[FFmpeg-devel] [PATCH] Fix issue when invalid nb_index_entries value is read - SPONSORED BY INA (Institut National de l'Audiovisuel)

[Cédric Le Barz]

Hi,

By "invalid", I mean that the written nb_index_entries value does not match with the real number of entries in the table. The number of entries in the table is lower than the specified one. This is a bug in commercial MXFTk software (at least version V 2.8.0.0.1) : it exists a lot a MXF files with this problem.  The proposed patch just robustify index table parsing.

Best regards,

Cédric

-----Message d'origine-----
De : ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> De la part de Marton Balint
Envoyé : mercredi 8 mars 2023 23:06
À : FFmpeg development discussions and patches <ffmpeg-devel at ffmpeg.org>
Objet : Re: [FFmpeg-devel] [PATCH] Fix issue when invalid nb_index_entries value is read - SPONSORED BY INA (Institut National de l'Audiovisuel)



On Wed, 8 Mar 2023, Cédric Le Barz wrote:

>
> Fix issue when invalid nb_index_entries value is read : in case of 
> false nb_index_entries value, ffmpeg exit. This patch fix this problem.

What do you mean invalid? Is the value wrong, or is the file truncated in the middle of the index segment? From the patch it looks like the latter.

Is there a single unluckily truncated file that you want to fix with this patch, or multiple files can be affected by this fix because e.g. they are written by the same broken muxer?

Overall I am not sure if we want this applied, the user can also opt to avoid parsing all partitions with -seekable 0 and read / remux the file sequentially ignoring any index, so if somebody wants to rescue the data, it can be done as is.

Regards,
Marton

>
>
> Signed-off-by: Cedric Le Barz <clebarz at ektacom.com>
> ---
>  ffmpeg/libavformat/mxfdec.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/ffmpeg/libavformat/mxfdec.c b/ffmpeg/libavformat/mxfdec.c 
> index 4530617..ffc8987 100644
> --- a/ffmpeg/libavformat/mxfdec.c
> +++ b/ffmpeg/libavformat/mxfdec.c
> @@ -1221,8 +1221,18 @@ static int 
> mxf_read_index_entry_array(AVIOContext *pb, MXFIndexTableSegment *seg
>      }
>      for (i = 0; i < segment->nb_index_entries; i++) {
> -        if(avio_feof(pb))
> -            return AVERROR_INVALIDDATA;
> +
> +        if(avio_feof(pb)) {
> +            if (i == 0) {
> +                return AVERROR_INVALIDDATA;
> +            } else {
> +                /* To be more robust to invalid nb_index_entries value,
> +                   fix the index entry number according to read entries */
> +                segment->nb_index_entries = i;
> +                return 0;
> +            }
> +        }
> +
>          segment->temporal_offset_entries[i] = avio_r8(pb);
>          avio_r8(pb);                                        /* 
> KeyFrameOffset */
>         segment->flag_entries[i] = avio_r8(pb);
> --
> 2.34.1
>
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