[FFmpeg-devel] [PATCH 2/2] avformat/apng: set max_fps to no limit by default

[James Almer]

On 3/21/2017 11:05 AM, Michael Niedermayer wrote:
> On Tue, Mar 21, 2017 at 10:03:48AM -0300, James Almer wrote:
>> On 3/21/2017 9:52 AM, Michael Niedermayer wrote:
>>> On Mon, Mar 20, 2017 at 11:03:23PM -0300, James Almer wrote:
>>>> Should fix ticket #6252
>>>>
>>>> Signed-off-by: James Almer <jamrial at gmail.com>
>>>> ---
>>>>  libavformat/apngdec.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/libavformat/apngdec.c b/libavformat/apngdec.c
>>>> index 7a284e32c2..75dcf74a0c 100644
>>>> --- a/libavformat/apngdec.c
>>>> +++ b/libavformat/apngdec.c
>>>> @@ -421,7 +421,7 @@ static const AVOption options[] = {
>>>>      { "ignore_loop", "ignore loop setting"                         , offsetof(APNGDemuxContext, ignore_loop),
>>>>        AV_OPT_TYPE_BOOL, { .i64 = 1 }              , 0, 1      , AV_OPT_FLAG_DECODING_PARAM },
>>>>      { "max_fps"    , "maximum framerate (0 is no limit)"           , offsetof(APNGDemuxContext, max_fps),
>>>> -      AV_OPT_TYPE_INT, { .i64 = DEFAULT_APNG_FPS }, 0, INT_MAX, AV_OPT_FLAG_DECODING_PARAM },
>>>> +      AV_OPT_TYPE_INT, { .i64 = 0 }, 0, INT_MAX, AV_OPT_FLAG_DECODING_PARAM },
>>>>      { "default_fps", "default framerate (0 is as fast as possible)", offsetof(APNGDemuxContext, default_fps),
>>>>        AV_OPT_TYPE_INT, { .i64 = DEFAULT_APNG_FPS }, 0, INT_MAX, AV_OPT_FLAG_DECODING_PARAM },
>>>>      { NULL },
>>>
>>> why was there a max fps set ?
>>> are there files which have huge and incorrect fps ?
>>
>> I have no idea. The author of the decoder may know. But apng is far from
>> a widespread format seeing it has been supported by only one browser until
>> like a week ago, so the chances of bad files like it could happen with
>> jpg or png is most likely low.
>>
>>> if so this may cause a regression and just increasing the default value for
>>> max_fps could be better.
>>
>> I guess 60 would be a saner max value than 15 as it is now, but i still
>> wonder why would we have a max fps set as default to begin with.
>>
> 
>> IMO, if the worry was about a broken/incorrect headers (fuzzing or such),
>> then checking the CRC field for correctness may be a better idea than
>> crippling decoding of valid files by default.
> 
> why do you think this could be related to fuzzing ?

Only reason i can think other than broken encodings to have the delay_num
and delay_den fields reporting bogus values.

> 
> i dont know why the check is there but i had suspected that it was
> a workaround for broken encoders. Possibly not apng encoders but
> a more widespread format like animated gif that is transcoded to apng
> 
> we have a similar max check in gifdec.c
> 
> if gif files need it, gif files converted to apng should need it too
> and i would suspect that animated gifs are a major source for apng
> files, but maybe iam wrong

So what would you say should be done? commit this patch, or make the
max_fps default to 60 instead?