In the case of most calls to end_ebml_master, the target is within the current segment, but in this case, it's in the first segment or header file, so if the context was marked as non-seekable manually (as in segment.c), avio_seek will ignore that and execute it anyway. This is fine in other uses of end_ebml_master, since both the current and target position are within the current segment, but breaks when ending mkv->segment, since `master.pos` refers to an offset in the first segment. So instead of failing the seek and returning, we end up seeking to an early point in the last segment, overwriting 8 unrelated bytes, and jumping back.
> On Mar 29, 2015, at 11:29, Michael Niedermayer <michae...@gmx.at> wrote: > > On Sat, Mar 28, 2015 at 07:25:21PM -0600, Rodger Combs wrote: >> --- >> libavformat/matroskaenc.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c >> index 6b2e390..c242a0e 100644 >> --- a/libavformat/matroskaenc.c >> +++ b/libavformat/matroskaenc.c >> @@ -1948,9 +1948,9 @@ static int mkv_write_trailer(AVFormatContext *s) >> put_ebml_float(pb, MATROSKA_ID_DURATION, mkv->duration); >> >> avio_seek(pb, currentpos, SEEK_SET); >> + end_ebml_master(pb, mkv->segment); >> } >> >> - end_ebml_master(pb, mkv->segment); > > the change should be ok but > why exactly does this fail ? > > if the output isnt seekable avio_seek() would fail and end_ebml_master > would return with just the failed seek. > why does this cause a problem ? > > [...] > > -- > Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB > > During times of universal deceit, telling the truth becomes a > revolutionary act. -- George Orwell > _______________________________________________ > ffmpeg-devel mailing list > ffmpeg-devel@ffmpeg.org <mailto:ffmpeg-devel@ffmpeg.org> > http://ffmpeg.org/mailman/listinfo/ffmpeg-devel > <http://ffmpeg.org/mailman/listinfo/ffmpeg-devel> _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
