[FFmpeg-devel] [PATCH 1/2] avcodec/avpacket: Use av_copy_packet_side_data() in av_packet_copy_props()

[wm4]

On Wed, 3 May 2017 15:16:27 +0200
Michael Niedermayer <michael at niedermayer.cc> wrote:

> On Wed, May 03, 2017 at 12:07:46PM +0200, wm4 wrote:
> > On Wed, 3 May 2017 11:55:16 +0200
> > Michael Niedermayer <michael at niedermayer.cc> wrote:
> >   
> > > On Wed, May 03, 2017 at 11:50:41AM +0200, Michael Niedermayer wrote:  
> > > > On Wed, May 03, 2017 at 11:37:35AM +0200, wm4 wrote:    
> > > > > On Wed, 3 May 2017 11:29:04 +0200
> > > > > Michael Niedermayer <michael at niedermayer.cc> wrote:
> > > > >     
> > > > > > On Wed, May 03, 2017 at 05:29:07AM +0200, wm4 wrote:    
> > > > > > > On Wed,  3 May 2017 05:21:50 +0200
> > > > > > > Michael Niedermayer <michael at niedermayer.cc> wrote:
> > > > > > >       
> > > > > > > > Fixes timeout
> > > > > > > > Fixes: 1293/clusterfuzz-testcase-minimized-6054752074858496
> > > > > > > > 
> > > > > > > > Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
> > > > > > > > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > > > > > > > ---
> > > > > > > >  libavcodec/avpacket.c | 3 +++
> > > > > > > >  1 file changed, 3 insertions(+)
> > > > > > > > 
> > > > > > > > diff --git a/libavcodec/avpacket.c b/libavcodec/avpacket.c
> > > > > > > > index 4bf830bb8a..ff7ee730a4 100644
> > > > > > > > --- a/libavcodec/avpacket.c
> > > > > > > > +++ b/libavcodec/avpacket.c
> > > > > > > > @@ -557,6 +557,9 @@ FF_ENABLE_DEPRECATION_WARNINGS
> > > > > > > >      dst->flags                = src->flags;
> > > > > > > >      dst->stream_index         = src->stream_index;
> > > > > > > >  
> > > > > > > > +    if (!dst->side_data_elems);
> > > > > > > > +        return av_copy_packet_side_data(dst, src);
> > > > > > > > +
> > > > > > > >      for (i = 0; i < src->side_data_elems; i++) {
> > > > > > > >           enum AVPacketSideDataType type = src->side_data[i].type;
> > > > > > > >           int size          = src->side_data[i].size;      
> > > > > > > 
> > > > > > > This doesn't look right...      
> > > > > > 
> > > > > > already fixed the ; locally
> > > > > > 
> > > > > > 
> > > > > > [...]    
> > > > > 
> > > > > I didn't see that, I was referring to the fact that you call
> > > > > av_copy_packet_side_data(), and only sometimes (at least by intention).
> > > > > That requires at least an explanation in the commit message.    
> > > > 
> > > > av_packet_copy_props() would add side data to the destination packet
> > > > it doesnt replace previously existing side data except in case of
> > > > error.
> > > > I dont know if that is intended but i didnt want to change it as that
> > > > would be unrelated to this patch    
> > > 
> > > added
> > > "av_copy_packet_side_data() is only used when it does not lead to a change in behaviour"
> > > to the commit message  
> > 
> > Sorry, that seems all kinds of hacky, especially with the "suspected"
> > reason that the function is unexpectedly faster than the existing code
> > with some memory analyzers. It just doesn't make sense to me.  
> 
> Its not unexpected that O(N) is signifiantly slower than O(1)

It is when N is bounded by a constant (number of defined side data
types). And this is not only theoretically speaking. The constant is
small.

> The "suspect" comes only from myself not profiling the memory analyzer
> to proof that the realloc() calls are what causes of the observed
> significant speed difference
>