[PATCH] f2fs: data: fix possible overflow in check_swap_activate()

[Sergey Shtylyov]

In check_swap_activate(), if the *while* loop exits early (0- or 1-page
long swap file), an overflow happens while calculating the value of the
span parameter as the lowest_pblock variable ends up being greater than
the highest_pblock variable. Let's set *span to 0 in this case...

Found by Linux Verification Center (linuxtesting.org) with the SVACE static
analysis tool.

Signed-off-by: Sergey Shtylyov <s.shtylyov@xxxxxx>
---
This patch is against the 'master' branch of Jaegeuk Kim's F2FS repo...

 fs/f2fs/data.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 916e317ac925..342cb0d5056d 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -4047,7 +4047,10 @@ static int check_swap_activate(struct swap_info_struct *sis,
 		cur_lblock += nr_pblocks;
 	}
 	ret = nr_extents;
-	*span = 1 + highest_pblock - lowest_pblock;
+	if (lowest_pblock <= highest_pblock)
+		*span = 1 + highest_pblock - lowest_pblock;
+	else
+		*span = 0;
 	if (cur_lblock == 0)
 		cur_lblock = 1;	/* force Empty message */
 	sis->max = cur_lblock;
-- 
2.26.3