Re: [RFC PATCH V6 2/2] Btrfs: Add a new ioctl to change the label of a mounted file system

On Tue, Dec 18, 2012 at 11:06:07AM +0800, Jeff Liu wrote:

+static int btrfs_ioctl_set_fslabel(struct file *file, void __user *arg)
+{
+	struct btrfs_root *root = BTRFS_I(fdentry(file)->d_inode)->root;
+	struct btrfs_super_block *super_block = root->fs_info->super_copy;
+	struct btrfs_trans_handle *trans;
+	char label[BTRFS_LABEL_SIZE];
+	int ret;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	if (copy_from_user(label, arg, sizeof(label)))
+		return -EFAULT;
+
+	if (strnlen(label, BTRFS_LABEL_SIZE) == BTRFS_LABEL_SIZE)
+		return -EINVAL;
+
+	ret = mnt_want_write_file(file);
+	if (ret)
+		return ret;
+
+	mutex_lock(&root->fs_info->volume_mutex);
+	trans = btrfs_start_transaction(root, 1);
+	if (IS_ERR(trans)) {
+		ret = PTR_ERR(trans);
+		goto out_unlock;
+	}
+
+	strcpy(super_block->label, label);
+	btrfs_end_transaction(trans, root);

If this fails, eg. with EIO, it will not be reported back to the user

        ret = btrfs_end_transaction(trans, root);

should fix it.

+
+out_unlock:
+	mutex_unlock(&root->fs_info->volume_mutex);
+	mnt_drop_write_file(file);
+	return ret;
+}