linux-fsdevel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: "J. Bruce Fields" <bfields@fieldses.org>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Jeff Layton <jlayton@kernel.org>
Subject: Re: [RFC][PATCH] nfsd: vfs_mkdir() might succeed leaving dentry negative unhashed
Date: Mon, 14 May 2018 12:27:04 -0400	[thread overview]
Message-ID: <20180514162704.GE7160@fieldses.org> (raw)
In-Reply-To: <20180514154550.GH30522@ZenIV.linux.org.uk>

On Mon, May 14, 2018 at 04:45:51PM +0100, Al Viro wrote:
> On Mon, May 14, 2018 at 11:32:16AM -0400, J. Bruce Fields wrote:
> > On Fri, May 11, 2018 at 10:13:39PM +0100, Al Viro wrote:
> > > [-stable fodder; as it is, one can e.g. add
> > > /mnt/cgroup     localhost(rw,no_root_squash,fsid=4242)
> > > to /etc/exports,
> > > mount -t cgroup none /mnt/cgroup
> > > mkdir /tmp/a
> > > mount -t nfs localhost://mnt/cgroup /tmp/a
> > > mkdir /tmp/a/foo
> > 
> > How is the cgroup filesystem exportable?  That sounds like a bug in
> > itself.  We don't want people using NFS as some kind of weird remote
> > configuration protocol.
> 
> You can't have open-by-fhandle without exportability.  And it's not
> the only fs like that.

We could separate the two--add a flag to export_operations, if
necessary.  I haven't formulated a strong argument, but exporting those
filesystems makes me *really* uncomfortable.

Poking around....  Looks like this was added by aa8188253474 "kernfs:
add exportfs operations", and they really do claim a use case for lookup
by filehandle.

> > > and have knfsd oops; the patch below deals with that.
> > > 
> > > Questions:
> > > 	1) is fh_update() use below legitimate, or should we
> > > do fh_put/fh_compose instead?
> > 
> > fh_update looks OK to me, but do we need it here?  There's already a
> > 
> > 	if (!err)
> > 		err = fh_update(reshp);
> > 
> > at the end of nfsd_create_locked.
> 
> Might be too late for that, though - the trouble hits when we hit
> nfsd_create_setattr().

Oh, got it.  Could move the bottom fh_update to just above the
nfsd_create_setattr(), though?

> > > 	2) is nfserr_serverfail valid for situation when
> > > directory created by such vfs_mkdir() manages to disappear
> > > under us immediately afterwards?  Or should we return nfserr_stale
> > > instead?
> > 
> > We just got a successful result on the create and the parent's still
> > locked, so if somebody hits this I think we want them reporting a bug,
> > not wasting time trying to find a mistake in their own logic.
> 
> No.  Suppose it's NFS-over-NFS (and yes, I agree that it's a bad idea;
> somebody *has* done that).  Lookup after successful mkdir can legitimately
> fail if it's been removed server-side.
> 
> And we *will* need to allow nfs_mkdir() to return that way in some cases
> (== success with dentry passed to it left unhashed negative), I'm afraid ;-/

Thanks, makes sense.

--b.

  reply	other threads:[~2018-05-14 16:27 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-11 21:13 [RFC][PATCH] nfsd: vfs_mkdir() might succeed leaving dentry negative unhashed Al Viro
2018-05-14 15:32 ` J. Bruce Fields
2018-05-14 15:45   ` Al Viro
2018-05-14 16:27     ` J. Bruce Fields [this message]
2018-05-14 17:00       ` Al Viro
2018-05-14 17:56         ` J. Bruce Fields
2018-05-14 16:47     ` Al Viro
2018-05-16 14:09       ` J. Bruce Fields

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20180514162704.GE7160@fieldses.org \
    --to=bfields@fieldses.org \
    --cc=jlayton@kernel.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-nfs@vger.kernel.org \
    --cc=torvalds@linux-foundation.org \
    --cc=viro@ZenIV.linux.org.uk \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).