This is experimental module that enables
`American Fuzzy Lop`_ fork server and instrumentation for pure-Python code.

.. _American Fuzzy Lop: https://lcamtuf.coredump.cx/afl/

HOWTO
-----

* Add this code (ideally, after all other modules are already imported) to
  the target program:

  .. code:: python

      import afl
      afl.init()

* The instrumentation is currently implemented with a `trace function`_,
  which is called whenever a new local scope is entered.
  You might need to wrap the code of the main program in a function
  to get it instrumented correctly.

.. _trace function:
   https://docs.python.org/2/library/sys.html#sys.settrace

* Optionally, add this code at the end of the target program:

  .. code:: python

      os._exit(0)

  This should speed up fuzzing considerably,
  at the risk of not catching bugs that could happen during normal exit.

* For persistent mode, wrap the tested code in this loop:

  .. code:: python

      while afl.loop(N):
         ...

  where ``N`` is the number of inputs to process before restarting.

  You shouldn't call ``afl.init()`` in this case.

  If you read input from ``sys.stdin``,
  you must rewind it in every loop iteration:

  .. code:: python

     sys.stdin.seek(0)

  afl-fuzz ≥ 1.82b is required for this feature.

* Use *py-afl-fuzz* instead of *afl-fuzz*::

     $ py-afl-fuzz [options] -- /path/to/fuzzed/python/script [...]

* The instrumentation is a bit slow at the moment,
  so you might want to enable the dumb mode (``-n``),
  while still leveraging the fork server.

  afl-fuzz ≥ 1.95b is required for this feature.

Environment variables
---------------------

The following environment variables affect *python-afl* behavior:

``PYTHON_AFL_SIGNAL``
   If this variable is set, *python-afl* installs an exception hook
   that kills the current process with the selected signal.
   That way *afl-fuzz* can treat unhandled exceptions as crashes.

   By default, *py-afl-fuzz*, *py-afl-showmap*, *python-afl-cmin*,
   and *py-afl-tmin* set this variable to ``SIGUSR1``.

   You can set ``PYTHON_AFL_SIGNAL`` to another signal;
   or set it to ``0`` to disable the exception hook.

``PYTHON_AFL_PERSISTENT``
   Persistent mode is enabled only if this variable is set.

   *py-afl-fuzz* sets this variable automatically,
   so there should normally no need to set it manually.

``PYTHON_AFL_TSTL``
   `TSTL`_ test harness code is ignored if this variable is set;
   relevant only to users of TSTL interface to python-afl.

.. _TSTL: https://github.com/agroce/tstl

Bugs
----

Multi-threaded code is not supported.

Further reading
---------------

* `Taking a look at python-afl <https://barro.github.io/2018/01/taking-a-look-at-python-afl/>`_ by Jussi Judin
* `Introduction to Fuzzing in Python with AFL <https://alexgaynor.net/2015/apr/13/introduction-to-fuzzing-in-python-with-afl/>`_ by Alex Gaynor
* `AFL's README <https://lcamtuf.coredump.cx/afl/README.txt>`_

Prerequisites
-------------

To build the module, you will need:

* Python 2.6+ or 3.2+
* Cython ≥ 0.19 (only at build time)

*py-afl-fuzz* requires AFL proper to be installed.

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