[FFmpeg-devel] [PATCH] Fix decoding crash on some trashed interlaced MPEG2 streams. This fixes issue 2367.

[Måns Rullgård]

Anatoly Nenashev <anatoly.nenashev at ovsoft.ru> writes:

> On 18.02.2011 18:38, M?ns Rullg?rd wrote:
>> Anatoly Nenashev<anatoly.nenashev at ovsoft.ru>  writes:
>>
>>    
>>> On 18.02.2011 15:26, M?ns Rullg?rd wrote:
>>>      
>>>> What is the actual problem you are trying to detect?  Missing reference
>>>> picture?
>>>>
>>>>
>>>>        
>>> The problem is available when second field of first decoded interlaced
>>> picture has P-type. In this case inter prediction can be done from the
>>> first field of current picture (works fine) or from the second field
>>> of previous  picture (crashes decoder). Sample exploit attached to
>>> issue 2367. This sample was specially prepared to show the problem.
>>>      
>> Couldn't that be checked per frame instead of per MB?  Sure, doing it
>> per MB might allow decoding some blocks, but is that really worth it?
>>
>>    
> I don't know how to  made this check per frame because there may be
> some macroblocks predicted from the first field of current picture and
> the other predicted from the second field of previous picture. I can't
> find this information without decoding each macroblock.

Two possibilities:

- ditch the entire frame if any possible references are missing
- substitute a dummy picture for missing references

The second of these is equivalent to your patch with less per-MB
overhead for undamaged files.

-- 
M?ns Rullg?rd
mans at mansr.com