Fuzz Tests for CPython
======================

These fuzz tests are designed to be included in Google's `OSS-Fuzz`_ project.

OSS-Fuzz works against a library exposing a function of the form
``int LLVMFuzzerTestOneInput(const uint8_t* data, size_t length)``. We provide
that library (``fuzzer.c``), and include a ``_xxtestfuzz`` module for testing with
some toy values -- no fuzzing occurs in Python's test suite.

OSS-Fuzz will regularly pull from CPython, discover all the tests in
``fuzz_tests.txt``, and run them -- so adding a new test here means it will
automatically be run in OSS-Fuzz, while also being smoke-tested as part of
CPython's test suite.

In addition, the tests are run on GitHub Actions using `CIFuzz
<https://google.github.io/oss-fuzz/getting-started/continuous-integration/>`_
for PRs to the ``main`` branch changing relevant files.


Adding a new fuzz test
----------------------

Add the test name on a new line in ``fuzz_tests.txt``.

In ``fuzzer.c``, add a function to be run::

    static int $fuzz_test_name(const char* data, size_t size) {
        ...
        return 0;
    }

And invoke it from ``LLVMFuzzerTestOneInput``::

    #if !defined(_Py_FUZZ_ONE) || defined(_Py_FUZZ_$fuzz_test_name)
        rv |= _run_fuzz(data, size, $fuzz_test_name);
    #endif

Don't forget to replace ``$fuzz_test_name`` with your actual test name.

``LLVMFuzzerTestOneInput`` will run in OSS-Fuzz, with each test in
``fuzz_tests.txt`` run separately.

Seed data (corpus) for the test can be provided in a subfolder called
``<test_name>_corpus`` such as ``fuzz_json_loads_corpus``. A wide variety
of good input samples allows the fuzzer to more easily explore a diverse
set of paths and provides a better base to find buggy input from.

Dictionaries of tokens (see the `libFuzzer documentation
<https://llvm.org/docs/LibFuzzer.html#dictionaries>`_ for more information) can
be placed in the ``dictionaries/`` folder with the name of the test.
For example, ``dictionaries/fuzz_json_loads.dict`` contains JSON tokens
to guide the fuzzer.


What makes a good fuzz test
---------------------------

Libraries written in C that might handle untrusted data are worthwhile. The
more complex the logic (e.g., parsing), the more likely this is to be a useful
fuzz test. See the existing examples for reference, and refer to the
`OSS-Fuzz`_ docs.

.. _OSS-Fuzz: https://github.com/google/oss-fuzz
