Re: [FFmpeg-devel] [PATCH] avformat/mpegts: correctly skip TP_extra_header in m2ts

[llyyr <llyyr.public@gmail.com>]

On 6/7/24 1:29 PM, Hendrik Leppkes wrote:
> On Fri, Jun 7, 2024 at 9:47 AM Hendrik Leppkes <h.leppkes@gmail.com> wrote:
>>
>> On Mon, May 27, 2024 at 3:47 PM llyyr <llyyr.public@gmail.com> wrote:
>>>
>>> instead of just resyncing and skipping a bunch of TS packets, leading to
>>> a loss of frames.
>>>
>>> Before this, a stray byte with the value of 0x47 in TP_extra_header
>>> would throw off the detection of where TS packets start.
>>>
>>> A typical file that could cause issues would look like this:
>>>
>>>      00000300: 238f 4780 4750 1110 0000 01e0 0000 84c0
>>>                     ^^   ^^
>>> The first four bytes here are TP_extra_header and the actual TS packet
>>> starts at offset 0x304
>>>
>>> FFmpeg would try to read a packet at 0x300 but since nothing skips the
>>> 4 byte TP_extra_header, find that the first byte is not 0x47 and
>>> immediately go into mpegts_resync, and incorrectly detect the stray 0x47
>>> in the TP_extra_header at 0x302 as the new sync byte.
>>>
>>> Fix this by correctly skipping the first 4 bytes if the source packet
>>> is 192 bytes.
>>>
>>> Signed-off-by: llyyr <llyyr.public@gmail.com>
>>> ---
>>>   libavformat/mpegts.c | 12 +++++++++++-
>>>   1 file changed, 11 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/libavformat/mpegts.c b/libavformat/mpegts.c
>>> index c66a1ea6ede0..0d80ad80f1aa 100644
>>> --- a/libavformat/mpegts.c
>>> +++ b/libavformat/mpegts.c
>>> @@ -2944,6 +2944,12 @@ static int read_packet(AVFormatContext *s, uint8_t *buf, int raw_packet_size,
>>>       AVIOContext *pb = s->pb;
>>>       int len;
>>>
>>> +    // 192 bytes source packet that start with a 4 bytes TP_extra_header
>>> +    // followed by 188 bytes of TS packet. The sync byte is at offset 4, so skip
>>> +    // the first 4 bytes otherwise we'll end up syncing to the wrong packet.
>>> +    if (raw_packet_size == TS_DVHS_PACKET_SIZE)
>>> +        avio_skip(pb, 4);
>>> +
>>
>> I think this doesn't work with mpegts_resync, since it always resyncs
>> for the packet start to be on the 0x47 marker, not 4 bytes before it.
>> So if sync is lost, it would never recover, if I read this right.
>>
> 
> Since we're dealing with a special case for 192 byte packets here
> anyway, maybe a special case in mpegts_resync that just checks if raw
> size = 192 && [4] = 0x47 would handle this with less potential
> fallout?

I've looked at this again and I don't see a way to gracefully handle it 
in mpegts_resync. It's much cleaner to just skip the header that's not 
relevant to us so we line up with the first packet being the sync byte.

FWIW this can be reproduced with

`ffmpeg -i <file> -c copy out.h264`

Sample file: https://0x0.st/XVJi.m2ts
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