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cbs_av1: Reject thirty-two zero bits in uvlc code

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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.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 7110a36ba07f85ca2996d3b99898a4819e72d9bb)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
This commit is contained in:
Mark Thompson 2023-10-22 19:35:52 +01:00 committed by Michael Niedermayer
parent 583b7afb7c
commit f4882ef9e5
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GPG Key ID: B18E8928B3948D64
1 changed files with 13 additions and 2 deletions
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15
libavcodec/cbs_av1.c
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@ -37,7 +37,7 @@ static int cbs_av1_read_uvlc(CodedBitstreamContext *ctx, GetBitContext *gbc,
position = get_bits_count(gbc);
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);
@ -50,7 +50,18 @@ static int cbs_av1_read_uvlc(CodedBitstreamContext *ctx, GetBitContext *gbc,
}
if (zeroes >= 32) {
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 "