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avformat/mpegts: correctly skip TP_extra_header in m2ts

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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>
Signed-off-by: Hendrik Leppkes <h.leppkes@gmail.com>
This commit is contained in:
llyyr 2024-05-27 18:52:31 +05:30 committed by Hendrik Leppkes
parent c893dcce31
commit 550c662a1f
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GPG Key ID: 846079A4B0A7C1B5
1 changed files with 11 additions and 1 deletions
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12
libavformat/mpegts.c
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@ -2961,6 +2961,12 @@ 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);
for (;;) {
len = ffio_read_indirect(pb, buf, TS_PACKET_SIZE, data);
if (len != TS_PACKET_SIZE)
@ -2983,7 +2989,11 @@ static int read_packet(AVFormatContext *s, uint8_t *buf, int raw_packet_size,
static void finished_reading_packet(AVFormatContext *s, int raw_packet_size)
{
AVIOContext *pb = s->pb;
int skip = raw_packet_size - TS_PACKET_SIZE;
int skip;
if (raw_packet_size == TS_DVHS_PACKET_SIZE)
skip = raw_packet_size - TS_DVHS_PACKET_SIZE;
else
skip = raw_packet_size - TS_PACKET_SIZE;
if (skip > 0)
avio_skip(pb, skip);
}