samba.git/lib/compression, branch talloc-2.3.4

lzxpress: compress shortcut if we've reached maximum length

2022-05-17T23:11:21+00:00

A simple degenerate case for our compressor has been a large number of
repeated bytes that will match the maximum length (~64k) at all 8192
search positions, 8191 of which searches are in vain because the
matches are not of greater length than the first one.

Here we recognise the inevitable and reduce runtime proportionately.

Credit to OSS-Fuzz.

REF: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=47428

Signed-off-by: Douglas Bagnall 
Reviewed-by: Andrew Bartlett 

Autobuild-User(master): Douglas Bagnall 
Autobuild-Date(master): Tue May 17 23:11:21 UTC 2022 on sn-devel-184

lzxpress/test: time performance of long boring sequences

2022-05-17T22:13:35+00:00

We get *very* slow when long runs of the bytes are the same. On this
laptop the test takes 18s; with the next commit it will be 0.006s.

Signed-off-by: Douglas Bagnall 
Reviewed-by: Andrew Bartlett

compression:tests: align test names with functions

2022-05-12T02:22:35+00:00

You'll thank me if you're ever debugging these and wondering why
'lzxpress4' calls 'lzxpress2' (or is it the other way round?).

Signed-off-by: Douglas Bagnall 
Reviewed-by: Andrew Bartlett

compression: add a few comments, including MS-XCA pointers.

2022-05-12T02:22:35+00:00

Signed-off-by: Douglas Bagnall 
Reviewed-by: Andrew Bartlett

compression: remove always false constant comparison

2022-05-12T02:22:35+00:00

We set `uncompressed_pos = 0;` unconditionally, just ~10 lines up.

Signed-off-by: Douglas Bagnall 
Reviewed-by: Andrew Bartlett

compression: lzxpress decompress empty string as empty string

2022-05-12T02:22:35+00:00

This mirrors the behaviour of lzxpress_compress, which "encodes" an
empty string as an empty string.

Signed-off-by: Douglas Bagnall 
Reviewed-by: Andrew Bartlett

compression: fix lzxpress decompress with trailing flags

2022-05-12T02:22:35+00:00

Every so often, lzxpress adds a 32-bit block of indicator flags to
help decode the next clump of 32 code words. A naive compressor (such
as we have) might do this at the very end for flags that aren't
actually used because there are no more bytes to decompress. If that
happens we need to stop processing, or we'll come to worse outcome at
the next CHECK_INPUT_BYTES.

Signed-off-by: Douglas Bagnall 
Reviewed-by: Andrew Bartlett

compression:tests: test lzxpress in some edge cases

2022-05-12T02:22:35+00:00

Empty strings and trailing flag blocks.

(found with Honggfuzz and a round-trip fuzzer that aborts if the
strings differ).

Signed-off-by: Douglas Bagnall 
Reviewed-by: Andrew Bartlett

compression: Move maximum length calculation out of inner loop

2022-05-12T02:22:35+00:00

This makes the code clearer.

Signed-off-by: Joseph Sutton 
Reviewed-by: Douglas Bagnall

compression: Use correct values for max len and offset

2022-05-12T02:22:35+00:00

Signed-off-by: Joseph Sutton 
Reviewed-by: Douglas Bagnall