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Subject: Re: [PATCH 2/3] fs: namei: Allow follow_down() to uncover auto
 mounts
From:   Jeff Layton <jlayton@kernel.org>
To:     Richard Weinberger <richard@nod.at>
Cc:     linux-nfs <linux-nfs@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        chuck lever <chuck.lever@oracle.com>, anna@kernel.org,
        trond myklebust <trond.myklebust@hammerspace.com>,
        Al Viro <viro@zeniv.linux.org.uk>, raven@themaw.net,
        chris chilvers <chris.chilvers@appsbroker.com>,
        david young <david.young@appsbroker.com>,
        luis turcitu <luis.turcitu@appsbroker.com>,
        david <david@sigma-star.at>
Date:   Thu, 17 Nov 2022 16:42:30 -0500
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On Thu, 2022-11-17 at 22:12 +0100, Richard Weinberger wrote:
> ----- Urspr=FCngliche Mail -----
> > Von: "Jeff Layton" <jlayton@kernel.org>
> > What happens when CROSSMOUNT isn't enabled and someone tries to stroll
> > into an automount point? I'm guessing the automount happens but the
> > export is denied?=20
>=20
> Exactly.
>=20
> On the other hand, why should knfsd not trigger automounts?
> Almost any userspace interaction would also do so.
>=20

I have no issue with knfsd activity triggering an automount, but I think
it'd be best if we don't do that when knfsd can't do anything with the
resulting filesystem. Automounts can be expensive.

> > It seems like LOOKUP_AUTOMOUNT ought to be conditional
> > on the parent export having CROSSMOUNT set.
> >=20
> > There's also another caller of follow_down too, the UNIX98 pty code.
> > This may be harmless for it, but it'd be best not to perturb that if we
> > can help it.
> >=20
> > Maybe follow_down can grow a lookupflags argument?
>=20
> So, in nfsd_cross_mnt() the follow_down() helper should use LOOKUP_AUTOMO=
UNT only
> if exp->ex_flags & NFSEXP_CROSSMOUNT is true?
> Sounds sane, thanks for the pointer.
>=20

Yeah, I think so. I do wonder if we ought to make any provision for
"nohide" exports, but since you have to enumerate those explicitly, it
shouldn't be a huge problem for someone to just ensure that they're
mounted beforehand.

--=20
Jeff Layton <jlayton@kernel.org>