The spec allows at least thirty-two zero bits followed by a one to mean 2^32-1, with no constraint on the number of zeroes. The libaom reference decoder does not match this, instead reading thirty-two zeroes but not the following one to mean 2^32-1. These two interpretations are incompatible and other implementations may follow one or the other. Therefore reject thirty-two zeroes because the intended behaviour is not clear. --- libaom, dav1d and SVT-AV1 all have the same nonstandard behaviour of stopping at thirty-two zeroes and not reading the one. gav1 just rejects thirty-two zeroes.
This is also a source of arbitrarily large single syntax elements to hit <https://lists.ffmpeg.org/pipermail/ffmpeg-devel/2023-October/315973.html>. libavcodec/cbs_av1.c | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-) diff --git a/libavcodec/cbs_av1.c b/libavcodec/cbs_av1.c index 1d9ac5ab44..13c749a25b 100644 --- a/libavcodec/cbs_av1.c +++ b/libavcodec/cbs_av1.c @@ -36,7 +36,7 @@ static int cbs_av1_read_uvlc(CodedBitstreamContext *ctx, GetBitContext *gbc, CBS_TRACE_READ_START(); zeroes = 0; - while (1) { + while (zeroes < 32) { if (get_bits_left(gbc) < 1) { av_log(ctx->log_ctx, AV_LOG_ERROR, "Invalid uvlc code at " "%s: bitstream ended.\n", name); @@ -49,10 +49,18 @@ static int cbs_av1_read_uvlc(CodedBitstreamContext *ctx, GetBitContext *gbc, } if (zeroes >= 32) { - // Note that the spec allows an arbitrarily large number of - // zero bits followed by a one bit in this case, but the - // libaom implementation does not support it. - value = MAX_UINT_BITS(32); + // The spec allows at least thirty-two zero bits followed by a + // one to mean 2^32-1, with no constraint on the number of + // zeroes. The libaom reference decoder does not match this, + // instead reading thirty-two zeroes but not the following one + // to mean 2^32-1. These two interpretations are incompatible + // and other implementations may follow one or the other. + // Therefore we reject thirty-two zeroes because the intended + // behaviour is not clear. + av_log(ctx->log_ctx, AV_LOG_ERROR, "Thirty-two zero bits in " + "%s uvlc code: considered invalid due to conflicting " + "standard and reference decoder behaviour.\n", name); + return AVERROR_INVALIDDATA; } else { if (get_bits_left(gbc) < zeroes) { av_log(ctx->log_ctx, AV_LOG_ERROR, "Invalid uvlc code at " -- 2.39.2 _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
