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

[Steven Liu]

> 在 2018年1月28日，上午9:09，Brendan McGrath <redmcg at redmandi.dyndns.org> 写道：
> 
> 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]
> 
will apply it
> 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;
>