Re: [FFmpeg-devel] [PATCH] avformat/flvdec: use avio operation instead of pb->buf_ptr use

[Steven Liu <lingjiujianke@gmail.com>]

Hendrik Leppkes <h.leppkes@gmail.com> 于2023年7月27日周四 14:47写道：
Hi Hendrik,

>
> On Thu, Jul 27, 2023 at 4:38 AM Steven Liu <lq@chinaffmpeg.org> wrote:
> >
> > fix segfaults:
> > READ of size 1 at 0x6100000003b7 thread T0
> >     #0 0x7f928d in flv_same_video_codec ffmpeg/libavformat/flvdec.c:317:29
> >     #1 0x7f928d in flv_read_packet ffmpeg/libavformat/flvdec.c:1177
> >     #2 0x6ff32f in ff_read_packet ffmpeg/libavformat/demux.c:575:15
> >     #3 0x70a2fd in read_frame_internal ffmpeg/libavformat/demux.c:1263:15
> >     #4 0x71d158 in avformat_find_stream_info ffmpeg/libavformat/demux.c:2634:15
> >     #5 0x4c821b in LLVMFuzzerTestOneInput ffmpeg/tools/target_dem_fuzzer.c:206:11
> >
> > Signed-off-by: Steven Liu <lq@chinaffmpeg.org>
> > ---
> >  libavformat/flvdec.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/libavformat/flvdec.c b/libavformat/flvdec.c
> > index 3fe21622f7..003e4d7691 100644
> > --- a/libavformat/flvdec.c
> > +++ b/libavformat/flvdec.c
> > @@ -313,8 +313,9 @@ static int flv_same_video_codec(AVFormatContext *s, AVCodecParameters *vpar, int
> >          return 1;
> >
> >      if (flv->exheader) {
> > -        uint8_t *codec_id_str = (uint8_t *)s->pb->buf_ptr;
> > -        uint32_t codec_id = codec_id_str[3] | codec_id_str[2] << 8 | codec_id_str[1] << 16 | codec_id_str[0] << 24;
> > +        int64_t pos = avio_tell(s->pb);
> > +        uint32_t codec_id = avio_rb32(s->pb);
> > +        avio_seek(s->pb, pos, SEEK_SET);
> >          switch(codec_id) {
> >              case MKBETAG('h', 'v', 'c', '1'):
> >                  return vpar->codec_id == AV_CODEC_ID_HEVC;
>
> You don't need to store the position, just do a relative seek
> backwards by 4 bytes.
> I would also recommend to include a call to ffio_ensure_seekback so it
> works with streams.

Thanks for your suggestions.
i have resubmit patch v2:
https://patchwork.ffmpeg.org/project/ffmpeg/patch/20230727073142.64813-1-lq@chinaffmpeg.org/


>
> eg. something like this:
> ffio_ensure_seekback(s->pb, 4);
> avio_rb32(s->pb);
> avio_seek(s->pb, -4, SEEK_CUR);
>
> Add appropriate error checking, in particular for ffio_ensure_seekback, etc.
>
> - Hendrik
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Thanks
Steven
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