Message-ID: <45A576E7.1070808@cs.columbia.edu>
Date: Wed, 10 Jan 2007 18:29:43 -0500
From: Shaya Potter <spotter@cs.columbia.edu>
User-Agent: Thunderbird 1.5.0.9 (X11/20070102)
MIME-Version: 1.0
To: Josef Sipek <jsipek@fsl.cs.sunysb.edu>
CC: Jan Kara <jack@suse.cz>, Erez Zadok <ezk@cs.sunysb.edu>,
       Andrew Morton <akpm@osdl.org>,
       "Josef 'Jeff' Sipek" <jsipek@cs.sunysb.edu>,
       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@cs.sunysb.edu>
Subject: Re: [PATCH 01/24] Unionfs: Documentation
References: <20070109122644.GB1260@atrey.karlin.mff.cuni.cz> <200701091734.l09HYRHB009290@agora.fsl.cs.sunysb.edu> <20070110161215.GB12654@atrey.karlin.mff.cuni.cz> <20070110232054.GB5088@filer.fsl.cs.sunysb.edu>
In-Reply-To: <20070110232054.GB5088@filer.fsl.cs.sunysb.edu>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1767
Lines: 35


Josef Sipek wrote:
> On Wed, Jan 10, 2007 at 05:12:15PM +0100, Jan Kara wrote:
>>   I see :). To me it just sounds as if you want to do remount-read-only
>> for source filesystems, which is operation we support perfectly fine,
>> and after that create union mount. But I agree you cannot do quite that
>> since you need to have write access later from your union mount. So
>> maybe it's not so easy as I thought.
>>   On the other hand, there was some effort to support read-only bind-mounts of
>> read-write filesystems (there were even some patches floating around but
>> I don't think they got merged) and that should be even closer to what
>> you'd need...
> 
> Since the RO flag is per-mount point, how do you guarantee that no one is
> messing with the fs? (I haven't looked at the patches that do per mount
> ro flag, but this would require some over-arching ro flag - in the
> superblock most likely.)

I thought about it, wrote an email, then cancelled it as it won't work.

what I thought was that you could a limited unionfs case would be with X 
layers read-only and the top layer read-write, and what you would do is 
dynamically make read only bind mounts for the the X layers and since 
you control the top layer hide it from the system.

However, read only bind mounts are great if you want a limit a process 
to accessing the files read-only, as they won't have access to the other 
vfs_mounts, but it does nothing for the other vfs_mounts that are using 
that same file system.  hence, does us no good.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/