[FFmpeg-devel] [PATCH] avformat/flvdec: check FLVHeader PreviousTagSize0

[Steven Liu]

2017-03-25 16:09 GMT+08:00 Hendrik Leppkes <h.leppkes at gmail.com>:

> On Sat, Mar 25, 2017 at 9:01 AM, Steven Liu <lq at chinaffmpeg.org> wrote:
> > refer to SPEC:
> > Annex E. The FLV File Format said:
> > E.3 TheFLVFileBody have a table:
> > Field            Type    Comment
> > PreviousTagSize0 UI32    Always 0
> >
> > Signed-off-by: Steven Liu <lq at chinaffmpeg.org>
> > ---
> >  libavformat/flvdec.c | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/libavformat/flvdec.c b/libavformat/flvdec.c
> > index cdcfb9c..ac74b45 100644
> > --- a/libavformat/flvdec.c
> > +++ b/libavformat/flvdec.c
> > @@ -709,6 +709,7 @@ static int flv_read_header(AVFormatContext *s)
> >      int flags;
> >      FLVContext *flv = s->priv_data;
> >      int offset;
> > +    int pre_tag_size = 0;
> >
> >      avio_skip(s->pb, 4);
> >      flags = avio_r8(s->pb);
> > @@ -719,7 +720,17 @@ static int flv_read_header(AVFormatContext *s)
> >
> >      offset = avio_rb32(s->pb);
> >      avio_seek(s->pb, offset, SEEK_SET);
> > -    avio_skip(s->pb, 4);
> > +
> > +    /* Annex E. The FLV File Format
> > +     * E.3 TheFLVFileBody
> > +     *     Field               Type    Comment
> > +     *     PreviousTagSize0    UI32    Always 0
> > +     * */
> > +    pre_tag_size = avio_rb32(s->pb);
> > +    if (pre_tag_size) {
> > +        av_log(s, AV_LOG_ERROR, "Read FLV header error, input file is
> not a standard flv format, first PreviousTagSize0 always is 0\n");
> > +        return AVERROR_INVALIDDATA;
> > +    }
> >
>
> What does this exactly break if its not 0?
> Demuxers should try to be forgiving on non-standard files, so checking
> some spec limitation like that only makes sense if it has an actual
> impact on playability of the file.
>

What about give a warning message and do not break?
After all the  PreviousTagSize0 should always 0.


> - Hendrik
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