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Commit 6b4d94c987 was a previous attempt
to fix this issue.
Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
The "inconvenience function" takes one type, and converts it to another
but the documentation was not updated.
Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
This allows the calculation of the worst case to be shared with callers.
Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
We shouldn't get a node with a zero code, and there's probably nothing
to do but stop.
CID 1517261 (#1-2 of 2): Bad bit shift operation
(BAD_SHIFT)11. negative_shift: In expression j >> offset - k,
shifting by a negative amount has undefined behavior. The shift
amount, offset - k, is -3.
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Mon Dec 19 23:29:04 UTC 2022 on sn-devel-184
Because we just wrote the intermediate representation to have no zero
distances, we can be sure it doesn't, but Coverity doesn't know. If
distance is zero, `bitlen_nonzero_16(distance)` would be bad.
CID 1517278 (#1 of 1): Bad bit shift operation
(BAD_SHIFT)41. large_shift: In expression 1 << code_dist, left
shifting by more than 31 bits has undefined behavior. The shift
amount, code_dist, is 65535.
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jeremy Allison <jra@samba.org>
Very long matches would be written instead as very very long matches.
We can't in fact hit this because we have a MAX_MATCH_LENGTH defined
as 64M, but if we could, it might make certain 2GB+ strings impossible
to compress.
CID 1517275 (#1 of 1): Unintended sign extension
(SIGN_EXTENSION)sign_extension: Suspicious implicit sign extension:
intermediate[i + 2UL] with type uint16_t (16 bits, unsigned) is
promoted in intermediate[i + 2UL] << 16 to type int (32 bits, signed),
then sign-extended to type unsigned long (64 bits, unsigned). If
intermediate[i + 2UL] << 16 is greater than 0x7FFFFFFF, the upper bits
of the result will all be 1.
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jeremy Allison <jra@samba.org>
We know that code is non-zero, because it comes from the combination of
the intermediate representation and the symbol tables that were generated
at the same time. But Coverity doesn't know that, and it thinks we could
be doing undefined things in the subsequent shift.
CID 1517302: Integer handling issues (BAD_SHIFT)
In expression "1 << code_bit_len", shifting by a negative amount has
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jeremy Allison <jra@samba.org>
The struct write_context bit_len attribute is always between 0 and 31,
but if the next patches are applied without this, SUSE GCC -O3 will
worry thusly:
../../lib/compression/lzxpress_huffman.c: In function
‘lzxpress_huffman_compress’:
../../lib/compression/lzxpress_huffman.c:953:5: error: assuming signed
overflow does not occur when simplifying conditional to constant
[-Werror=strict-overflow]
if (wc->bit_len > 16) {
^
cc1: all warnings being treated as errors
Inspection tell us that the invariant holds. Nevertheless, we can
safely use an unsigned type and insist that over- or under- flow is
bad.
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jeremy Allison <jra@samba.org>
We had
output[output_pos - distance];
where output_pos and distance are size_t and distance can be greater
than output_pos (because it refers to a place in the previous block).
The underflow is defined, leading to a big number, and when
sizeof(size_t) == sizeof(*uint8_t) the subsequent overflow works as
expected. But if size_t is smaller than a pointer, bad things will
happen.
This was found by OSSFuzz with
'UBSAN_OPTIONS=print_stacktrace=1:silence_unsigned_overflow=1'.
Credit to OSSFuzz.
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
If you need to see a Huffman tree (and sometimes you do), set
DEBUG_HUFFMAN_TREE to true at the top of lzxpress_huffman.c, and run:
make bin/test_lzx_huffman && bin/test_lzx_huffman
Actually, that will show you hundreds of trees, and you'll be glad of
that if you are ever trying to understand this.
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Encoding without LZ77 matches is valid, and it is useful for isolating
bugs.
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>
This compresses files as described in MS-XCA 2.2, and as decompressed
by the decompressor in the previous commit.
As with the decompressor, there are two public functions -- one that
uses a talloc context, and one that uses pre-allocated memory. The
compressor requires a tightly bound amount of auxillary memory
(>220kB) in a few different buffers, which is all gathered together in
the public struct lzxhuff_compressor_mem. An instantiated but not
initialised copy of this struct is required by the non-talloc
function; it can be used over and over again.
Our compression speed is about the same as the decompression speed
(between 20 and 500 MB/s on this laptop, depending on the data), and
our compression ratio is very similar to that of Windows.
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>
This format is described in [MS-XCA] 2.1 and 2.2, with exegesis in
many posts on the cifs-protocol list[1].
The two public functions are:
ssize_t lzxpress_huffman_decompress(const uint8_t *input,
size_t input_size,
uint8_t *output,
size_t output_size);
uint8_t *lzxpress_huffman_decompress_talloc(TALLOC_CTX *mem_ctx,
const uint8_t *input_bytes,
size_t input_size,
size_t output_size);
In both cases the caller needs to know the *exact* decompressed size,
which is essential for decompression. The _talloc version allocates
the buffer for you, and uses the talloc context to allocate a 128k
working buffer. THe non-talloc function will allocate the working
buffer on the stack.
This compression format gives better compression for messages of
several kilobytes than the "plain" LXZPRESS compression, but is
probably a bit slower to decompress and is certainly worse for very
short messages, having a fixed 256 byte overhead for the first Huffman
table.
Experiments show decompression rates between 20 and 500 MB per second,
depending on the compression ratio and data size, on an i5-1135G7 with
no compiler optimisations.
This compression format is used in AD claims and in SMB, but that
doesn't happen with this commit.
I will not try to describe LZ77 or Huffman encoding here. Don't expect
an answer in MS-XCA either; instead read the code and/or Wikipedia.
[1] Much of that starts here:
https://lists.samba.org/archive/cifs-protocol/2022-October/
but there's more earlier, particularly in June/July 2020, when
Aurélien Aptel was working on an implementation that ended up in
Wireshark.
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Pair-programmed-with: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>
