Re: How to set the NFSv4 "HIDDEN" attribute on Linux?

[Chuck Lever <chuck.lever@oracle.com>]

On Wed, Nov 22, 2023 at 11:42:43PM +0100, Cedric Blancher wrote:
> On Mon, 20 Nov 2023 at 12:46, Jeff Layton <jlayton@kernel.org> wrote:
> >
> > On Sun, 2023-11-19 at 17:51 +0100, Cedric Blancher wrote:
> > > On Sat, 18 Nov 2023 at 12:56, Jeff Layton <jlayton@kernel.org> wrote:
> > > >
> > > > On Sat, 2023-11-18 at 07:24 +0100, Cedric Blancher wrote:
> > > > > Good morning!
> > > > >
> > > > > NFSv4 has a "hidden" filesystem object attribute. How can I set that
> > > > > on a Linux NFSv4 server, or in a filesystem exported on Linux via
> > > > > NFSv4, so that the NFSv4 client gets this attribute for a file?
> > > > >
> > > >
> > > > You can't. RFC 8881 defines that as "TRUE, if the file is considered
> > > > hidden with respect to the Windows API." There is no analogous Linux
> > > > inode attribute.
> > >
> > > Can we use setfattr and getfattr to set/get the NFSv4.1 HIDDEN and
> > > ARCHIVE? We have Windows NFSv4 clients (and kofemann/Roland's codebase
> > > supports this), and that means we need to be able to set/get and
> > > backup/restore these flags on the NFSv4 server side.
> > >
> >
> > No. They would need to be stored in the inode on the server somehow and
> > there is no place to store them. These attributes are simply not
> > supported by the Linux NFS server.
> 
> Linux has xattrs, which are per inode, and can be backuped and
> restored via tar --xattrs. That would be good enough

This being upstream, we are more concerned with a properly
architected long-term solution rather than a proof of concept.

If Linux were to support them, I would rather see these treated as
first-class file attributes -- such attributes might be used by
Samba, the SMB client, and local accessors as well as NFSD, so they
would need a common and consistent API, through the VFS.

Since they are not POSIX attributes, I think they would have to be
plumbed through statx(), and each file system would need to
determine their own mechanism for storing them. The VFS API would
also need to indicate whether the file system supports them at all.

Again, this is not as easy as just stuffing these things into an
xattr and all of the above is outside the purview of NFSD. As we
don't have any Linux-native application use cases, I can't see
implementing support for them as a priority. But as always, patches
are welcome.


-- 
Chuck Lever