linux-kernel - Re: [PATCH 01/24] Unionfs: Documentation

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Date:	Wed, 10 Jan 2007 18:29:43 -0500
From:	Shaya Potter <spotter@...columbia.edu>
To:	Josef Sipek <jsipek@....cs.sunysb.edu>
CC:	Jan Kara <jack@...e.cz>, Erez Zadok <ezk@...sunysb.edu>,
	Andrew Morton <akpm@...l.org>,
	"Josef 'Jeff' Sipek" <jsipek@...sunysb.edu>,
	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@...sunysb.edu>
Subject: Re: [PATCH 01/24] Unionfs: Documentation

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.
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