[Python-Dev] segfaults due to hash randomization in C OrderedDict

MRAB python at mrabarnett.plus.com
Thu May 21 19:17:05 CEST 2015

On 2015-05-21 15:55, Eric Snow wrote:
> (see http://bugs.python.org/issue16991)
> I an working on resolving an intermittent segfault that my C
> OrderedDict patch introduces.  The failure happens in
> test_configparser (RawConfigParser uses OrderedDict internally), but
> only sporadically.  However, Ned pointed out to me that it appears to
> be related to hash randomization, which I have verified.  I'm looking
> into it.
> In the meantime, here's a specific question.  What would lead to the
> pattern of failures I'm seeing?  I've verified that the segfault
> happens consistently for certain hash randomization seeds and never
> for the rest.  I don't immediately recognize the pattern but expect
> that it would shed some light on where the problem lies.  I ran the
> following command with the OrderedDict patch applied:
>    for i in `seq 1 100`; do echo $i; PYTHONHASHSEED=$i ./python -m
> test.regrtest -m test_basic test_configparser ; done
> Through 100 I get segfaults with seeds of 7, 15, 35, 37, 39, 40, 42,
> 47, 50, 66, 67, 85, 87, 88, and 92.  I expect the distribution across
> all seeds is uniform, but I haven't verified that.
> Thoughts?
In "_odict_get_index", for example (there are others), you're caching

     PyDictKeysObject *keys = ((PyDictObject *)od)->ma_keys;

If it resizes, you go back to the label "start", which is after that
line, but could "ma_keys" change when it's resized?

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