[FFmpeg-devel] [PATCH 0/1] Handle ASS format subtitle encoding ambiguity

[Tim Angus]

Some matroska files embed ASS format subtitles. The header for said subtitles
include the header for the subtitle stream in the "codec private data" section.
It appears to be optional whether or not the last byte of this data is 0, i.e.
a null terminator for the string data. Using ffmpeg to extract subtitles for
such a file, this header is copied directly to the output file, including the
null terminator, if it was present. This results in a file in which there is a
null terminator after the header, but preceeding the actual content of the
subtitle file. Obviously this is not correct.

As a data point, of the ~600 mkvs I have locally, 22 of them have ASS
subtitles, and of them 20 include the null terminator, so it doesn't appear to
be a rare phenomenon.

As another data point, the tool mkvextract from mkvtoolnix avoids the ambiguity
by first assuming that the source buffer is *not* null terminated, and then
manually adding a (possibly second) null terminator. The buffer is then
interpreted as a null terminated string and processed that way.
(https://gitlab.com/mbunkus/mkvtoolnix/-/blob/main/src/extract/xtr_textsubs.cpp#L117)

My change here simply avoids copying the trailing null terminator(s) present.
FATE succeeds as there are no mkvs in the suite that have ASS subtitles
embedded.

Tim Angus (1):
  avformat/assenc: fix incorrect copy of null terminator

 libavformat/assenc.c | 6 ++++++
 1 file changed, 6 insertions(+)

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2.25.1