[PATCH] block: eliminate potential for infinite loop in blkdev_issue_discard

[Mike Snitzer <snitzer@redhat.com>]

Due to the recently identified overflow in read_capacity_16() it was
possible for max_discard_sectors to be zero but still have discards
enabled on the associated device's queue.

Eliminate the possibility for blkdev_issue_discard to infinitely loop.

Interestingly this issue wasn't identified until a device, whose
discard_granularity was 0 due to read_capacity_16 overflow, was consumed
by blk_stack_limits() to construct limits for a higher-level DM
multipath device.  The multipath device's resulting limits never had the
discard limits stacked because blk_stack_limits() will only do so if
the bottom device's discard_granularity != 0.  This resulted in the
multipath device's limits.max_discard_sectors being 0.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 block/blk-lib.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/block/blk-lib.c b/block/blk-lib.c
index c392029..186f249 100644
--- a/block/blk-lib.c
+++ b/block/blk-lib.c
@@ -56,7 +56,10 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
 	 * granularity
 	 */
 	max_discard_sectors = min(q->limits.max_discard_sectors, UINT_MAX >> 9);
-	if (q->limits.discard_granularity) {
+	if (unlikely(!max_discard_sectors)) {
+		/* Avoid infinite loop (below) */
+		return -EOPNOTSUPP;
+	} else if (q->limits.discard_granularity) {
 		unsigned int disc_sects = q->limits.discard_granularity >> 9;
 
 		max_discard_sectors &= ~(disc_sects - 1);