[PATCH 29/29] nfsd: only allow filesystems that set EXPORT_OP_STABLE_HANDLES

[Jeff Layton]

On Thu, 2026-01-15 at 20:23 +0100, Amir Goldstein wrote:
> On Thu, Jan 15, 2026 at 6:51 PM Jeff Layton <jlayton at kernel.org> wrote:
> > 
> > Some filesystems have grown export operations in order to provide
> > filehandles for local usage. Some of these filesystems are unsuitable
> > for use with nfsd, since their filehandles are not persistent across
> > reboots.
> > 
> > In __fh_verify, check whether EXPORT_OP_STABLE_HANDLES is set
> > and return nfserr_stale if it isn't.
> > 
> > Signed-off-by: Jeff Layton <jlayton at kernel.org>
> > ---
> >  fs/nfsd/nfsfh.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
> > index ed85dd43da18e6d4c4667ff14dc035f2eacff1d6..da9d5fb2e6613c2707195da2e8678b3fcb3d444d 100644
> > --- a/fs/nfsd/nfsfh.c
> > +++ b/fs/nfsd/nfsfh.c
> > @@ -334,6 +334,10 @@ __fh_verify(struct svc_rqst *rqstp,
> >         dentry = fhp->fh_dentry;
> >         exp = fhp->fh_export;
> > 
> > +       error = nfserr_stale;
> > +       if (!(dentry->d_sb->s_export_op->flags & EXPORT_OP_STABLE_HANDLES))
> > +               goto out;
> > +
> >         trace_nfsd_fh_verify(rqstp, fhp, type, access);
> > 
> 
> IDGI. Don't you want  to deny the export of those fs in check_export()?
> By the same logic that check_export() checks for can_decode_fh()
> not for can_encode_fh().
> 

It certainly won't hurt to add a check for this to check_export(), and
I've gone ahead and done so. To be clear, doing that won't prevent the
filesystem from being exported, but you will get a warning like this
when you try:

    exportfs: /sys/fs/cgroup does not support NFS export

That export will still show up in mountd though, so this is just a
warning. Trying to mount it though will fail.

Thanks,
-- 
Jeff Layton <jlayton at kernel.org>