From 59771079c18c44e39106f0f30054025acafadb41 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Wed, 19 Dec 2012 07:18:35 -0800 Subject: [PATCH] blk: avoid divide-by-zero with zero discard granularity Commit 8dd2cb7e880d ("block: discard granularity might not be power of 2") changed a couple of 'binary and' operations into modulus operations. Which turned the harmless case of a zero discard_granularity into a possible divide-by-zero. The code also had a much more subtle bug: it was doing the modulus of a value in bytes using 'sector_t'. That was always conceptually wrong, but didn't actually matter back when the code assumed a power-of-two granularity: we only looked at the low bits anyway. But with potentially arbitrary sector numbers, using a 'sector_t' to express bytes is very very wrong: depending on configuration it limits the starting offset of the device to just 32 bits, and any overflow would result in a wrong value if the modulus wasn't a power-of-two. So re-write the code to not only protect against the divide-by-zero, but to do the starting sector arithmetic in sectors, and using the proper types. [ For any mathematicians out there: it also looks monumentally stupid to do the 'modulo granularity' operation *twice*, never mind having a "+ granularity" in the second modulus op. But that's the easiest way to avoid negative values or overflow, and it is how the original code was done. ] Reported-by: Ingo Molnar Reported-by: Doug Anderson Cc: Neil Brown Cc: Shaohua Li Acked-by: Jens Axboe Signed-off-by: Linus Torvalds --- include/linux/blkdev.h | 19 +++++++++++++++---- 1 file changed, 15 insertions(+), 4 deletions(-) diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h index acb4f7bbbd32..f94bc83011ed 100644 --- a/include/linux/blkdev.h +++ b/include/linux/blkdev.h @@ -1188,14 +1188,25 @@ static inline int queue_discard_alignment(struct request_queue *q) static inline int queue_limit_discard_alignment(struct queue_limits *lim, sector_t sector) { - sector_t alignment = sector << 9; - alignment = sector_div(alignment, lim->discard_granularity); + unsigned int alignment, granularity, offset; if (!lim->max_discard_sectors) return 0; - alignment = lim->discard_granularity + lim->discard_alignment - alignment; - return sector_div(alignment, lim->discard_granularity); + /* Why are these in bytes, not sectors? */ + alignment = lim->discard_alignment >> 9; + granularity = lim->discard_granularity >> 9; + if (!granularity) + return 0; + + /* Offset of the partition start in 'granularity' sectors */ + offset = sector_div(sector, granularity); + + /* And why do we do this modulus *again* in blkdev_issue_discard()? */ + offset = (granularity + alignment - offset) % granularity; + + /* Turn it back into bytes, gaah */ + return offset << 9; } static inline int bdev_discard_alignment(struct block_device *bdev)