demux_mkv: improve robustness by explicitly checking for level 1 elements

Matroska makes it pretty hard to resync correctly on broken files:
random data returns "valid" EBML IDs with a high probability, and when
trying to skip them it's likely that you skip a random amount of data
(instead of considering the element length invalid).

Improve upon this by skipping known level 1 elements only. Consider
everything else invalid and call the resync code. This might result in
annoying behavior when Matroska adds new level 1 elements, although it
won't be particularly harmful. Matroska doesn't really allow us to do
better (even mkvtoolnix explicitly checks for known level 1 elements).

Since we now don't always want to combine EBML element skipping and
resyncing, remove ebml_read_skip_or_resync_cluster(), and make
ebml_read_skip() more tolerant against skipping broken elements.

Also, don't resync when reading sub-elements, and instead do resyncing
when reading them results in an error.

1 files changed, 2 insertions, 3 deletions

diff --git a/demux/ebml.h b/demux/ebml.h
index 2259563340..b934ee5c34 100644
--- a/demux/ebml.h
+++ b/demux/ebml.h
@@ -92,6 +92,7 @@ struct ebml_parse_ctx {
 #define EBML_FLOAT_INVALID  -1000000000.0
 
 
+bool ebml_is_mkv_level1_id(uint32_t id);
 uint32_t ebml_read_id (stream_t *s, int *length);
 uint64_t ebml_read_vlen_uint (bstr *buffer);
 int64_t ebml_read_vlen_int (bstr *buffer);
@@ -101,9 +102,7 @@ int64_t ebml_read_int (stream_t *s, uint64_t *length);
 double ebml_read_float (stream_t *s, uint64_t *length);
 char *ebml_read_ascii (stream_t *s, uint64_t *length);
 char *ebml_read_utf8 (stream_t *s, uint64_t *length);
-int ebml_read_skip (stream_t *s, uint64_t *length);
-int ebml_read_skip_or_resync_cluster(struct mp_log *log, int64_t end,
-                                     stream_t *s);
+int ebml_read_skip(struct mp_log *log, int64_t end, stream_t *s);
 int ebml_resync_cluster(struct mp_log *log, stream_t *s);
 uint32_t ebml_read_master (stream_t *s, uint64_t *length);