linux-kernel - Re: [PATCH 0/5] hybrid union filesystem prototype

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Date:	Tue, 31 Aug 2010 16:19:47 -0400
From:	Trond Myklebust <Trond.Myklebust@...app.com>
To:	Valerie Aurora <vaurora@...hat.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Neil Brown <neilb@...e.de>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
	jblunck@...e.de, hch@...radead.org
Subject: Re: [PATCH 0/5] hybrid union filesystem prototype

On Tue, 2010-08-31 at 15:18 -0400, Valerie Aurora wrote:
> On Mon, Aug 30, 2010 at 02:20:47PM +0200, Miklos Szeredi wrote:
> > On Mon, 30 Aug 2010, Neil Brown wrote:
> > 
> > > Val has been following that approach and asking if it is possible to make an
> > > NFS filesystem really-truly read-only. i.e. no changes.
> > > I don't believe it is.
> > 
> > Perhaps it doesn't matter.  The nasty cases can be prevented by just
> > disallowing local modification.  For the rest NFS will return ESTALE:
> > "though luck, why didn't you follow the rules?"
> 
> I agree: Ask the server to keep it read-only, but also detect if it
> lied to prevent kernel bugs on the client.
> 
> Is detecting ESTALE and failing the mount sufficient to detect all
> cases of a cached directory being altered?

No. Files can be altered without being unlinked.

>   I keep trying to trap an
> NFS developer and beat the answer out of him but they usually get hung
> up on the impossibility of 100% enforcement of the read-only server
> option. (Agreed, impossible, just give the sysadmin a mount option so
> that it doesn't happen accidentally.)

Remind me again why mounting the filesystem '-oro' on the server (and
possibly exporting it 'ro') isn't an option?

Trond
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