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Subject: Re: [PATCH 01/24] Unionfs: Documentation
From: Shaya Potter <spotter@cs.columbia.edu>
To: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: Jan Kara <jack@suse.cz>, Josef Sipek <jsipek@fsl.cs.sunysb.edu>,
       Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org,
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       Erez Zadok <ezk@cs.sunysb.edu>
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Date: Tue, 09 Jan 2007 11:41:33 -0500
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On Tue, 2007-01-09 at 11:30 -0500, Trond Myklebust wrote:
> On Tue, 2007-01-09 at 13:15 +0100, Jan Kara wrote:
> > > On Mon, Jan 08, 2007 at 11:18:52AM -0800, Andrew Morton wrote:
> > > > On Sun,  7 Jan 2007 23:12:53 -0500
> > > > "Josef 'Jeff' Sipek" <jsipek@cs.sunysb.edu> wrote:
> > > > 
> >   <snip>
> > 
> > > > > Any such change can cause Unionfs to oops, or stay
> > > > > silent and even RESULT IN DATA LOSS.
> > > > 
> > > > With a rather rough user interface ;)
> > > 
> > > That statement is meant to scare people away from modifying the lower fs :)
> > > I tortured unionfs quite a bit, and it can oops but it takes some effort.
> >   But isn't it then potential DOS? If you happen to union two filesystems
> > and an untrusted user has write access to both original filesystem and
> > the union, then you say he'd be able to produce oops? That does not
> > sound very secure to me... And if any secure use of unionfs requires
> > limitting access to the original trees, then I think it's a good reason
> > to implement it in unionfs itself. Just my 2 cents.
> 
> You mean somebody like, say, a perfectly innocent process working on the
> NFS server or some other client that is oblivious to the existence of
> unionfs stacks on your particular machine?
> To me, this has always sounded like a showstopper for using unionfs with
> a remote filesystem.

Again, what about fibre channel support?  Imagine I have multiple blades
connected to a SAN.  For whatever reason I format the san w/ ext3 (I've
actually done this when we didn't need sharing, just needed a huge disk,
for instance for doing benchmarks where I needed a large data set that
was bigger than the 40GB disk that the blades came with).  I better not
touch that disk from any of the other blades.

All you are saying is unionfs should always make sure its data is sane,
never make assumptions about it being correct.

Put it this way, imagine I have an ext3 fs on a SAN, I can only use it
frm one machine.  Lets say I want to be smart and mount the FS read-only
from another machine,  should I have any expectation that it will work?
Nope.  

Now, under what conditions can one expect unionfs to work?  Basically,
where the underlying FS isn't being modified (though possible others).
Is that a reasonable condition.  I believe so.  If you disobey the
condition, the machine shouldn't oops, but it should detect it and tell
you and shut down usage of the FS.

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