[Python-Dev] Disabling string interning for null and single-char causes segfaults

[Nick Coghlan]

On Sat, Mar 2, 2013 at 1:24 AM, Stefan Bucur <stefan.bucur at gmail.com> wrote:
> Hi,
>
> I'm working on an automated bug finding tool that I'm trying to apply on the
> Python interpreter code (version 2.7.3). Because of early prototype
> limitations, I needed to disable string interning in stringobject.c. More
> precisely, I modified the PyString_FromStringAndSize and PyString_FromString
> to no longer check for the null and single-char cases, and create instead a
> new string every time (I can send the patch if needed).
>
> However, after applying this modification, when running "make test" I get a
> segfault in the test___all__ test case.
>
> Before digging deeper into the issue, I wanted to ask here if there are any
> implicit assumptions about string identity and interning throughout the
> interpreter implementation. For instance, are two single-char strings having
> the same content supposed to be identical objects?
>
> I'm assuming that it's either this, or some refcount bug in the interpreter
> that manifests only when certain strings are no longer interned and thus
> have a higher chance to get low refcount values.

In theory, interning is supposed to be a pure optimisation, but it
wouldn't surprise me if there are cases that assume the described
strings are always interned (especially the null string case). Our
test suite would never detect such bugs, as we never disable the
interning.

Whether or not we're interested in fixing such bugs would depend on
the size of the patches needed to address them. From our point of
view, such bugs are purely theoretical (as the assumption is always
valid in an unpatched CPython build), so if the problem is too hard to
diagnose or fix, we're more likely to declare that interning of at
least those kinds of string values is required for correctness when
creating modified versions of CPython.

I'm not sure what kind of analyser you are writing, but if it relates
to the CPython C API, you may be interested in
https://gcc-python-plugin.readthedocs.org/en/latest/cpychecker.html

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia