Message-ID: <45A79862.5060303@cs.columbia.edu>
Date: Fri, 12 Jan 2007 09:17:06 -0500
From: Shaya Potter <spotter@cs.columbia.edu>
User-Agent: Thunderbird 1.5.0.9 (X11/20070102)
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To: Pavel Machek <pavel@suse.cz>
CC: Trond Myklebust <trond.myklebust@fys.uio.no>, Jan Kara <jack@suse.cz>,
       Josef Sipek <jsipek@fsl.cs.sunysb.edu>, Andrew Morton <akpm@osdl.org>,
       linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
       hch@infradead.org, viro@ftp.linux.org.uk, torvalds@osdl.org,
       mhalcrow@us.ibm.com, David Quigley <dquigley@fsl.cs.sunysb.edu>,
       Erez Zadok <ezk@cs.sunysb.edu>
Subject: Re: unionfs unusable on multiuser systems (was Re: [PATCH 01/24]
 Unionfs: Documentation)
References: <1168229596580-git-send-email-jsipek@cs.sunysb.edu> <1168229596875-git-send-email-jsipek@cs.sunysb.edu> <20070108111852.ee156a90.akpm@osdl.org> <20070108231524.GA1269@filer.fsl.cs.sunysb.edu> <20070109121552.GA1260@atrey.karlin.mff.cuni.cz> <1168360219.6054.14.camel@lade.trondhjem.org> <20070111142956.GA6843@ucw.cz>
In-Reply-To: <20070111142956.GA6843@ucw.cz>
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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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