[FFmpeg-devel] [PATCH] avcodec/pngdec: fix possible race condition with APNG decoding

[Anton Khirnov]

Quoting Paul B Mahol (2021-02-14 11:48:14)
> On Sun, Feb 14, 2021 at 11:44 AM Anton Khirnov <anton at khirnov.net> wrote:
> 
> > Quoting Paul B Mahol (2021-02-14 11:41:20)
> > > On Sun, Feb 14, 2021 at 11:21 AM Anton Khirnov <anton at khirnov.net>
> > wrote:
> > >
> > > > Quoting Paul B Mahol (2021-02-11 22:58:33)
> > > > > Fixes #9017
> > > > >
> > > > > Signed-off-by: Paul B Mahol <onemda at gmail.com>
> > > > > ---
> > > > >  libavcodec/pngdec.c | 4 ++--
> > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/libavcodec/pngdec.c b/libavcodec/pngdec.c
> > > > > index 395b86bbe7..61642b7cbe 100644
> > > > > --- a/libavcodec/pngdec.c
> > > > > +++ b/libavcodec/pngdec.c
> > > > > @@ -711,13 +711,13 @@ static int decode_idat_chunk(AVCodecContext
> > > > *avctx, PNGDecContext *s,
> > > > >              s->bpp += byte_depth;
> > > > >          }
> > > > >
> > > > > -        if ((ret = ff_thread_get_buffer(avctx, &s->picture,
> > > > AV_GET_BUFFER_FLAG_REF)) < 0)
> > > > > -            return ret;
> > > > >          if (avctx->codec_id == AV_CODEC_ID_APNG &&
> > s->last_dispose_op
> > > > != APNG_DISPOSE_OP_PREVIOUS) {
> > > > >              ff_thread_release_buffer(avctx, &s->previous_picture);
> > > > >              if ((ret = ff_thread_get_buffer(avctx,
> > > > &s->previous_picture, AV_GET_BUFFER_FLAG_REF)) < 0)
> > > > >                  return ret;
> > > > >          }
> > > > > +        if ((ret = ff_thread_get_buffer(avctx, &s->picture,
> > > > AV_GET_BUFFER_FLAG_REF)) < 0)
> > > > > +            return ret;
> > > > >          p->pict_type        = AV_PICTURE_TYPE_I;
> > > > >          p->key_frame        = 1;
> > > > >          p->interlaced_frame = !!s->interlace_type;
> > > > > --
> > > > > 2.17.1
> > > >
> > > > It's pretty non-obvious what the race is and how is it fixed by
> > > > reordering the calls.
> > > >
> > >
> > > Before patch hash of decoded output would differ with single threading
> > and
> > > multiple threads.
> > > Now it does not.
> >
> > That still does not explain where the bug is and why this specific
> > change is the right way to fix it.
> >
> 
> Speedup gain is still there with multiple threads.
> And when using helgrind one of items was gone and no new items were
> introduced.

That still does not explain where the bug was. If you used helgrind, you
might as well explain what it found.

-- 
Anton Khirnov