From: Darrick J. Wong <darrick.wong@oracle.com>
Date: Wed, 29 Jan 2014 00:57:39 +0000 (-0800)
Subject: bcache: fix BUG_ON due to integer overflow with GC_SECTORS_USED
X-Git-Tag: v3.14-rc3~22^2~21^2~2
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=947174476701fbc84ea8c7ec9664270f9d80b076;p=pandora-kernel.git

bcache: fix BUG_ON due to integer overflow with GC_SECTORS_USED

The BUG_ON at the end of __bch_btree_mark_key can be triggered due to
an integer overflow error:

BITMASK(GC_SECTORS_USED, struct bucket, gc_mark, 2, 13);
...
SET_GC_SECTORS_USED(g, min_t(unsigned,
	     GC_SECTORS_USED(g) + KEY_SIZE(k),
	     (1 << 14) - 1));
BUG_ON(!GC_SECTORS_USED(g));

In bcache.h, the SECTORS_USED bitfield is defined to be 13 bits wide.
While the SET_ code tries to ensure that the field doesn't overflow by
clamping it to (1<<14)-1 == 16383, this is incorrect because 16383
requires 14 bits.  Therefore, if GC_SECTORS_USED() + KEY_SIZE() =
8192, the SET_ statement tries to store 8192 into a 13-bit field.  In
a 13-bit field, 8192 becomes zero, thus triggering the BUG_ON.

Therefore, create a field width constant and a max value constant, and
use those to create the bitfield and check the inputs to
SET_GC_SECTORS_USED.  Arguably the BITMASK() template ought to have
BUG_ON checks for too-large values, but that's a separate patch.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
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