[PATCH] ext4: Avoid underflow of in ext4_trim_fs()

[Lukas Czerner <lczerner@redhat.com>]

Currently if len argument in ext4_trim_fs() is smaller than one block,
the 'end' variable underflow. Avoid that by exiting right away if len
is smaller than one file system block.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
---
 fs/ext4/mballoc.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index f8b27bf..06c8526 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4989,13 +4989,18 @@ int ext4_trim_fs(struct super_block *sb, struct fstrim_range *range)
 	int ret = 0;
 
 	start = range->start >> sb->s_blocksize_bits;
-	end = start + (range->len >> sb->s_blocksize_bits) - 1;
 	minlen = EXT4_NUM_B2C(EXT4_SB(sb),
 			      range->minlen >> sb->s_blocksize_bits);
 
 	if (unlikely(minlen > EXT4_CLUSTERS_PER_GROUP(sb)) ||
 	    unlikely(start >= max_blks))
 		return -EINVAL;
+
+	end = range->len >> sb->s_blocksize_bits;
+	if (0 == end)
+		goto out;
+	end += start - 1;
+
 	if (end >= max_blks)
 		end = max_blks - 1;
 	if (end <= first_data_blk)
-- 
1.7.7.6