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

[Michael Niedermayer]

On Wed, May 03, 2017 at 03:28:53PM +0200, Hendrik Leppkes wrote:
> On Wed, May 3, 2017 at 3:23 PM, Michael Niedermayer
> <michael at niedermayer.cc> wrote:
> > On Wed, May 03, 2017 at 12:54:53PM +0200, Hendrik Leppkes wrote:
> >> On Wed, May 3, 2017 at 11:50 AM, Michael Niedermayer
> >> <michael at niedermayer.cc> 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
> >> >
> >>
> >> This behavior seems odd at best, so maybe we should just change that
> >> and make the behavior more logical, and fix your issue at the same
> >> time?
> >
> > That can be done and makes alot of sense after the patch.
> >
> > we need to fix this issue in our releases too
> > a simple bugfix and a seperate behavior change afterwards allows us
> > to simply backport the bugfix from master.
> >
> 
> If anything your "bugfix" here is a performance improvement, and I
> don't think that warrants backporting either way.

Before the patch the sample file takes
51seconds to decode, after it, it takes 203 milliseconds.
The difference is caused by only 2 calls to the copy code

blocking a machine for 51 seconds for something that decodes in 203ms
otherwise, is IMO worth backporting a fix for.

We can add a bound to the number of side data elements if theres
consens about doing that and doing it in releases.
still, no code should call realloc() in a loop when it can do one
(re)allocation call at the top.


[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Into a blind darkness they enter who follow after the Ignorance,
they as if into a greater darkness enter who devote themselves
to the Knowledge alone. -- Isha Upanishad
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