Re: [PATCH v2 2/2] xfs: add FS_IOC_GETFSUUID ioctl

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Nov 22, 2022 at 08:02:23AM +1100, Dave Chinner wrote:
> On Fri, Nov 18, 2022 at 01:14:08PM -0800, Catherine Hoang wrote:
> > Add a new ioctl to retrieve the UUID of a mounted xfs filesystem. This is a
> > precursor to adding the SETFSUUID ioctl.
> > 
> > Signed-off-by: Catherine Hoang <catherine.hoang@oracle.com>
> > Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
> > ---
> >  fs/xfs/xfs_ioctl.c | 36 ++++++++++++++++++++++++++++++++++++
> >  1 file changed, 36 insertions(+)
> > 
> > diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> > index 1f783e979629..cf77715afe9e 100644
> > --- a/fs/xfs/xfs_ioctl.c
> > +++ b/fs/xfs/xfs_ioctl.c
> > @@ -1865,6 +1865,39 @@ xfs_fs_eofblocks_from_user(
> >  	return 0;
> >  }
> >  
> > +static int
> > +xfs_ioctl_getuuid(
> > +	struct xfs_mount	*mp,
> > +	struct fsuuid __user	*ufsuuid)
> > +{
> > +	struct fsuuid		fsuuid;
> > +	__u8			uuid[UUID_SIZE];
> 
> uuid_t, please, not an open coded uuid_t.
> 
> > +
> > +	if (copy_from_user(&fsuuid, ufsuuid, sizeof(fsuuid)))
> > +		return -EFAULT;
> 
> I still think this failing to copy the flex array member and then
> having to declare a local uuid buffer is an ugly wart, not just on
> the API side of things.
> 
> > +	if (fsuuid.fsu_len == 0) {
> > +		fsuuid.fsu_len = UUID_SIZE;
> 
> XFS uses sizeof(uuid_t) for the size of it's uuids, not UUID_SIZE.
> 
> > +		if (copy_to_user(&ufsuuid->fsu_len, &fsuuid.fsu_len,
> > +					sizeof(fsuuid.fsu_len)))
> > +			return -EFAULT;
> > +		return 0;
> > +	}
> > +
> > +	if (fsuuid.fsu_len < UUID_SIZE || fsuuid.fsu_flags != 0)
> > +		return -EINVAL;
> > +
> > +	spin_lock(&mp->m_sb_lock);
> > +	memcpy(uuid, &mp->m_sb.sb_uuid, UUID_SIZE);
> > +	spin_unlock(&mp->m_sb_lock);
> 
> Hmmmm. Shouldn't we be promoting xfs_fs_get_uuid() to xfs_super.c
> (without the pNFS warning!) and calling that here, rather than open
> coding another "get the XFS superblock UUID" function here?

I disagree that it's worth the effort to create a helper function to
wrap four lines of code, particularly since the pnfs code has this extra
weird wart of returning the byte offset(?) of the uuid location.

> i.e.
> 	if (fsuuid.fsu_flags != 0)
> 		return -EINVAL;
> 
> 	error = xfs_fs_get_uuid(&mp->m_sb, uuid, &fsuuid.fsu_len, NULL);
> 	if (error)
> 		return -EINVAL;
> 
> Also, uuid_copy()?

Why does xfs_fs_get_uuid use memcpy then?  Did the compiler reject the
u8* -> uuid_t * type conversion?

Alternately there's export_uuid().

--D

> Cheers,
> 
> Dave.
> 
> -- 
> Dave Chinner
> david@fromorbit.com