Date:	Thu, 04 Feb 2010 08:38:45 +1100
From:	Nigel Cunningham <ncunningham@...a.org.au>
To:	linux-crypto@...r.kernel.org,
	TuxOnIce Devel List <tuxonice-devel@...ts.tuxonice.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: LZO irreversible output?

Hi all.

(Not sent to LKML yesterday; no reply from linux-crypto yet, so resending).

A while back now, I stopped supplying the LZF compression algorithm with
TuxOnIce and made LZO the default algorithm. Around the same time, we
started getting occasional errors when reading images; decompression
failures.

I've finally managed to find the time to properly look at this, and have
managed to find a data page that LZO compresses, but seems to be unable
to decompress back to the original contents. I'm wondering whether this
is because I'm doing something wrong, or because there really is some
data the LZO (or the kernel implementation) can't do reversible
compression on.

I've turned the test case and the way TuxOnIce invokes the compression
and decompression code into a standalone kernel module (attached). On my
 64 bit machine, insmoding the module results in the error path being
triggered (skipping the dump of the original page):

[52205.333463] Output 716 bytes. Result 0.
[52205.333468] Compressed to: 06 f8 8f 09 00 00 ea ff ff 00 a1 00 40 7a
00 ff
[52205.333473] Compressed to: ff 27 3c 00 78 01 2b 0c 00 01 30 90 09 00
7c 05
[52205.333478] Compressed to: fc 00 d0 03 3f dd 00 68 dc 06 fc 00 20 07
dd 00
[52205.333483] Compressed to: a0 dc 06 fc 00 20 07 dd 00 d8 dc 06 fc 00
20 07
[52205.333488] Compressed to: de 00 10 91 bc 1b fc 00 20 07 dd 00 48 dc
06 fc
[52205.333493] Compressed to: 00 20 07 dd 00 80 dc 06 fc 00 20 07 dd 00
b8 dc
[52205.333498] Compressed to: 06 fc 00 20 07 dd 00 f0 dc 06 fc 00 20 07
de 00
[52205.333503] Compressed to: 28 92 bc 22 fc 00 20 07 dd 00 60 dc 06 fc
00 20
[52205.333508] Compressed to: 07 dd 00 98 dc 06 fc 00 20 07 dd 00 d0 dc
06 fc
[52205.333512] Compressed to: 00 20 07 de 00 08 93 bc 1b fc 00 20 07 dd
00 40
[52205.333517] Compressed to: dc 06 fc 00 20 07 dd 00 78 dc 06 fc 00 20
07 dd
[52205.333522] Compressed to: 00 b0 dc 06 fc 00 20 07 dd 00 e8 dc 06 fc
00 20
[52205.333527] Compressed to: 07 de 00 20 94 bc 22 fc 00 20 07 dd 00 58
dc 06
[52205.333532] Compressed to: fc 00 20 07 dd 00 90 dc 06 fc 00 20 07 dd
00 c8
[52205.333537] Compressed to: dc 06 fc 00 20 07 de 00 00 95 bc 1b 27 1c
00 a0
[52205.333542] Compressed to: 07 3f 3d 13 38 dc 06 fc 00 b9 06 00 3f dd
00 70
[52205.333546] Compressed to: dc 06 fc 00 20 07 dd 00 a8 dc 06 fc 00 20
07 dd
[52205.333551] Compressed to: 00 e0 dc 06 fc 00 20 07 de 00 18 96 bc 22
fc 00
[52205.333556] Compressed to: 20 07 dd 00 50 dc 06 fc 00 20 07 dd 00 88
dc 06
[52205.333561] Compressed to: fc 00 20 07 dd 00 c0 dc 06 fc 00 20 07 dd
00 f8
[52205.333566] Compressed to: dc 06 fc 00 20 07 de 00 30 97 bc 22 fc 00
20 07
[52205.333571] Compressed to: dd 00 68 dc 06 fc 00 20 07 dd 00 a0 dc 06
fc 00
[52205.333576] Compressed to: 20 07 dd 00 d8 dc 06 fc 00 20 07 de 00 10
98 bc
[52205.333581] Compressed to: 1b fc 00 20 07 dd 00 48 dc 06 fc 00 20 07
dd 00
[52205.333586] Compressed to: 80 dc 06 fc 00 20 07 dd 00 b8 dc 06 fc 00
20 07
[52205.333591] Compressed to: dd 00 f0 dc 06 fc 00 20 07 de 00 28 99 bc
22 fc
[52205.333596] Compressed to: 00 20 07 dd 00 60 dc 06 fc 00 20 07 dd 00
98 dc
[52205.333601] Compressed to: 06 fc 00 20 07 dd 00 d0 dc 06 fc 00 20 07
de 00
[52205.333606] Compressed to: 08 9a bc 1b fc 00 20 07 dd 00 40 dc 06 fc
00 20
[52205.333611] Compressed to: 07 dd 00 78 dc 06 fc 00 20 07 dd 00 b0 dc
06 fc
[52205.333616] Compressed to: 00 20 07 dd 00 e8 dc 06 fc 00 20 07 de 00
20 9b
[52205.333621] Compressed to: bc 22 fc 00 20 07 dd 00 58 dc 06 fc 00 20
07 dd
[52205.333625] Compressed to: 00 90 dc 06 fc 00 20 07 dd 00 c8 dc 06 fc
00 20
[52205.333630] Compressed to: 07 de 00 00 9c bc 1b 27 1c 00 a0 07 3f 1d
1b 38
[52205.333635] Compressed to: dc 06 fc 00 b8 06 20 02 fc 1b dd 06 70 dc
00 20
[52205.333640] Compressed to: 07 dd 00 a8 dc 05 fc 00 20 07 dd 00 e0 dc
06 fc
[52205.333645] Compressed to: 00 20 07 de 00 18 9d bc 22 fc 00 20 07 dd
00 50
[52205.333650] Compressed to: dc 06 fc 00 20 07 dd 00 88 dc 06 fc 00 20
07 dd
[52205.333655] Compressed to: 00 c0 dc 06 fc 00 20 07 dd 00 f8 dc 06 fc
00 20
[52205.333660] Compressed to: 07 de 00 30 9e bc 22 fc 00 20 07 dd 00 68
dc 06
[52205.333665] Compressed to: fc 00 20 07 dd 00 a0 dc 06 fc 00 20 07 dd
00 d8
[52205.333670] Compressed to: dc 06 fc 00 20 07 de 00 10 9f bc 1b fc 00
20 07
[52205.333675] Compressed to: dd 00 48 dc 06 fc 00 20 07 dd 00 80 dc 06
fc 00
[52205.333679] Compressed to: 20 07 dd 00 b8 dc 06 fc 00 20 07 dd 00 f0
dc 06
[52205.333684] Compressed to: 05 f0 9f 09 00 00 ea ff ff 11 00 00
[52205.333689]
[52205.333691] Restored to 0 bytes, result code -22.

Would someone be willing and able to tell me what (if anything) I'm
doing wrong, or whether there is something wrong with the algo or its
implementation?

Thanks!

Nigel


View attachment "940-lzo-test.patch" of type "text/x-diff" (21237 bytes)

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