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

[Anton Khirnov]

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.

-- 
Anton Khirnov