Btrfs: Add a new ioctl to get the label of a mounted file system

Add a new ioctl(2) BTRFS_IOC_GET_FSLABLE, so that we can get the label upon a mounted filesystem.
Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Cc: Miao Xie <miaox@cn.fujitsu.com>
Cc: Goffredo Baroncelli <kreijack@inwind.it>
Cc: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/ioctl.c

@@ -3917,6 +3917,25 @@ static long btrfs_ioctl_set_received_subvol(struct file *file,
 	return ret;
 }
 
+static int btrfs_ioctl_get_fslabel(struct file *file, void __user *arg)
+{
+	struct btrfs_root *root = BTRFS_I(fdentry(file)->d_inode)->root;
+	const char *label = root->fs_info->super_copy->label;
+	size_t len = strnlen(label, BTRFS_LABEL_SIZE);
+	int ret;
+
+	if (len == BTRFS_LABEL_SIZE) {
+		pr_warn("btrfs: label is too long, return the first %zu bytes\n",
+			--len);
+	}
+
+	mutex_lock(&root->fs_info->volume_mutex);
+	ret = copy_to_user(arg, label, len);
+	mutex_unlock(&root->fs_info->volume_mutex);
+
+	return ret ? -EFAULT : 0;
+}
+
 long btrfs_ioctl(struct file *file, unsigned int
 		cmd, unsigned long arg)
 {
@@ -4017,6 +4036,8 @@ long btrfs_ioctl(struct file *file, unsigned int
 		return btrfs_ioctl_qgroup_limit(file, argp);
 	case BTRFS_IOC_DEV_REPLACE:
 		return btrfs_ioctl_dev_replace(root, argp);
+	case BTRFS_IOC_GET_FSLABEL:
+		return btrfs_ioctl_get_fslabel(file, argp);
 	}
 
 	return -ENOTTY;

include/uapi/linux/btrfs.h

@@ -502,6 +502,8 @@ struct btrfs_ioctl_send_args {
 			       struct btrfs_ioctl_qgroup_create_args)
 #define BTRFS_IOC_QGROUP_LIMIT _IOR(BTRFS_IOCTL_MAGIC, 43, \
 			       struct btrfs_ioctl_qgroup_limit_args)
+#define BTRFS_IOC_GET_FSLABEL _IOR(BTRFS_IOCTL_MAGIC, 49, \
+				   char[BTRFS_LABEL_SIZE])
 #define BTRFS_IOC_GET_DEV_STATS _IOWR(BTRFS_IOCTL_MAGIC, 52, \
 				      struct btrfs_ioctl_get_dev_stats)
 #define BTRFS_IOC_DEV_REPLACE _IOWR(BTRFS_IOCTL_MAGIC, 53, \