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author | Andreas Rheinhardt <andreas.rheinhardt@gmail.com> | 2020-07-31 08:32:55 +0200 |
---|---|---|
committer | Andreas Rheinhardt <andreas.rheinhardt@gmail.com> | 2020-08-08 01:08:47 +0200 |
commit | 06fef1e9f11a45df809e7cc4522f7174f48a664f (patch) | |
tree | 48b08c7a14197a0beb93cd0374b08601a6a60dd2 /libavcodec/put_bits.h | |
parent | 6fdf3cc53b5e90524ccfde534ae4a91ee87814aa (diff) |
avcodec/put_bits: Make skip_put_bits() less dangerous
Before c63c303a1f2b58677d480505ec93a90f77dd25b5 (the commit which
introduced a typedef for the type of the buffer of a PutBitContext)
skip_put_bits() was as follows:
static inline void skip_put_bits(PutBitContext *s, int n)
{
s->bit_left -= n;
s->buf_ptr -= 4 * (s->bit_left >> 5);
s->bit_left &= 31;
}
If s->bit_left was negative after the first subtraction, then the next
line will divide this by 32 with rounding towards -inf and multiply by
four; the result will be negative, of course.
The aforementioned commit changed this to:
static inline void skip_put_bits(PutBitContext *s, int n)
{
s->bit_left -= n;
s->buf_ptr -= sizeof(BitBuf) * ((unsigned)s->bit_left / BUF_BITS);
s->bit_left &= (BUF_BITS - 1);
}
Casting s->bit_left to unsigned meant that the rounding is still towards
-inf; yet the right side is now always positive (it transformed the
arithmetic shift into a logical shift), so that s->buf_ptr will always
be decremented (by about UINT_MAX / 8 unless n is huge) which leads to
segfaults on further usage and is already undefined pointer arithmetic
before that. This can be reproduced with the mpeg4 encoder with the
AV_CODEC_FLAG2_NO_OUTPUT flag set.
Furthermore, the earlier version as well as the new version share
another bug: s->bit_left will be in the range of 0..(BUF_BITS - 1)
afterwards, although the assumption throughout the other PutBitContext
functions is that it is in the range of 1..BUF_BITS. This might lead to
a shift by BUF_BITS in little-endian mode. This has been fixed, too.
The new version is furthermore able to skip zero bits, too.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Diffstat (limited to 'libavcodec/put_bits.h')
-rw-r--r-- | libavcodec/put_bits.h | 8 |
1 files changed, 4 insertions, 4 deletions
diff --git a/libavcodec/put_bits.h b/libavcodec/put_bits.h index ddd97906b2..3ba9549948 100644 --- a/libavcodec/put_bits.h +++ b/libavcodec/put_bits.h @@ -364,13 +364,13 @@ static inline void skip_put_bytes(PutBitContext *s, int n) /** * Skip the given number of bits. * Must only be used if the actual values in the bitstream do not matter. - * If n is 0 the behavior is undefined. + * If n is < 0 the behavior is undefined. */ static inline void skip_put_bits(PutBitContext *s, int n) { - s->bit_left -= n; - s->buf_ptr -= sizeof(BitBuf) * ((unsigned)s->bit_left / BUF_BITS); - s->bit_left &= (BUF_BITS - 1); + unsigned bits = BUF_BITS - s->bit_left + n; + s->buf_ptr += sizeof(BitBuf) * (bits / BUF_BITS); + s->bit_left = BUF_BITS - (bits & (BUF_BITS - 1)); } /** |