linux-kernel - Re: [PATCH v3] SUNRPC: set desired file system root before connecting local transports

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Message-ID: <20121114214236.GB539@fieldses.org>
Date:	Wed, 14 Nov 2012 16:42:36 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
Cc:	Stanislav Kinsbursky <skinsbursky@...allels.com>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...nvz.org" <devel@...nvz.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH v3] SUNRPC: set desired file system root before
 connecting local transports

On Wed, Nov 14, 2012 at 09:36:33PM +0000, Myklebust, Trond wrote:
> On Wed, 2012-11-14 at 16:01 -0500, J. Bruce Fields wrote:
> > On Mon, Nov 12, 2012 at 12:37:54PM +0400, Stanislav Kinsbursky wrote:
> > > 07.11.2012 22:33, J. Bruce Fields пишет:
> > > >On Tue, Nov 06, 2012 at 08:36:05AM -0500, J. Bruce Fields wrote:
> > > >>On Tue, Nov 06, 2012 at 08:10:18AM -0500, Christoph Hellwig wrote:
> > > >>>On Tue, Nov 06, 2012 at 08:07:06AM -0500, J. Bruce Fields wrote:
> > > >>>>So you're worried that a bug in the nfs code could modify the root and
> > > >>>>then not restore it?
> > > >>>
> > > >>>At least the link you pointed to earlier never sets it back.
> > > >>
> > > >>This? http://thread.gmane.org/gmane.linux.kernel/1259986/focus=47687
> > > >>
> > > >>	+	get_fs_root(current->fs, &root);
> > > >>	+	set_fs_root(current->fs, &transport->root);
> > > >>	+
> > > >>	 	status = xs_local_finish_connecting(xprt, sock);
> > > >>	+
> > > >>	+	set_fs_root(current->fs, &root);
> > > >>	+	path_put(&root);
> > > >>
> > > >>>Instead
> > > >>>of messing with it I'd rather have the sunrpc code use vfs_path_lookup
> > > >>>and not care about current->fs->root at all.
> > > >>
> > > >>The annoyance is that the lookup happens somewhere lower down in the
> > > >>networking code (net/unix/af_unix.c:unix_find_other, I think).  So we'd
> > > >>need some new (internal) API.  We'd likely be the only user of that new
> > > >>API.
> > > >
> > > >So, if the only drawback is really just the risk of introducing a bug
> > > >that leaves the fs_root changed--the above seems simple enough for that
> > > >not to be a great risk, right?
> > > >
> > > 
> > > If we unshare rpciod fs struct (which is exported already), then we
> > 
> > I'm not sure what you mean by that.  Do workqueues actually have their
> > own dedicated set of associated tasks?  I thought all workqueues shared
> > a common pool of tasks these days.
> > 
> > > won't affect other kthreads by root swapping.
> > > But would be great to hear Trond's opinion about this approach.
> > > 
> > > Trond, could you tell us your feeling about all this?
> > 
> > I think it's often easier to get people to comment on an actual patch,
> > and this one would be quite short, so try that....
> 
> unshare() would break expectations for other users of workqueue threads
> unless you "reshare()" afterwards. Either way that's going to be
> seriously ugly.
> 
> OK, let's look at this again. Do we ever use AF_LOCAL connections for
> anything other than synchronous rpc calls to the local rpcbind daemon in
> order to register/unregister new services?

Simo's patches use them for upcalls to svcgssd.  Those will always be
done from server threads.

> If not, then let's just move
> the AF_LOCAL connection back into the process context and out of rpciod.

Remind me how this helps?

--b.

> 
> That implies that the process needs to be privileged, but it needs
> privileges in order to start RPC daemons anyway.
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