[PATCH 31/31] swapfile: avoid NULL pointer dereference in swapon when s_bdev is NULL

[Suresh Jayaraman <sjayaraman@suse.de>]

While testing Swap over NFS patchset, I noticed an oops that was triggered
during swapon. Investigating further, the NULL pointer deference is due to the
SSD device check/optimization in the swapon code that assumes s_bdev is not
NULL.

inode->i_sb->s_bdev could be NULL in a few cases. For e.g. one such case is
loopback NFS mount, there could be others as well. Fix this by ensuring s_bdev
is not NULL before we try to deference s_bdev.

Signed-off-by: Suresh Jayaraman <sjayaraman@suse.de>
---
 mm/swapfile.c |   26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

Index: mmotm/mm/swapfile.c
===================================================================
--- mmotm.orig/mm/swapfile.c
+++ mmotm/mm/swapfile.c
@@ -160,10 +160,12 @@ static int discard_swap(struct swap_info
 				continue;
 		}
 
-		err = blkdev_issue_discard(si->bdev, start_block,
+		if (si->bdev) {
+			err = blkdev_issue_discard(si->bdev, start_block,
 						nr_blocks, GFP_KERNEL);
-		if (err)
-			break;
+			if (err)
+				break;
+		}
 
 		cond_resched();
 	}
@@ -199,9 +201,11 @@ static void discard_swap_cluster(struct
 
 			start_block <<= PAGE_SHIFT - 9;
 			nr_blocks <<= PAGE_SHIFT - 9;
-			if (blkdev_issue_discard(si->bdev, start_block,
+			if (si->bdev) {
+				if (blkdev_issue_discard(si->bdev, start_block,
 							nr_blocks, GFP_NOIO))
-				break;
+					break;
+			}
 		}
 
 		lh = se->list.next;
@@ -1991,12 +1995,14 @@ SYSCALL_DEFINE2(swapon, const char __use
 		goto bad_swap;
 	}
 
-	if (blk_queue_nonrot(bdev_get_queue(p->bdev))) {
-		p->flags |= SWP_SOLIDSTATE;
-		p->cluster_next = 1 + (random32() % p->highest_bit);
+	if (p->bdev) {
+		if (blk_queue_nonrot(bdev_get_queue(p->bdev))) {
+			p->flags |= SWP_SOLIDSTATE;
+			p->cluster_next = 1 + (random32() % p->highest_bit);
+		}
+		if (discard_swap(p) == 0)
+			p->flags |= SWP_DISCARDABLE;
 	}
-	if (discard_swap(p) == 0)
-		p->flags |= SWP_DISCARDABLE;
 
 	mutex_lock(&swapon_mutex);
 	spin_lock(&swap_lock);

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