[FFmpeg-devel] [PATCH v3] avcodec/v4l2_m2m_dec: Remove redundant packet and fix double free

[Andriy Gelman]

On Sat, 09. May 13:35, Andriy Gelman wrote:
> From: Andriy Gelman <andriy.gelman at gmail.com>
> 
> v4l2_receive_frame() uses two packets s->buf_pkt and avpkt. If avpkt
> cannot be enqueued, the packet is buffered in s->buf_pkt and enqueued in
> the next call. Currently the ownership transfer between the two packets
> is not properly handled. A double free occurs if
> ff_v4l2_context_enqueue_packet() returns EAGAIN and v4l2_try_start
> returns EINVAL.
> 
> In fact, having two AVPackets is not needed and everything can be
> handled by s->buf_pkt.
> 
> This commit removes the local avpkt from v4l2_receive_frame(), meaning
> that the ownership transfer doesn't need to be handled and the double
> free is fixed.
> 
> Signed-off-by: Andriy Gelman <andriy.gelman at gmail.com>
> ---
> 
> Sorry, forgot to squash the commit from v1 so v2 didn't apply. This is correct version.
> 
> Supersedes:
> https://patchwork.ffmpeg.org/project/ffmpeg/patch/20200505055454.28683-1-andriy.gelman@gmail.com/
> 
> 
>  libavcodec/v4l2_m2m_dec.c | 37 +++++++++++++++----------------------
>  1 file changed, 15 insertions(+), 22 deletions(-)
> 
> diff --git a/libavcodec/v4l2_m2m_dec.c b/libavcodec/v4l2_m2m_dec.c
> index 3e17e0fcac..b038efed9c 100644
> --- a/libavcodec/v4l2_m2m_dec.c
> +++ b/libavcodec/v4l2_m2m_dec.c
> @@ -138,14 +138,10 @@ static int v4l2_receive_frame(AVCodecContext *avctx, AVFrame *frame)
>      V4L2m2mContext *s = ((V4L2m2mPriv*)avctx->priv_data)->context;
>      V4L2Context *const capture = &s->capture;
>      V4L2Context *const output = &s->output;
> -    AVPacket avpkt = {0};
>      int ret;
>  
> -    if (s->buf_pkt.size) {
> -        avpkt = s->buf_pkt;
> -        memset(&s->buf_pkt, 0, sizeof(AVPacket));
> -    } else {
> -        ret = ff_decode_get_packet(avctx, &avpkt);
> +    if (!s->buf_pkt.size) {
> +        ret = ff_decode_get_packet(avctx, &s->buf_pkt);
>          if (ret < 0 && ret != AVERROR_EOF)
>              return ret;
>      }
> @@ -153,32 +149,29 @@ static int v4l2_receive_frame(AVCodecContext *avctx, AVFrame *frame)
>      if (s->draining)
>          goto dequeue;
>  
> -    ret = ff_v4l2_context_enqueue_packet(output, &avpkt);
> -    if (ret < 0) {
> -        if (ret != AVERROR(EAGAIN))
> -           return ret;
> +    ret = ff_v4l2_context_enqueue_packet(output, &s->buf_pkt);
> +    if (ret < 0 && ret != AVERROR(EAGAIN))
> +        goto fail;
>  
> -        s->buf_pkt = avpkt;
> -        /* no input buffers available, continue dequeing */
> -    }
> +    /* if EAGAIN don't unref packet and try to enqueue in the next iteration */
> +    if (ret != AVERROR(EAGAIN))
> +        av_packet_unref(&s->buf_pkt);
>  
> -    if (avpkt.size) {
> +    if (!s->draining) {
>          ret = v4l2_try_start(avctx);
>          if (ret) {
> -            av_packet_unref(&avpkt);
> -
>              /* cant recover */
> -            if (ret == AVERROR(ENOMEM))
> -                return ret;
> -
> -            return 0;
> +            if (ret != AVERROR(ENOMEM))
> +                ret = 0;
> +            goto fail;
>          }
>      }
>  
>  dequeue:
> -    if (!s->buf_pkt.size)
> -        av_packet_unref(&avpkt);
>      return ff_v4l2_context_dequeue_frame(capture, frame, -1);
> +fail:
> +    av_packet_unref(&s->buf_pkt);
> +    return ret;
>  }
>  
>  static av_cold int v4l2_decode_init(AVCodecContext *avctx)
> -- 
> 2.25.1
> 

ping

-- 
Andriy