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

[Anton Khirnov]

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.

-- 
Anton Khirnov