[FFmpeg-devel] [PATCH] avutil/mathematics: correct documentation for av_gcd

[Ganesh Ajjanagadde]

On Wed, Oct 28, 2015 at 10:09 AM, Ganesh Ajjanagadde
<gajjanagadde at gmail.com> wrote:
> On Tue, Oct 27, 2015 at 8:18 PM, Ganesh Ajjanagadde
> <gajjanagadde at gmail.com> wrote:
>> av_gcd is now always defined regardless of input. This documents this
>> change in the "documented API". Two benefits (closely related):
>> 1. The function is robust, and there is no need to worry about INT64_MIN, etc.
>>
>> 2. Clients of av_gcd, like av_reduce, can now be made fully correct. Currently,
>> av_reduce can trigger undefined behavior if e.g num is INT64_MIN due to
>> integer overflow in the FFABS. Furthermore, this undefined behavior is
>> completely undocumented, and could be a fuzzer's paradise. The FFABS was needed in the past as
>> av_gcd was undefined for negative inputs. In order to make av_reduce
>> robust, it is essential to guarantee that av_gcd works for all int64_t.
>>
>> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
>> ---
>>  libavutil/mathematics.h | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/libavutil/mathematics.h b/libavutil/mathematics.h
>> index ac94488..6fc2577 100644
>> --- a/libavutil/mathematics.h
>> +++ b/libavutil/mathematics.h
>> @@ -77,9 +77,9 @@ enum AVRounding {
>>  };
>>
>>  /**
>> - * Return the greatest common divisor of a and b.
>> - * If both a and b are 0 or either or both are <0 then behavior is
>> - * undefined.
>> + * Compute the greatest common divisor of a and b.
>> + *
>> + * @return gcd of a and b up to sign; if a and b are both zero returns 0
>>   */
>>  int64_t av_const av_gcd(int64_t a, int64_t b);
>>
>> --
>> 2.6.2
>>
>
> Patch dropped for now, undefined behavior is still possible with
> av_gcd: take a and b to be both INT64_MIN. Need to examine this a
> little more closely to make it robust without losing performance.

Request to reconsider; patch making av_gcd more robust sent.