[FFmpeg-devel] [PATCH] avformat/mpegts: Shuffle avio_seek

[Michael Niedermayer]

On Sun, May 17, 2020 at 02:11:19PM +0200, Marton Balint wrote:
> 
> 
> On Thu, 7 May 2020, Michael Niedermayer wrote:
> 
> >On Thu, May 07, 2020 at 02:09:43PM +0200, Anton Khirnov wrote:
> >>Quoting Michael Niedermayer (2020-05-07 12:38:26)
> >>>This avoids accessing an old, no longer valid buffer.
> >>>Fixes: out of array access
> >>>Fixes: crash_audio-2020
> >>>
> >>>Found-by: le wu <shoulewoba at gmail.com>
> >>>Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> >>>---
> >>> libavformat/mpegts.c | 7 ++++---
> >>> 1 file changed, 4 insertions(+), 3 deletions(-)
> >>>
> >>>diff --git a/libavformat/mpegts.c b/libavformat/mpegts.c
> >>>index 0833d62ea5..a065c61c40 100644
> >>>--- a/libavformat/mpegts.c
> >>>+++ b/libavformat/mpegts.c
> >>>@@ -2881,15 +2881,16 @@ static int mpegts_resync(AVFormatContext *s, int seekback, const uint8_t *curren
> >>>     AVIOContext *pb = s->pb;
> >>>     int c, i;
> >>>     uint64_t pos = avio_tell(pb);
> >>>-
> >>>-    avio_seek(pb, -FFMIN(seekback, pos), SEEK_CUR);
> >>>+    int64_t back = FFMIN(seekback, pos);
> >>>
> >>>     //Special case for files like 01c56b0dc1.ts
> >>>     if (current_packet[0] == 0x80 && current_packet[12] == 0x47) {
> >>>-        avio_seek(pb, 12, SEEK_CUR);
> >>>+        avio_seek(pb, 12 - back, SEEK_CUR);
> >>>         return 0;
> >>>     }
> >>>
> >>>+    avio_seek(pb, -back, SEEK_CUR);
> >>>+
> >>
> >>This seems pretty non-obvious - why would ordering seeks in a specific
> >>way result in invalid memorry access?
> >
> >because current_packet in one case points to the avio internal buffer
> >and doing any "state changing" avio could change that buffer.
> >so all accesses to the buffer must happen before any avio seeks
> 
> Yuck. Patch LGTM though.

will apply

thx

[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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