Btrfs: get write access when setting the default subvolume

When wen want to set the default subvolume, we must get write access, or
we will change the R/O file system.
Signed-off-by: NMiao Xie <miaox@cn.fujitsu.com>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>

fs/btrfs/ioctl.c

@@ -2843,12 +2843,19 @@ static long btrfs_ioctl_default_subvol(struct file *file, void __user *argp)
 	struct btrfs_disk_key disk_key;
 	u64 objectid = 0;
 	u64 dir_id;
+	int ret;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
-	if (copy_from_user(&objectid, argp, sizeof(objectid)))
-		return -EFAULT;
+	ret = mnt_want_write_file(file);
+	if (ret)
+		return ret;
+
+	if (copy_from_user(&objectid, argp, sizeof(objectid))) {
+		ret = -EFAULT;
+		goto out;
+	}
 
 	if (!objectid)
 		objectid = root->root_key.objectid;
@@ -2858,21 +2865,28 @@ static long btrfs_ioctl_default_subvol(struct file *file, void __user *argp)
 	location.offset = (u64)-1;
 
 	new_root = btrfs_read_fs_root_no_name(root->fs_info, &location);
-	if (IS_ERR(new_root))
-		return PTR_ERR(new_root);
+	if (IS_ERR(new_root)) {
+		ret = PTR_ERR(new_root);
+		goto out;
+	}
 
-	if (btrfs_root_refs(&new_root->root_item) == 0)
-		return -ENOENT;
+	if (btrfs_root_refs(&new_root->root_item) == 0) {
+		ret = -ENOENT;
+		goto out;
+	}
 
 	path = btrfs_alloc_path();
-	if (!path)
-		return -ENOMEM;
+	if (!path) {
+		ret = -ENOMEM;
+		goto out;
+	}
 	path->leave_spinning = 1;
 
 	trans = btrfs_start_transaction(root, 1);
 	if (IS_ERR(trans)) {
 		btrfs_free_path(path);
-		return PTR_ERR(trans);
+		ret = PTR_ERR(trans);
+		goto out;
 	}
 
 	dir_id = btrfs_super_root_dir(root->fs_info->super_copy);
@@ -2883,7 +2897,8 @@ static long btrfs_ioctl_default_subvol(struct file *file, void __user *argp)
 		btrfs_end_transaction(trans, root);
 		printk(KERN_ERR "Umm, you don't have the default dir item, "
 		       "this isn't going to work\n");
-		return -ENOENT;
+		ret = -ENOENT;
+		goto out;
 	}
 
 	btrfs_cpu_key_to_disk(&disk_key, &new_root->root_key);
@@ -2893,8 +2908,9 @@ static long btrfs_ioctl_default_subvol(struct file *file, void __user *argp)
 
 	btrfs_set_fs_incompat(root->fs_info, DEFAULT_SUBVOL);
 	btrfs_end_transaction(trans, root);
-
-	return 0;
+out:
+	mnt_drop_write_file(file);
+	return ret;
 }
 
 void btrfs_get_block_group_info(struct list_head *groups_list,