[FFmpeg-devel] [PATCH 1/2] avformat/demux: Make read_frame_internal() return AVERREOR(EAGAIN) on stuck empty input parser
Marton Balint
cus at passwd.hu
Fri Jun 17 22:53:13 EEST 2022
On Tue, 8 Feb 2022, Michael Niedermayer wrote:
> Fixes: read_frame_internal() which does not return even though both demuxer and parser do return
> Fixes: 43717/clusterfuzz-testcase-minimized-ffmpeg_IO_DEMUXER_fuzzer-5206008287330304
>
> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> ---
> libavformat/demux.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/libavformat/demux.c b/libavformat/demux.c
> index ec34b65288..dd42d32710 100644
> --- a/libavformat/demux.c
> +++ b/libavformat/demux.c
> @@ -1222,11 +1222,15 @@ static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
> FFFormatContext *const si = ffformatcontext(s);
> int ret, got_packet = 0;
> AVDictionary *metadata = NULL;
> + int empty = 0;
>
> while (!got_packet && !si->parse_queue.head) {
> AVStream *st;
> FFStream *sti;
>
> + if (empty > 1)
> + return AVERROR(EAGAIN);
> +
> /* read next packet */
> ret = ff_read_packet(s, pkt);
> if (ret < 0) {
> @@ -1317,6 +1321,8 @@ static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
> }
> got_packet = 1;
> } else if (st->discard < AVDISCARD_ALL) {
> + if (pkt->size == 0)
> + empty ++;
> if ((ret = parse_packet(s, pkt, pkt->stream_index, 0)) < 0)
> return ret;
> st->codecpar->sample_rate = sti->avctx->sample_rate;
> --
> 2.17.1
Sorry, just noticed this patchset, and it is very hackish.
For starters the meaning of AVERROR(EAGAIN) for
av_read_frame()/read_frame_internal() is not very well defined. Should the
user retry immediately? Should the user sleep() sometime and the retry?
Can the user expect that a loop of av_read_frame() will eventually return
something different than AVERROR(EAGAIN)?
I am not sure I understand the original issue entirely, but it looks
that instead of fixing the component which returns infinite amount
of zero sized packets you implemented a check that makes the user get an
EAGAIN() on the second zero-sized packet.
And this helps the user how? Instead of a busy CPU loop in the library he
either gets a busy CPU loop in its application or a non-busy CPU loop in
its application (if he sleeps() on EAGAIN).
Is there a file which causes this? Can't the underlying components be
fixed instead?
Thanks,
Marton
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