[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: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>

1 files changed, 6 insertions, 4 deletions

diff --git a/block/elevator.c b/block/elevator.c
index 73aa46b6db49..cacfff7418e4 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -762,13 +762,15 @@ error:
 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) {