Re: [FFmpeg-devel] [PATCH 1/2] avformat/demux: Make read_frame_internal() return AVERREOR(EAGAIN) on stuck empty input parser

[Marton Balint <cus@passwd.hu>]

On Sat, 18 Jun 2022, Michael Niedermayer wrote:

> On Fri, Jun 17, 2022 at 09:53:13PM +0200, Marton Balint wrote:
>>
>>
>> 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@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.
>
> yes thats why i waited so many month before i applied it
> some patchsets i forget but this i kept pushing away
>
>>
>> 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)?
>
> In the context of this problem the idea is to give the user an opertunity
> to do something else in a single threaded environment or error out if
> its taking too long not produingh anything
>
>
>>
>> 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.
>
> IIRC the problem was that the demuxer produced 0 sized packets and alot of them
> for a file only a few hundread bytes large.

AFAIK a demuxer is not supposed to generate 0-sized packets. Or do you 
think otherwise?

E.g. a simple patch like this seems the better approach:

--- a/libavformat/mov.c
+++ b/libavformat/mov.c
@@ -8677,8 +8677,12 @@ static int mov_read_packet(AVFormatContext *s, 
AVPacket *pkt)

          if (st->codecpar->codec_id == AV_CODEC_ID_EIA_608 && sample->size > 8)
              ret = get_eia608_packet(sc->pb, pkt, sample->size);
-        else
+        else if (sample->size > 0) {
              ret = av_get_packet(sc->pb, pkt, sample->size);
+        } else {
+            av_log(mov->fc, AV_LOG_ERROR, "Invalid sample size, corrupt index?\n");
+            return AVERROR_INVALIDDATA;
+        }
          if (ret < 0) {
              if (should_retry(sc->pb, ret)) {
                  mov_current_sample_dec(sc);

Regards,
Marton
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