UBI: add more checks to chdev open

When opening UBI volumes by their character device names, make
sure we are opening character devices, not block devices or any
other inode type.

Signed-off-by: Artem Bityutskiy <[e-mail]>

drivers/mtd/ubi/kapi.c

···
  */
 struct ubi_volume_desc *ubi_open_volume_path(const char *pathname, int mode)
 {
-	int error, ubi_num, vol_id;
-	struct ubi_volume_desc *ret;
+	int error, ubi_num, vol_id, mod;
 	struct inode *inode;
 	struct path path;
 
···
 		return ERR_PTR(error);
 
 	inode = path.dentry->d_inode;
+	mod = inode->i_mode;
 	ubi_num = ubi_major2num(imajor(inode));
 	vol_id = iminor(inode) - 1;
-
-	if (vol_id >= 0 && ubi_num >= 0)
-		ret = ubi_open_volume(ubi_num, vol_id, mode);
-	else
-		ret = ERR_PTR(-ENODEV);
-
 	path_put(&path);
-	return ret;
+
+	if (!S_ISCHR(mod))
+		return ERR_PTR(-EINVAL);
+	if (vol_id >= 0 && ubi_num >= 0)
+		return ubi_open_volume(ubi_num, vol_id, mode);
+	return ERR_PTR(-ENODEV);
 }
 EXPORT_SYMBOL_GPL(ubi_open_volume_path);