pyfuzzer 0.19.0
Fuzz test Python modules with libFuzzer.
License: MIT
Keywords: fuzz,fuzzying,test

|buildstatus|_
|coverage|_

About
=====

Use `libFuzzer`_ to fuzz test Python 3.6+ C extension modules.

Installation
============

`clang 8` or later is required.

.. code-block:: text

   $ apt install clang
   $ pip install pyfuzzer

Example Usage
=============

Hello world
-----------

Use the default mutator ``pyfuzzer.mutators.generic`` when testing the
module ``hello_world``.

.. code-block:: text

   $ cd examples/hello_world
   $ pyfuzzer run -l max_total_time=1 hello_world.c
   <lots of libFuzzer output>

Print the function calls that found new code paths. This information
is usually useful when writing unit tests.

.. code-block:: text

   $ pyfuzzer print_corpus
   corpus/25409981b15b978c9fb5a5a2f4dab0c4b04e295f:
       tell(b'') = 5
   corpus/a8a4e6c9abfd3c6cba171579190702ddc1317df0:
       tell(b'\xfd#') = b'Hello!'
   corpus/80f87702ef9fbe4baf17095c79ff928b9fa1ea14:
       tell(b'\x00') = True
   corpus/be3d1b7df189727b2cecd6526aa8f24abbf6df10:
       tell(b'\x00\xfd\x00') = 0
   corpus/defd8787d638f271cd83362eafe7fdeed9fa4a8f:
       tell(None) raises:
       Traceback (most recent call last):
         File "/home/erik/workspace/pyfuzzer/pyfuzzer/mutators/utils.py", line 35, in print_callable
           res = obj(*args)
       TypeError: expected bytes, NoneType found

See the `hello_world`_ for all files.

Hello world fatal error
-----------------------

Similar to the previous example, but triggers a fatal error when
``tell()`` is called with a bytes object longer than 2 bytes as its
first argument.

.. code-block:: text

   $ cd examples/hello_world_fatal_error
   $ pyfuzzer run hello_world.c
   ...
   Fatal Python error: deallocating None

   Current thread 0x00007f7ca99c2780 (most recent call first):
   ...

Print the function call that caused the crash. Just as expected, the
first argument is clearly longer than 2 bytes.

.. code-block:: text

   $ pyfuzzer print_crashes
   crash-1013ed88cd71fd14407b2bdbc17b95d7bc317c21:
       tell(b'\n\xbf+') = None

See the `hello_world_fatal_error`_ for all files.

Custom mutator
--------------

Use the custom mutator ``hello_world_mutator`` when testing the module
``hello_world``.

Testing with a custom mutator is often more efficient than using a
generic one.

.. code-block:: text

   $ cd examples/hello_world_custom_mutator
   $ pyfuzzer run -l max_total_time=1 -m hello_world_mutator.py hello_world.c
   ...

See the `hello_world_custom_mutator`_ for all files.

Mutators
========

A mutator module uses data from `libFuzzer`_ to test a module. A
mutator module must implement the function ``setup(module)``, where
``module`` is the module under test. It shall return a mutator
instance that implements the methods ``test_one_input(self, data)``
and ``test_one_input_print(self, data)``, where ``data`` is the data
generated by `libFuzzer`_ (as a bytes object).

``test_one_input(self, data)`` performs the actual fuzz testing, while
``test_one_input_print(self, data)`` prints corpus and crashes.

A minimal mutator fuzz testing a CRC-32 algorithm could look like
below. It simply calls ``crc_32()`` with ``data`` as its only
argument.

.. code-block:: python

   from pyfuzzer.mutators.generic import print_callable

   class Mutator:

       def __init__(self, module):
           self._module = module

       def test_one_input(self, data):
           return module.crc_32(data)

       def test_one_input_print(self, data):
           print_callable(self._module.crc_32, [data])

    def setup(module):
        return Mutator(module)

Ideas
=====

- Add support to fuzz test pure Python modules by generating C code
  using Cython.

.. |buildstatus| image:: https://travis-ci.org/eerimoq/pyfuzzer.svg
.. _buildstatus: https://travis-ci.org/eerimoq/pyfuzzer

.. |coverage| image:: https://coveralls.io/repos/github/eerimoq/pyfuzzer/badge.svg?branch=master
.. _coverage: https://coveralls.io/github/eerimoq/pyfuzzer

.. _libFuzzer: https://llvm.org/docs/LibFuzzer.html

.. _hello_world: https://github.com/eerimoq/pyfuzzer/tree/master/examples/hello_world

.. _hello_world_fatal_error: https://github.com/eerimoq/pyfuzzer/tree/master/examples/hello_world_fatal_error

.. _hello_world_custom_mutator: https://github.com/eerimoq/pyfuzzer/tree/master/examples/hello_world_custom_mutator