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

[Hendrik Leppkes <h.leppkes@gmail.com>]

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?

- Hendrik
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