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

[Eric Snow]

Good catch.  Unfortunately, sticking "keys = ((PyDictObject
*)od)->ma_keys;" right after "hash = ..." did not make a difference.
I still get the same segfault.

-eric

On Thu, May 21, 2015 at 11:17 AM, MRAB <python at mrabarnett.plus.com> wrote:
> 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
> "ma_keys":
>
>     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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