[FFmpeg-devel] [PATCH] Fix MPEG video lowres crash

Anatoly Nenashev anatoly.nenashev
Thu Dec 16 09:36:00 CET 2010

On 16.12.2010 04:48, Michael Niedermayer wrote:
> On Wed, Dec 15, 2010 at 02:41:22PM +0300, Anatoly Nenashev wrote:
>> Hi!
>> A little fix for possible crash of MPEG video as discribed in issue
>> https://roundup.ffmpeg.org/issue2421.
>> After that valgrind reports no error.
> what makes you think this fix is correct
> The first hunk needs explanation from you what that is supposed to do and why
> the rest looks wrong.
The main problem is that if lowres>0 then picture sizes in input stream 
and output stream are different.
In test example when lowres=1 it makes ist->codec->width=704 and 
That fact cause to problem in av_image_copy (imgutils.c:230) where 
src_linesize=dst_linesize=704 but source picture from ost context has 
Also the difference in sizes between input and output streams' pictures 
cause ffmpeg to use rescaling filter which is really unneeded.
As I can see from AVCodecContext API, if lowres>0 then width must be set 
as coded_width>>lowres. So if you want to know the original size of 
coded picture then look at the coded_width but not at the width.

> MV=0 does not need the emu code but your change looks
> like it would call it. I guess theres rather a oversight related to the length
> of the MC filter
This fix may by ugly but it was caused by SSSE3/MMX implementation of 
h264_chroma_mc4. The closest look at the code shows that if mc4 applyed 
in bottom macroblock's line of picture then overrun from source buffer 
is available even if MV=0. That issue can be fixed by enlarging 
picture's buffer size but I've decided that this is not a good solution 
corresponded to flag CODEC_FLAG_EMU_EDGE.

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