testing/libfuzzer/dictionary_generator.py

#!/usr/bin/env python3
#
# Copyright 2016 The Chromium Authors
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.
"""Generate a dictionary for libFuzzer or AFL-based fuzzer.

Invoked manually using a fuzzer binary and target format/protocol specification.
Works better for text formats or protocols. For binary ones may be useless.
"""

import argparse

# This is a Python 2-only import despite the file using a Python 3 shebang. This
# implies that this file has been unused for years and has not been properly
# converted to Python 3.
import HTMLParser
import logging
import os
import re
import shutil
import subprocess
import sys
import tempfile

ENCODING_TYPES = ['ascii', 'utf_16_be', 'utf_16_le', 'utf_32_be', 'utf_32_le']
MIN_STRING_LENGTH = 4


def DecodeHTML(html_data):
  """HTML-decoding of the data."""
  html_parser = HTMLParser.HTMLParser()
  data = html_parser.unescape(html_data.decode('ascii', 'ignore'))
  return data.encode('ascii', 'ignore')


def EscapeDictionaryElement(element):
  """Escape all unprintable and control characters in an element."""
  element_escaped = element.encode('string_escape')
  # Remove escaping for single quote because it breaks libFuzzer.
  element_escaped = element_escaped.replace("\\'", "'")
  # Add escaping for double quote.
  element_escaped = element_escaped.replace('"', '\\"')
  return element_escaped


def ExtractWordsFromBinary(filepath, min_length=MIN_STRING_LENGTH):
  """Extract words (splitted strings) from a binary executable file."""
  rodata = PreprocessAndReadRodata(filepath)
  words = []

  strings_re = re.compile(r'[^\x00-\x1F\x7F-\xFF]{%d,}' % min_length)
  # Use different encodings for strings extraction.
  for encoding in ENCODING_TYPES:
    data = rodata.decode(encoding, 'ignore').encode('ascii', 'ignore')
    raw_strings = strings_re.findall(data)
    for splitted_line in map(lambda line: line.split(), raw_strings):
      words += splitted_line

  return set(words)


def ExtractWordsFromLines(lines):
  """Extract all words from a list of strings."""
  words = set()
  for line in lines:
    for word in line.split():
      words.add(word)

  return words


def ExtractWordsFromSpec(filepath, is_html):
  """Extract words from a specification."""
  data = ReadSpecification(filepath, is_html)
  words = data.split()
  return set(words)


def FindIndentedText(text):
  """Find space-indented text blocks, e.g. code or data samples in RFCs."""
  lines = text.split('\n')
  indented_blocks = []
  current_block = ''
  previous_number_of_spaces = 0

  # Go through every line and concatenate space-indented blocks into lines.
  for i in range(0, len(lines), 1):
    if not lines[i]:
      # Ignore empty lines.
      continue

    # Space-indented text blocks have more leading spaces than regular text.
    n = FindNumberOfLeadingSpaces(lines[i])

    if n > previous_number_of_spaces:
      # Beginning of a space-indented text block, start concatenation.
      current_block = lines[i][n:]
    elif n == previous_number_of_spaces and current_block:
      # Or continuation of a space-indented text block, concatenate lines.
      current_block += '\n' + lines[i][n:]

    if n < previous_number_of_spaces and current_block:
      # Current line is not indented, save previously concatenated lines.
      indented_blocks.append(current_block)
      current_block = ''

    previous_number_of_spaces = n

  return indented_blocks


def FindNumberOfLeadingSpaces(line):
  """Calculate number of leading whitespace characters in the string."""
  n = 0
  while n < len(line) and line[n].isspace():
    n += 1

  return n


def GenerateDictionary(path_to_binary, path_to_spec, strategy, is_html=False):
  """Generate a dictionary for given pair of fuzzer binary and specification."""
  for filepath in [path_to_binary, path_to_spec]:
    if not os.path.exists(filepath):
      logging.error("%s doesn't exist. Exit.", filepath)
      sys.exit(1)

