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From: ebiederm@xmission.com (Eric W. Biederman)
To: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: Dave Hansen <haveblue@us.ibm.com>, "Serge E. Hallyn" <serue@us.ibm.com>,
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       Andrey Savochkin <saw@sw.ru>, Herbert Poetzl <herbert@13thfloor.at>,
       Sam Vilain <sam.vilain@catalyst.net.nz>
Subject: Re: [PATCH -mm 5/7] add user namespace
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Date: Fri, 14 Jul 2006 12:40:05 -0600
In-Reply-To: <1152900911.5729.30.camel@lade.trondhjem.org> (Trond Myklebust's
	message of "Fri, 14 Jul 2006 14:15:11 -0400")
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Trond Myklebust <trond.myklebust@fys.uio.no> writes:

> On Fri, 2006-07-14 at 11:36 -0600, Eric W. Biederman wrote:
>> Dave Hansen <haveblue@us.ibm.com> writes:
>> 
>> > On Fri, 2006-07-14 at 12:08 -0500, Serge E. Hallyn wrote:
>> >> yes, of course, vfsmount, which I assume is what Eric meant?
>> >> 
>> >> Which means we'd have to do this at permission() using the nameidata, or
>> >> pass nd to generic_permission. 
>> >
>> > Yeah, I think so.  But, this is well into Al territory, and there might
>> > be a better way.
>> 
>> Well until we get that sorted out I will keep picking on i_sb.
>
> Don't bother: labelling superblocks with process-specific data is always
> going to be unacceptable.

It's not process specific data.  It is a pointer to global context in
which uid's on the filesystem uniquely specify a user.  This is
something that would get set when the filesystem is mounted.

> In order to avoid aliased superblocks, you would have to be able
> guarantee to be the sole owner of the data on the device that it refers
> to. You'd have to own the device in order to do that, in which case you
> are better off just labelling the device instead.

Now I do agree if I can set the information in vfsmount and not in
the superblock it is probably better.  But even with nfs mount superblock
collapsing (which I almost understand) I don't see it as a real
problem, as long as I could prevent the superblock from collapsing.

Eric