xfs: implement online get/set fs label

The GET ioctl is trivial, just return the current label.

The SET ioctl is more involved:
It transactionally modifies the superblock to write a new filesystem
label to the primary super.

A new variant of xfs_sync_sb then writes the superblock buffer
immediately to disk so that the change is visible from userspace.

It then invalidates any page cache that userspace might have previously
read on the block device so that i.e. blkid can see the change
immediately, and updates all secondary superblocks as userspace relable
does.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
[darrick: use dchinner's new xfs_update_secondary_sbs function]
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

1 files changed, 86 insertions, 0 deletions

diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index 89fb1eb80aae..f9c97ea52961 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -1811,6 +1811,88 @@ xfs_ioc_swapext(
 	return error;
 }
 
+static int
+xfs_ioc_getlabel(
+	struct xfs_mount	*mp,
+	char			__user *user_label)
+{
+	struct xfs_sb		*sbp = &mp->m_sb;
+	char			label[XFSLABEL_MAX + 1];
+
+	/* Paranoia */
+	BUILD_BUG_ON(sizeof(sbp->sb_fname) > FSLABEL_MAX);
+
+	spin_lock(&mp->m_sb_lock);
+	strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));
+	spin_unlock(&mp->m_sb_lock);
+
+	/* xfs on-disk label is 12 chars, be sure we send a null to user */
+	label[XFSLABEL_MAX] = '\0';
+	if (copy_to_user(user_label, label, sizeof(sbp->sb_fname)))
+		return -EFAULT;
+	return 0;
+}
+
+static int
+xfs_ioc_setlabel(
+	struct file		*filp,
+	struct xfs_mount	*mp,
+	char			__user *newlabel)
+{
+	struct xfs_sb		*sbp = &mp->m_sb;
+	char			label[XFSLABEL_MAX + 1];
+	size_t			len;
+	int			error;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+	/*
+	 * The generic ioctl allows up to FSLABEL_MAX chars, but XFS is much
+	 * smaller, at 12 bytes.  We copy one more to be sure we find the
+	 * (required) NULL character to test the incoming label length.
+	 * NB: The on disk label doesn't need to be null terminated.
+	 */
+	if (copy_from_user(label, newlabel, XFSLABEL_MAX + 1))
+		return -EFAULT;
+	len = strnlen(label, XFSLABEL_MAX + 1);
+	if (len > sizeof(sbp->sb_fname))
+		return -EINVAL;
+
+	error = mnt_want_write_file(filp);
+	if (error)
+		return error;
+
+	spin_lock(&mp->m_sb_lock);
+	memset(sbp->sb_fname, 0, sizeof(sbp->sb_fname));
+	strncpy(sbp->sb_fname, label, sizeof(sbp->sb_fname));
+	spin_unlock(&mp->m_sb_lock);
+
+	/*
+	 * Now we do several things to satisfy userspace.
+	 * In addition to normal logging of the primary superblock, we also
+	 * immediately write these changes to sector zero for the primary, then
+	 * update all backup supers (as xfs_db does for a label change), then
+	 * invalidate the block device page cache.  This is so that any prior
+	 * buffered reads from userspace (i.e. from blkid) are invalidated,
+	 * and userspace will see the newly-written label.
+	 */
+	error = xfs_sync_sb_buf(mp);
+	if (error)
+		goto out;
+	/*
+	 * growfs also updates backup supers so lock against that.
+	 */
+	mutex_lock(&mp->m_growlock);
+	error = xfs_update_secondary_sbs(mp);
+	mutex_unlock(&mp->m_growlock);
+
+	invalidate_bdev(mp->m_ddev_targp->bt_bdev);
+
+out:
+	mnt_drop_write_file(filp);
+	return error;
+}
+
 /*
  * Note: some of the ioctl's return positive numbers as a
  * byte count indicating success, such as readlink_by_handle.
@@ -1834,6 +1916,10 @@ xfs_file_ioctl(
 	switch (cmd) {
 	case FITRIM:
 		return xfs_ioc_trim(mp, arg);
+	case FS_IOC_GETFSLABEL:
+		return xfs_ioc_getlabel(mp, arg);
+	case FS_IOC_SETFSLABEL:
+		return xfs_ioc_setlabel(filp, mp, arg);
 	case XFS_IOC_ALLOCSP:
 	case XFS_IOC_FREESP:
 	case XFS_IOC_RESVSP: