[BLOCK] fix string handling in elv_iosched_store

elv_iosched_store doesn't terminate string passed from userspace if
it's too long.  Also, if the written length is zero (probably not
possible), it accesses elevator_name[-1].  This patch fixes both bugs.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Signed-off-by: NJens Axboe <axboe@suse.de>

block/elevator.c

@@ -762,13 +762,15 @@ static void elevator_switch(request_queue_t *q, struct elevator_type *new_e)
 ssize_t elv_iosched_store(request_queue_t *q, const char *name, size_t count)
 {
 	char elevator_name[ELV_NAME_MAX];
+	size_t len;
 	struct elevator_type *e;
 
-	memset(elevator_name, 0, sizeof(elevator_name));
-	strncpy(elevator_name, name, sizeof(elevator_name));
+	elevator_name[sizeof(elevator_name) - 1] = '\0';
+	strncpy(elevator_name, name, sizeof(elevator_name) - 1);
+	len = strlen(elevator_name);
 
-	if (elevator_name[strlen(elevator_name) - 1] == '\n')
-		elevator_name[strlen(elevator_name) - 1] = '\0';
+	if (len && elevator_name[len - 1] == '\n')
+		elevator_name[len - 1] = '\0';
 
 	e = elevator_get(elevator_name);
 	if (!e) {