[PATCH 00/29] fs: require filesystems to explicitly opt-in to nfsd export support

[Darrick J. Wong]

On Thu, Jan 22, 2026 at 07:12:36AM -0500, Jeff Layton wrote:
> On Wed, 2026-01-21 at 22:37 -0800, Christoph Hellwig wrote:
> > On Wed, Jan 21, 2026 at 10:18:00AM -0500, Jeff Layton wrote:
> > > > fat seems to be an exception as far as the 'real' file systems go.
> > > > And it did sound to me like some of the synthetic ones had similar
> > > > issues.
> > > > 
> > > 
> > > Not sure what we can do about FAT without changing the filehandle
> > > format in some fashion. The export ops just use
> > > generic_encode_ino32_fh, and FAT doesn't have stable inode numbers.
> > > The "nostale" ops seem sane enough but it looks like they only work
> > > with the fs in r/o mode.
> > 
> > Yeah.  I guess we need to ignore this because of <history>
> > 
> 
> Yep. This is a case where the handles are not PERSISTENT but I don't
> think we can get away with making FAT unexportable. We're probably
> stuck with it.
> 
> > > > I think Amirs patch would take care of that.  Although userland nfs
> > > > servers or other storage applications using the handle syscalls would
> > > > still see them.  Then again fixing the problem that some handles
> > > > did not fulfill the long standing (but not documented well enough)
> > > > semantics probably is a good fix on it's own.
> > > 
> > > Agreed. We should try to ensure uniqueness and persistence in all
> > > filehandles both for nfsd and userland applications.
> > 
> > Sounds good to me.
> 
> 
> Unfortunately, there are already exceptions. Apparently pidfs and
> cgroupfs handles (at least) can't be extended because of userspace
> expectations:
> 
> https://lore.kernel.org/linux-nfs/20260120-irrelevant-zeilen-b3c40a8e6c30@brauner/

systemd cracking file handles??  Yeesh, I thought userspace was supposed
to treat a file handle as an opaque N-byte blob and nothing more, and
only certain "special" tools (e.g. xfsprogs on XFS) could do more than
that.

--D

> My personal take is that we should try to make handle uniqueness a goal
> for most existing filesystems, but we're going to have some that can't
> achieve that. For them we probably want to be able to flag them so they
> can be id'ed by userland.
> 
> So, we will need an export_operations flag of some sort
> (EXPORT_OP_UNIQUE_HANDLES?). At that point, we'll have to decide
> whether to deny nfsd export based on that flag:
> 
> We could deny export of any fs that doesn't set the flag, but NFSv4
> actually allows the server to advertise that it can't guarantee handle
> uniqueness. There isn't much guidance for the client on how to handle
> that though and the attribute seems to have the scope of the entire NFS
> server.
> 
> -- 
> Jeff Layton <jlayton at kernel.org>
>