glibc/elf/tst-relro-symbols.py

#!/usr/bin/python3
# Verify that certain symbols are covered by RELRO.
# Copyright (C) 2022 Free Software Foundation, Inc.
# This file is part of the GNU C Library.
#
# The GNU C Library is free software; you can redistribute it and/or
# modify it under the terms of the GNU Lesser General Public
# License as published by the Free Software Foundation; either
# version 2.1 of the License, or (at your option) any later version.
#
# The GNU C Library is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
# Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public
# License along with the GNU C Library; if not, see
# <https://www.gnu.org/licenses/>.

"""Analyze a (shared) object to verify that certain symbols are
present and covered by the PT_GNU_RELRO segment.

"""

import argparse
import os.path
import sys

# Make available glibc Python modules.
sys.path.append(os.path.join(
    os.path.dirname(os.path.realpath(__file__)), os.path.pardir, 'scripts'))

import glibcelf

def find_relro(path: str, img: glibcelf.Image) -> (int, int):
    """Discover the address range of the PT_GNU_RELRO segment."""
    for phdr in img.phdrs():
        if phdr.p_type == glibcelf.Pt.PT_GNU_RELRO:
            # The computation is not entirely accurate because
            # _dl_protect_relro in elf/dl-reloc.c rounds both the
            # start end and downwards using the run-time page size.
            return phdr.p_vaddr, phdr.p_vaddr + phdr.p_memsz
    sys.stdout.write('{}: error: no PT_GNU_RELRO segment\n'.format(path))
    sys.exit(1)

def check_in_relro(kind, relro_begin, relro_end, name, start, size, error):
    """Check if a section or symbol falls within in the RELRO segment."""
    end = start + size - 1
    if not (relro_begin <= start < end < relro_end):
        error(
            '{} {!r} of size {} at 0x{:x} is not in RELRO range [0x{:x}, 0x{:x})'.format(
                kind, name.decode('UTF-8'), start, size,
                relro_begin, relro_end))

def get_parser():
    """Return an argument parser for this script."""
    parser = argparse.ArgumentParser(description=__doc__)
    parser.add_argument('object', help='path to object file to check')
    parser.add_argument('--required', metavar='NAME', action='append',
                        default=[], help='required symbol names')
    parser.add_argument('--optional', metavar='NAME', action='append',
                        default=[], help='required symbol names')
    return parser

def main(argv):
    """The main entry point."""
    parser = get_parser()
    opts = parser.parse_args(argv)
    img = glibcelf.Image.readfile(opts.object)

    required_symbols = frozenset([sym.encode('UTF-8')
                                  for sym in opts.required])
    optional_symbols = frozenset([sym.encode('UTF-8')
                                  for sym in opts.optional])
    check_symbols = required_symbols | optional_symbols

    # Tracks the symbols in check_symbols that have been found.
    symbols_found = set()

    # Discover the extent of the RELRO segment.
    relro_begin, relro_end = find_relro(opts.object, img)
    symbol_table_found = False

    errors = False
    def error(msg: str) -> None:
        """Record an error condition and write a message to standard output."""
        nonlocal errors
        errors = True
        sys.stdout.write('{}: error: {}\n'.format(opts.object, msg))

    # Iterate over section headers to find the symbol table.
    for shdr in img.shdrs():
        if shdr.sh_type == glibcelf.Sht.SHT_SYMTAB:
            symbol_table_found = True
            for sym in img.syms(shdr):
                if sym.st_name in check_symbols:
                    symbols_found.add(sym.st_name)

                    # Validate symbol type, section, and size.
                    if sym.st_info.type != glibcelf.Stt.STT_OBJECT:
                        error('symbol {!r} has wrong type {}'.format(
                            sym.st_name.decode('UTF-8'), sym.st_info.type))
                    if sym.st_shndx in glibcelf.Shn:
                        error('symbol {!r} has reserved section {}'.format(
                            sym.st_name.decode('UTF-8'), sym.st_shndx))
                        continue
                    if sym.st_size == 0:
                        error('symbol {!r} has size zero'.format(
                            sym.st_name.decode('UTF-8')))
                        continue

                    check_in_relro('symbol', relro_begin, relro_end,
                                   sym.st_name, sym.st_value, sym.st_size,
                                   error)
            continue # SHT_SYMTAB
        if shdr.sh_name == b'.data.rel.ro' \
           or shdr.sh_name.startswith(b'.data.rel.ro.'):
            check_in_relro('section', relro_begin, relro_end,
                           shdr.sh_name, shdr.sh_addr, shdr.sh_size,
                           error)
            continue

    if required_symbols - symbols_found:
        for sym in sorted(required_symbols - symbols_found):
            error('symbol {!r} not found'.format(sym.decode('UTF-8')))

    if errors:
        sys.exit(1)

    if not symbol_table_found:
        sys.stdout.write(
            '{}: warning: no symbol table found (stripped object)\n'.format(
                opts.object))
        sys.exit(77)

if __name__ == '__main__':
    main(sys.argv[1:])