objcopy binary',
metavar='FILE')
parser.add_argument(
'--unstripped-output',
required=True,
help='Unstripped output file',
metavar='FILE')
parser.add_argument(
'--stripped-output',
required=True,
help='Stripped output file',
metavar='FILE')
parser.add_argument('--split-dwarf', action='store_true')
parser.add_argument('input', help='Input file')
args = parser.parse_args()
_ExtractPartition(args.objcopy, args.input, args.unstripped_output,
args.partition)
subprocess.check_call([
args.objcopy,
'--strip-all',
args.unstripped_output,
args.stripped_output,
])
# Debug info for partitions is the same as for the main library, so just
# symlink the .dwp files.
if args.split_dwarf:
dest = args.unstripped_output + '.dwp'
try:
os.unlink(dest)
except OSError:
pass
relpath = os.path.relpath(args.input + '.dwp', os.path.dirname(dest))
os.symlink(relpath, dest)
if __name__ == '__main__':
sys.exit(main())
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