Michael Niedermayer:
> The calling code does not handle failures and will fail with assertion
> failures later.
> Seeking can always fail even when the position was previously read.
>
> Fixes: Assertion failure
> Fixes:
> 35253/clusterfuzz-testcase-minimized-ffmpeg_dem_MATROSKA_fuzzer-4693059982983168
>
> Found-by: continuous fuzzing process
> https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> Signed-off-by: Michael Niedermayer <mich...@niedermayer.cc>
> ---
> libavformat/matroskadec.c | 19 ++++++++++++-------
> 1 file changed, 12 insertions(+), 7 deletions(-)
>
> diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
> index 356a02339c..a0e6e0cf8b 100644
> --- a/libavformat/matroskadec.c
> +++ b/libavformat/matroskadec.c
> @@ -804,20 +804,22 @@ static int matroska_read_close(AVFormatContext *s);
> static int matroska_reset_status(MatroskaDemuxContext *matroska,
> uint32_t id, int64_t position)
> {
> + int64_t err = 0;
> if (position >= 0) {
> - int64_t err = avio_seek(matroska->ctx->pb, position, SEEK_SET);
> - if (err < 0)
> - return err;
> - }
> + err = avio_seek(matroska->ctx->pb, position, SEEK_SET);
> + if (err > 0)
> + err = 0;
> + } else
> + position = avio_tell(matroska->ctx->pb);
>
> matroska->current_id = id;
> matroska->num_levels = 1;
> matroska->unknown_count = 0;
> - matroska->resync_pos = avio_tell(matroska->ctx->pb);
> + matroska->resync_pos = position;
> if (id)
> matroska->resync_pos -= (av_log2(id) + 7) / 8;
>
> - return 0;
> + return err;
The changes here will make the demuxer update its internal state as if
it had seeked to its target level-1-element, even though it didn't. Is
this really good?
> }
>
> static int matroska_resync(MatroskaDemuxContext *matroska, int64_t last_pos)
> @@ -1873,6 +1875,7 @@ static int
> matroska_parse_seekhead_entry(MatroskaDemuxContext *matroska,
> uint32_t saved_id = matroska->current_id;
> int64_t before_pos = avio_tell(matroska->ctx->pb);
> int ret = 0;
> + int ret2;
>
> /* seek */
> if (avio_seek(matroska->ctx->pb, pos, SEEK_SET) == pos) {
> @@ -1897,7 +1900,9 @@ static int
> matroska_parse_seekhead_entry(MatroskaDemuxContext *matroska,
> }
> /* Seek back - notice that in all instances where this is used
> * it is safe to set the level to 1. */
> - matroska_reset_status(matroska, saved_id, before_pos);
> + ret2 = matroska_reset_status(matroska, saved_id, before_pos);
> + if (ret >= 0)
> + ret = ret2;
>
> return ret;
> }
>
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