block/rnbd-srv: Check for unlikely string overflow

Since "dev_search_path" can technically be as large as PATH_MAX,
there was a risk of truncation when copying it and a second string
into "full_path" since it was also PATH_MAX sized. The W=1 builds were
reporting this warning:

drivers/block/rnbd/rnbd-srv.c: In function 'process_msg_open.isra':
drivers/block/rnbd/rnbd-srv.c:616:51: warning: '%s' directive output may be truncated writing up to 254 bytes into a region of size between 0 and 4095 [-Wformat-truncation=]
  616 |                 snprintf(full_path, PATH_MAX, "%s/%s",
      |                                                   ^~
In function 'rnbd_srv_get_full_path',
    inlined from 'process_msg_open.isra' at drivers/block/rnbd/rnbd-srv.c:721:14: drivers/block/rnbd/rnbd-srv.c:616:17: note: 'snprintf' output between 2 and 4351 bytes into a destination of size 4096
  616 |                 snprintf(full_path, PATH_MAX, "%s/%s",
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  617 |                          dev_search_path, dev_name);
      |                          ~~~~~~~~~~~~~~~~~~~~~~~~~~

To fix this, unconditionally check for truncation (as was already done
for the case where "%SESSNAME%" was present).
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312100355.lHoJPgKy-lkp@intel.com/
Cc: Md. Haris Iqbal <haris.iqbal@ionos.com>
Cc: Jack Wang <jinpu.wang@ionos.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc:  <linux-block@vger.kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Acked-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20231212214738.work.169-kees@kernel.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/rnbd/rnbd-srv.c

@@ -586,6 +586,7 @@ static char *rnbd_srv_get_full_path(struct rnbd_srv_session *srv_sess,
 {
 	char *full_path;
 	char *a, *b;
+	int len;
 
 	full_path = kmalloc(PATH_MAX, GFP_KERNEL);
 	if (!full_path)
@@ -597,19 +598,19 @@ static char *rnbd_srv_get_full_path(struct rnbd_srv_session *srv_sess,
 	 */
 	a = strnstr(dev_search_path, "%SESSNAME%", sizeof(dev_search_path));
 	if (a) {
-		int len = a - dev_search_path;
+		len = a - dev_search_path;
 
 		len = snprintf(full_path, PATH_MAX, "%.*s/%s/%s", len,
 			       dev_search_path, srv_sess->sessname, dev_name);
-		if (len >= PATH_MAX) {
-			pr_err("Too long path: %s, %s, %s\n",
-			       dev_search_path, srv_sess->sessname, dev_name);
-			kfree(full_path);
-			return ERR_PTR(-EINVAL);
-		}
 	} else {
-		snprintf(full_path, PATH_MAX, "%s/%s",
-			 dev_search_path, dev_name);
+		len = snprintf(full_path, PATH_MAX, "%s/%s",
+			       dev_search_path, dev_name);
+	}
+	if (len >= PATH_MAX) {
+		pr_err("Too long path: %s, %s, %s\n",
+		       dev_search_path, srv_sess->sessname, dev_name);
+		kfree(full_path);
+		return ERR_PTR(-EINVAL);
 	}
 
 	/* eliminitate duplicated slashes */