diff options
author | Dave Rodgman <dave.rodgman@arm.com> | 2020-06-11 17:34:54 -0700 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2020-06-11 18:17:47 -0700 |
commit | b5265c813ce4efbfa2e46fd27cdf9a7f44a35d2e (patch) | |
tree | 28486c04a70a504864e588d1b341dab1140558b9 /Documentation/lzo.txt | |
parent | fce1affe4e39b9ca8346bdbb5171e60ac303c355 (diff) |
lib/lzo: fix ambiguous encoding bug in lzo-rle
In some rare cases, for input data over 32 KB, lzo-rle could encode two
different inputs to the same compressed representation, so that
decompression is then ambiguous (i.e. data may be corrupted - although
zram is not affected because it operates over 4 KB pages).
This modifies the compressor without changing the decompressor or the
bitstream format, such that:
- there is no change to how data produced by the old compressor is
decompressed
- an old decompressor will correctly decode data from the updated
compressor
- performance and compression ratio are not affected
- we avoid introducing a new bitstream format
In testing over 12.8M real-world files totalling 903 GB, three files
were affected by this bug. I also constructed 37M semi-random 64 KB
files totalling 2.27 TB, and saw no affected files. Finally I tested
over files constructed to contain each of the ~1024 possible bad input
sequences; for all of these cases, updated lzo-rle worked correctly.
There is no significant impact to performance or compression ratio.
Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Dave Rodgman <dave.rodgman@arm.com>
Cc: Willy Tarreau <w@1wt.eu>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Markus F.X.J. Oberhumer <markus@oberhumer.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Chao Yu <yuchao0@huawei.com>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/20200507100203.29785-1-dave.rodgman@arm.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/lzo.txt')
-rw-r--r-- | Documentation/lzo.txt | 8 |
1 files changed, 6 insertions, 2 deletions
diff --git a/Documentation/lzo.txt b/Documentation/lzo.txt index ca983328976b..f65b51523014 100644 --- a/Documentation/lzo.txt +++ b/Documentation/lzo.txt @@ -159,11 +159,15 @@ Byte sequences distance = 16384 + (H << 14) + D state = S (copy S literals after this block) End of stream is reached if distance == 16384 + In version 1 only, to prevent ambiguity with the RLE case when + ((distance & 0x803f) == 0x803f) && (261 <= length <= 264), the + compressor must not emit block copies where distance and length + meet these conditions. In version 1 only, this instruction is also used to encode a run of - zeros if distance = 0xbfff, i.e. H = 1 and the D bits are all 1. + zeros if distance = 0xbfff, i.e. H = 1 and the D bits are all 1. In this case, it is followed by a fourth byte, X. - run length = ((X << 3) | (0 0 0 0 0 L L L)) + 4. + run length = ((X << 3) | (0 0 0 0 0 L L L)) + 4 0 0 1 L L L L L (32..63) Copy of small block within 16kB distance (preferably less than 34B) |