Static Analysis of CPython Extensions
I'm working on a static analysis tool that can detect common mistakes made in Python extension modules written in C. We'll run it on all such code in Fedora, fixing any problems we find, and send the patches to the appropriate upstream projects.
- Name: Dave Malcolm
- Email: firstname.lastname@example.org
- Targeted release: [[Releases/<number> | Fedora <number> ]]
- Last updated: 2011-05-22
- Percentage of completion: 10%
Python makes it relatively easy to write wrapper code for C and C++ libraries, acting as a "glue" from which programs can be created.
Unfortunately, there are various mistakes that are commonly made in such wrapper code, and these mistakes can lead to /usr/bin/python leaking memory or segfaulting. There are other mistakes that only manifest as bugs when run on less common CPU architectures.
I'm working on static analysis code for C, to detect common errors in C extension modules for Python. The plan is to integrate this with Fedora's packaging, so that all C extension modules packaged for Python 2 and Python 3 can be guaranteed free of such errors (by adding hooks to the python-devel and python3-devel packages). We can also send fixes for this code as needed to upstream projects, when it reports problems.
For this to be viable, we'll need the tool to achieve a good signal:noise ratio. Part of this will need to involve having "good" error messages, spelling out how the problem occurs, what the impact is, and how to fix.
This will also benefit PyPy. PyPy has its own implementation of the CPython extension API, and certain bugs in extension code can lead to more severe symptoms with PyPy than with CPython. Specifically, some reference-counting bugs that are harmless on CPython can lead to segfaults of PyPy. So by fixing these kinds of bug, we also help PyPy.
Benefit to Fedora
Fedora is already a great environment for doing Python development - having a good-quality static analysis tool integrated into Fedora's build system for python extension modules will make Fedora even more compelling for Python developers. (Naturally the tool will be Free Software, and thus usable on other platforms; but we'll have it first).
The presence of the tool should also make it easier to fix certain awkward bugs, and make it easier to support secondary CPU architectures.
- writing the tool
- ensuring that it works well on historical bugs (examples of real bugs that are now fixed)
- tuning it to achieve a good signal:noise ratio:
- testing it on everything in Fedora:
- analyzing the issues that it reports
- fixing bugs in the tool
- fixing bugs in the software-under-test
- generating a test suite for the tool
- testing it on everything in Fedora:
- integrating it into the python 2 and python 3 build of Fedora RPMs (python-devel and python3-devel)
- ensuring that it does not substantially increase the time it takes to build the software-under-test
- the selftest suite for the tool will need a performance component; we also need to be careful how we integrate it into Fedora's build system
The bugs I intend for the tool to detect are:
- ob_refcnt errors: missing Py_INCREF/Py_DECREF etc
- tp_traverse errors (which can mess up the garbage collector); missing it altogether, or omitting fields
- errors in PyArg_ParseTuple and friends (often leads to flaws on big-endian 64-bit architectures)
How To Test
Non-technical end-users of Fedora should see no difference (other than more a robust operating system).
Python users/developers should see additional warnings/errors when building Python extension modules that contain bugs. The exact experience will depend on how much we can be sure that an issue is a real problem; we don't want to impact the ability for people to do automated buildouts from PyPI.
I'm planning to do this via a GCC plugin that embeds Python, so that I can write the checker in Python itself.
FWIW I also investigated a few other approaches to doing this:
- as a patch to LLVM's static analysis tool (packaged as part of llvm.src.rpm)
- using sparse
- using CIL (see e.g. the work we did to detect errors in libvirt).
- using Coccinelle, like my experiment on PyArg_ParseTuple from November 2009
- using a Python library to parse C, e.g. pycparser or pyclibrary
There can be various levels of fallback:
- the ability to set a flag in an rpm specfile that turns off testing for this rpm build
- the ability to set a variable in the environment to suppress testing (perhaps this is the other way around: the extra tests are only run when a value is set)
- (worst case) fully removing the testing hooks from python-devel and python3-devel if the feature proves problematic and is impeding getting the release out of the door.
I'm not yet sure what the structures of opt-in/opt-out and per-test/per-file/per-build should be.