Re: [PATCH 3/5 V2] xfs: implement online get/set fs label

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, May 14, 2018 at 12:39:19PM -0500, Eric Sandeen wrote:
> 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>

Looks ok, to me....
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
> 
> V2, rework the max length handling a bit, add comments re: userspace,
> do the blkdev flush at the end, sleep for growfs lock, add locking around
> getlabel, add define for max label length ...
> 
>  fs/xfs/libxfs/xfs_format.h |  7 +++-
>  fs/xfs/libxfs/xfs_sb.c     | 30 +++++++++++++
>  fs/xfs/libxfs/xfs_sb.h     |  1 +
>  fs/xfs/xfs_ioctl.c         | 86 ++++++++++++++++++++++++++++++++++++++
>  4 files changed, 122 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
> index 42956d8d95ed..c1cb29a5f4f6 100644
> --- a/fs/xfs/libxfs/xfs_format.h
> +++ b/fs/xfs/libxfs/xfs_format.h
> @@ -98,6 +98,9 @@ struct xfs_ifork;
>  	 XFS_SB_VERSION2_PROJID32BIT	| \
>  	 XFS_SB_VERSION2_FTYPE)
>  
> +/* Maximum size of the xfs filesystem label, no terminating NULL */
> +#define XFSLABEL_MAX			12
> +
>  /*
>   * Superblock - in core version.  Must match the ondisk version below.
>   * Must be padded to 64 bit alignment.
> @@ -122,7 +125,7 @@ typedef struct xfs_sb {
>  	uint16_t	sb_sectsize;	/* volume sector size, bytes */
>  	uint16_t	sb_inodesize;	/* inode size, bytes */
>  	uint16_t	sb_inopblock;	/* inodes per block */
> -	char		sb_fname[12];	/* file system name */
> +	char		sb_fname[XFSLABEL_MAX]; /* file system name */
>  	uint8_t		sb_blocklog;	/* log2 of sb_blocksize */
>  	uint8_t		sb_sectlog;	/* log2 of sb_sectsize */
>  	uint8_t		sb_inodelog;	/* log2 of sb_inodesize */
> @@ -213,7 +216,7 @@ typedef struct xfs_dsb {
>  	__be16		sb_sectsize;	/* volume sector size, bytes */
>  	__be16		sb_inodesize;	/* inode size, bytes */
>  	__be16		sb_inopblock;	/* inodes per block */
> -	char		sb_fname[12];	/* file system name */
> +	char		sb_fname[XFSLABEL_MAX]; /* file system name */
>  	__u8		sb_blocklog;	/* log2 of sb_blocksize */
>  	__u8		sb_sectlog;	/* log2 of sb_sectsize */
>  	__u8		sb_inodelog;	/* log2 of sb_inodesize */
> diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
> index d9b94bd5f689..0bc0a40764e1 100644
> --- a/fs/xfs/libxfs/xfs_sb.c
> +++ b/fs/xfs/libxfs/xfs_sb.c
> @@ -888,6 +888,36 @@ xfs_sync_sb(
>  	return xfs_trans_commit(tp);
>  }
>  
> +/*
> + * Same behavior as xfs_sync_sb, except that it is always synchronous and it
> + * also writes the superblock buffer to disk sector 0 immediately.
> + */
> +int
> +xfs_sync_sb_buf(
> +	struct xfs_mount	*mp)
> +{
> +	struct xfs_trans	*tp;
> +	int			error;
> +
> +	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_sb, 0, 0, 0, &tp);
> +	if (error)
> +		return error;
> +
> +	xfs_log_sb(tp);
> +	xfs_trans_bhold(tp, mp->m_sb_bp);
> +	xfs_trans_set_sync(tp);
> +	error = xfs_trans_commit(tp);
> +	if (error)
> +		goto out;
> +	/*
> +	 * write out the sb buffer to get the changes to disk
> +	 */
> +	error = xfs_bwrite(mp->m_sb_bp);
> +out:
> +	xfs_buf_relse(mp->m_sb_bp);
> +	return error;
> +}
> +
>  int
>  xfs_fs_geometry(
>  	struct xfs_sb		*sbp,
> diff --git a/fs/xfs/libxfs/xfs_sb.h b/fs/xfs/libxfs/xfs_sb.h
> index 63dcd2a1a657..226827281760 100644
> --- a/fs/xfs/libxfs/xfs_sb.h
> +++ b/fs/xfs/libxfs/xfs_sb.h
> @@ -29,6 +29,7 @@ extern int	xfs_initialize_perag_data(struct xfs_mount *, xfs_agnumber_t);
>  
>  extern void	xfs_log_sb(struct xfs_trans *tp);
>  extern int	xfs_sync_sb(struct xfs_mount *mp, bool wait);
> +extern int	xfs_sync_sb_buf(struct xfs_mount *mp);
>  extern void	xfs_sb_mount_common(struct xfs_mount *mp, struct xfs_sb *sbp);
>  extern void	xfs_sb_from_disk(struct xfs_sb *to, struct xfs_dsb *from);
>  extern void	xfs_sb_to_disk(struct xfs_dsb *to, struct xfs_sb *from);
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index 89fb1eb80aae..5e7116ff3dd3 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_supers(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:
> -- 
> 2.17.0
> 
> 
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