Subject: Re: [PATCH] VFS: Suppress automount on [l]stat, [l]getxattr, etc.
From: Trond Myklebust <Trond.Myklebust@netapp.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ian Kent <raven@themaw.net>, Jeff Layton <jlayton@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        David Howells <dhowells@redhat.com>, viro@zeniv.linux.org.uk,
        gregkh@suse.de, linux-nfs@vger.kernel.org, leonardo.lists@gmail.com
Date: Mon, 26 Sep 2011 18:33:44 -0400
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On Mon, 2011-09-26 at 15:24 -0700, Linus Torvalds wrote: 
> On Mon, Sep 26, 2011 at 2:31 PM, Trond Myklebust
> <Trond.Myklebust@netapp.com> wrote:
> >
> > A bind mount doesn't actually want us to set up OPEN state on the NFSv4
> > server and return a fully initialised 'struct file' in the struct
> > nameidata. If it did, we would already have set the LOOKUP_OPEN flag
> > together with the accompanying open intent structure in the struct
> > nameidata.
> >
> > Ditto for the quotactl().
> 
> Umm. As far as I can see, adding LOOKUP_OPEN doesn't actually even
> *do* what you claim.
> 
> The whole "allocate struct file in struct nameidata" happens in
> path_openat(), it doesn't happen in the normal path creation
> (do_path_lookup() and friends). The only thing NFS does with
> LOOKUP_OPEN is to check whether it needs to do a revalidate
> regardless.

Please see the 'is_atomic_open()' helper and the way that
nfs_atomic_lookup() will substitute an OPEN call for the usual LOOKUP if
it sees that the caller is trying to open a file that might potentially
be a regular file.

> But even more importantly than your apparent confusion about filling
> in the 'nameidata.intent' fields, even *if* we were to do unnecessary
> filp allocations etc (which we don't do, as mentioned), from a user
> standpoint, what would be the actual downside?

Lookup permission checks would be replaced with open permission checks
on the server.

IOW: the operation could potentially fail due to a completely unrelated
issue.

Cheers
  Trond

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com