  words_from_binary = ExtractWordsFromBinary(path_to_binary)
  words_from_spec = ExtractWordsFromSpec(path_to_spec, is_html)

  dictionary_words = set()

  if 'i' in strategy:
    # Strategy i: only words which are common for binary and for specification.
    dictionary_words = words_from_binary.intersection(words_from_spec)

  if 'q' in strategy:
    # Strategy q: add words from all quoted strings from specification.
    # TODO(mmoroz): experimental and very noisy. Not recommended to use.
    spec_data = ReadSpecification(path_to_spec, is_html)
    quoted_strings = FindIndentedText(spec_data)
    quoted_words = ExtractWordsFromLines(quoted_strings)
    dictionary_words = dictionary_words.union(quoted_words)

  if 'u' in strategy:
    # Strategy u: add all uppercase words from specification.
    uppercase_words = set(w for w in words_from_spec if w.isupper())
    dictionary_words = dictionary_words.union(uppercase_words)

  return dictionary_words


def PreprocessAndReadRodata(filepath):
  """Create a stripped copy of the binary and extract .rodata section."""
  stripped_file = tempfile.NamedTemporaryFile(prefix='.stripped_')
  stripped_filepath = stripped_file.name
  shutil.copyfile(filepath, stripped_filepath)

  # Strip all symbols to reduce amount of redundant strings.
  strip_cmd = ['strip', '--strip-all', stripped_filepath]
  result = subprocess.call(strip_cmd)
  if result:
    logging.warning('Failed to strip the binary. Using the original version.')
    stripped_filepath = filepath

  # Extract .rodata section to reduce amount of redundant strings.
  rodata_file = tempfile.NamedTemporaryFile(prefix='.rodata_')
  rodata_filepath = rodata_file.name
  objcopy_cmd = ['objcopy', '-j', '.rodata', stripped_filepath, rodata_filepath]

  # Hide output from stderr since objcopy prints a warning.
  with open(os.devnull, 'w') as devnull:
    result = subprocess.call(objcopy_cmd, stderr=devnull)

  if result:
    logging.warning('Failed to extract .rodata section. Using the whole file.')
    rodata_filepath = stripped_filepath

  with open(rodata_filepath) as file_handle:
    data = file_handle.read()

  stripped_file.close()
  rodata_file.close()

  return data


def ReadSpecification(filepath, is_html):
  """Read a specification file and return its contents."""
  with open(filepath, 'r') as file_handle:
    data = file_handle.read()

  if is_html:
    data = DecodeHTML(data)

  return data


def WriteDictionary(dictionary_path, dictionary):
  """Write given dictionary to a file."""
  with open(dictionary_path, 'wb') as file_handle:
    file_handle.write('# This is an automatically generated dictionary.\n')
    for word in dictionary:
      if not word:
        continue
      line = '"%s"\n' % EscapeDictionaryElement(word)
      file_handle.write(line)


def main():
  parser = argparse.ArgumentParser(description='Generate fuzzer dictionary.')
  parser.add_argument(
    '--fuzzer',
    required=True,
    help='Path to a fuzzer binary executable. It is '
    'recommended to use a binary built with '
    '"use_libfuzzer=false is_asan=false" to get a better '
    'dictionary with fewer number of redundant elements.',
  )
  parser.add_argument(
    '--spec',
    required=True,
    help='Path to a target specification (in textual form).',
  )
  parser.add_argument(
    '--html',
    default=0,
    help='Decode HTML [01] (0 is default value): '
    '1 - if specification has HTML entities to be decoded.',
  )
  parser.add_argument(
    '--out', required=True, help='Path to a file to write a dictionary into.'
  )
  parser.add_argument(
    '--strategy',
    default='iu',
    help='Generation strategy [iqu] ("iu" is default value): '
    'i - intersection, q - quoted, u - uppercase.',
  )
  args = parser.parse_args()

  dictionary = GenerateDictionary(
    args.fuzzer, args.spec, args.strategy, is_html=bool(args.html)
  )
  WriteDictionary(args.out, dictionary)


if __name__ == '__main__':
  main()