[FFmpeg-devel] [PATCH v2] avformat/flvdec: check FLVHeader PreviousTagSize0

[Bodecs Bela]

2017.03.25. 14:15 keltezéssel, Steven Liu írta:
> 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 | 12 +++++++++++-
>   1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/libavformat/flvdec.c b/libavformat/flvdec.c
> index cdcfb9c..94c9e28 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 +720,16 @@ 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_WARNING, "Read FLV header error, input file is not a standard flv format, first PreviousTagSize0 always is 0\n");
> +    }
>   
>       s->start_time = 0;
>       flv->sum_flv_tag_size = 0;
I think this forgiving behaviour is better, than the earlier rigorous 
one. I agree that showing warning is more useful than returning error 
and stopping execution because if

PreviousTagSize0 is not zero that causes no harm.

bb