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

[Hendrik Leppkes]

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.

- Hendrik