linux-kernel - Re: unionfs unusable on multiuser systems (was Re: [PATCH 01/24] Unionfs: Documentation)

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Date:	Fri, 12 Jan 2007 09:17:06 -0500
From:	Shaya Potter <spotter@...columbia.edu>
To:	Pavel Machek <pavel@...e.cz>
CC:	Trond Myklebust <trond.myklebust@....uio.no>,
	Jan Kara <jack@...e.cz>,
	Josef Sipek <jsipek@....cs.sunysb.edu>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, hch@...radead.org,
	viro@....linux.org.uk, torvalds@...l.org, mhalcrow@...ibm.com,
	David Quigley <dquigley@....cs.sunysb.edu>,
	Erez Zadok <ezk@...sunysb.edu>
Subject: Re: unionfs unusable on multiuser systems (was Re: [PATCH 01/24]
 Unionfs: Documentation)



Pavel Machek wrote:
> Hi!
> 
>>>> 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.
> 
> Actually, it is worse than that. find / (and updatedb) *will* write to
> all the filesystems (atime).
> 
> Expecting sysadmins to know/prevent this seems like expecting quite a
> lot from them. Sounds like a show stopper to me :-(....

a modified atime will not affect unionfs at all (at least from my 
experience)
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