[FFmpeg-devel] [PATCH] dashdec: Fix segfault on decoding segment timeline

[Brendan McGrath]

Just making sure this patch hasn't been overlooked.

To better explain the existing issue - the value of 'pls->first_seq_no' 
comes from the 'startNumber' attribute of the 'SegmentTemplate' element.

The 'pls->timelines' array represents the elements within the 
'SegmentTimeline' element.

So for this example:
   <SegmentTemplate timescale="48000" 
initialization="init-stream$RepresentationID$.m4s" 
media="chunk-stream$RepresentationID$-$Number%05d$.m4s" startNumber="7">
     <SegmentTimeline>
       <S t="2196480" d="480256" r="1" />
     </SegmentTimeline>
   </SegmentTemplate>

'pls->first_seq_no' will be 7 but the 'pls->timelines' array will only 
have the one element - hence:
pls->timelines[pls->first_seq_no]

causes a segfault.

This patch removes the use of 'pls->first_seq_no' within the 
'pls->timelines' array

On 20/01/18 18:07, Brendan McGrath wrote:
> If first_seq_no is not within the bounds of timelines then a segfault
> will occur.
>
> This patch removes the use of first_seq_no within the timelines array
>
> It also adds first_seq_no to the value returned by calc_next_seg_no_from_timelines
> (which allows for different values of 'startNumber')
>
> Signed-off-by: Brendan McGrath <redmcg at redmandi.dyndns.org>
> ---
>
> This can be reproduced by creating a dash file with the following command:
> ffmpeg -i in.ts -f dash -window_size 2 out.mpd
>
> and then after about 10 seconds run ffprobe against it.
>
> The SegementTemplatei of out.mpd will look something like this:
>    <SegmentTemplate timescale="48000" initialization="init-stream$RepresentationID$.m4s" media="chunk-stream$RepresentationID$-$Number%05d$.m4s" startNumber="7">
>      <SegmentTimeline>
>        <S t="2196480" d="480256" r="1" />
>      </SegmentTimeline>
>    </SegmentTemplate>
>
> So the code was trying to access the 7th element within the timeline
> array (which only had one element).
>
> The patch doesn't worry about checking for a negative value after deducting the 60
> seconds as calc_next_seg_no_from_timelines accepts a signed int and works correctly with
> negative values.
>
>   libavformat/dashdec.c | 11 ++++-------
>   1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/libavformat/dashdec.c b/libavformat/dashdec.c
> index 2492f1d..cdb7ba5 100644
> --- a/libavformat/dashdec.c
> +++ b/libavformat/dashdec.c
> @@ -1083,15 +1083,12 @@ static int64_t calc_cur_seg_no(AVFormatContext *s, struct representation *pls)
>           if (pls->n_fragments) {
>               num = pls->first_seq_no;
>           } else if (pls->n_timelines) {
> -            start_time_offset = get_segment_start_time_based_on_timeline(pls, 0xFFFFFFFF) - pls->timelines[pls->first_seq_no]->starttime; // total duration of playlist
> -            if (start_time_offset < 60 * pls->fragment_timescale)
> -                start_time_offset = 0;
> -            else
> -                start_time_offset = start_time_offset - 60 * pls->fragment_timescale;
> -
> -            num = calc_next_seg_no_from_timelines(pls, pls->timelines[pls->first_seq_no]->starttime + start_time_offset);
> +            start_time_offset = get_segment_start_time_based_on_timeline(pls, 0xFFFFFFFF) - 60 * pls->fragment_timescale; // 60 seconds before end
> +            num = calc_next_seg_no_from_timelines(pls, start_time_offset);
>               if (num == -1)
>                   num = pls->first_seq_no;
> +            else
> +                num += pls->first_seq_no;
>           } else if (pls->fragment_duration){
>               if (pls->presentation_timeoffset) {
>                   num = pls->presentation_timeoffset * pls->fragment_timescale / pls->fragment_duration;