reversion_glibc.py

#!/usr/bin/env python3
# Copyright 2021 The Chromium Authors
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

"""Rewrite incompatible default symbols in glibc.
"""

import re
import subprocess
import sys

# This constant comes from the oldest glibc version in
# //chrome/installer/linux/debian/dist_package_versions.json and
# //chrome/installer/linux/rpm/dist_package_provides.json
MAX_ALLOWED_GLIBC_VERSION = [2, 26]

VERSION_PATTERN = re.compile('GLIBC_([0-9\.]+)')
SECTION_PATTERN = re.compile(r'^ *\[ *[0-9]+\] +(\S+) +\S+ + ([0-9a-f]+) .*$')

# Some otherwise disallowed symbols are referenced in the linux-chromeos build.
# To continue supporting it, allow these symbols to remain enabled.
SYMBOL_ALLOWLIST = {
    'fts64_close',
    'fts64_open',
    'fts64_read',
    'memfd_create',
}

# The two dictionaries below map from symbol name to
# (symbol version, symbol index).
#
# The default version for a given symbol (which may be unsupported).
default_version = {}
# The max supported symbol version for a given symbol.
supported_version = {}

# The file name of the binary we're going to rewrite.
BIN_FILE = sys.argv[1]

# Populate |default_version| and |supported_version| with data from readelf.
stdout = subprocess.check_output(['readelf', '--dyn-syms', '--wide', BIN_FILE])
for line in stdout.decode("utf-8").split('\n'):
  cols = re.split('\s+', line)
  # Skip the preamble.
  if len(cols) < 9:
    continue

  index = cols[1].rstrip(':')
  # Skip the header.
  if not index.isdigit():
    continue

  index = int(index)
  name = cols[8].split('@')
  # Ignore unversioned symbols.
  if len(name) < 2:
    continue

  base_name = name[0]
  version = name[-1]
  # The default version will have '@@' in the name.
  is_default = len(name) > 2

  if version.startswith('XCRYPT_'):
    # Prefer GLIBC_* versioned symbols over XCRYPT_* ones.  Set the version to
    # something > MAX_ALLOWED_GLIBC_VERSION so this symbol will not be picked.
    version = [float('inf')]
  else:
    match = re.match(VERSION_PATTERN, version)
    # Ignore symbols versioned with GLIBC_PRIVATE.
    if not match:
      continue
    version = [int(part) for part in match.group(1).split('.')]

  if version < MAX_ALLOWED_GLIBC_VERSION:
    old_supported_version = supported_version.get(base_name, ([-1], -1))
    supported_version[base_name] = max((version, index), old_supported_version)
  if is_default:
    default_version[base_name] = (version, index)

# Get the offset into the binary of the .gnu.version section from readelf.
stdout = subprocess.check_output(['readelf', '--sections', '--wide', BIN_FILE])
for line in stdout.decode("utf-8").split('\n'):
  if match := SECTION_PATTERN.match(line):
    section_name, address = match.groups()
    if section_name == '.gnu.version':
      gnu_version_addr = int(address, base=16)
      break
else:
  print('No .gnu.version section found', file=sys.stderr)
  sys.exit(1)

# Rewrite the binary.
bin_data = bytearray(open(BIN_FILE, 'rb').read())
for name, (version, index) in default_version.items():
  # No need to rewrite the default if it's already an allowed version.
  if version <= MAX_ALLOWED_GLIBC_VERSION:
    continue

  if name in SYMBOL_ALLOWLIST:
    continue
  elif name in supported_version:
    _, supported_index = supported_version[name]
  else:
    supported_index = -1

  # The .gnu.version section is divided into 16-bit chunks that give the
  # symbol versions.  The 16th bit is a flag that's false for the default
  # version.  The data is stored in little-endian so we need to add 1 to
  # get the address of the byte we want to flip.
  #
  # Disable the unsupported symbol.
  old_default = gnu_version_addr + 2 * index + 1
  assert (bin_data[old_default] & 0x80) == 0
  bin_data[old_default] ^= 0x80

  # If we found a supported version, enable that as default.
  if supported_index != -1:
    new_default = gnu_version_addr + 2 * supported_index + 1
    assert (bin_data[new_default] & 0x80) == 0x80
    bin_data[new_default] ^= 0x80

open(BIN_FILE, 'wb').write(bin_data)