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>
diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 98cc0a8..7d42118 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -1167,7 +1167,7 @@
/* guard against overflow */
SET_GC_SECTORS_USED(g, min_t(unsigned,
GC_SECTORS_USED(g) + KEY_SIZE(k),
- (1 << 14) - 1));
+ MAX_GC_SECTORS_USED));
BUG_ON(!GC_SECTORS_USED(g));
}