Date: Thu, 13 Jan 2005 10:29:09 -0800
From: "Paul E. McKenney" <paulmck@us.ibm.com>
To: Greg KH <greg@kroah.com>
Cc: Al Viro <viro@parcelfarce.linux.theplanet.co.uk>,
       Arjan van de Ven <arjan@infradead.org>, akpm@osdl.org,
       linux-kernel@vger.kernel.org, jtk@us.ibm.com, wtaber@us.ibm.com,
       pbadari@us.ibm.com, markv@us.ibm.com, tytso@us.ibm.com,
       suparna@in.ibm.com
Subject: Re: [PATCH] fs: Restore files_lock and set_fs_root exports
Message-ID: <20050113182909.GF1269@us.ibm.com>
Reply-To: paulmck@us.ibm.com
References: <1105039259.4468.9.camel@laptopd505.fenrus.org> <20050106201531.GJ1292@us.ibm.com> <20050106203258.GN26051@parcelfarce.linux.theplanet.co.uk> <20050106210408.GM1292@us.ibm.com> <20050106212417.GQ26051@parcelfarce.linux.theplanet.co.uk> <20050107010119.GS1292@us.ibm.com> <20050113025157.GA2849@us.ibm.com> <20050113170712.GA867@us.ibm.com> <20050113174428.GD1269@us.ibm.com> <20050113175520.GA24349@kroah.com>
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On Thu, Jan 13, 2005 at 09:55:20AM -0800, Greg KH wrote:
> On Thu, Jan 13, 2005 at 09:44:28AM -0800, Paul E. McKenney wrote:
> > On Thu, Jan 13, 2005 at 09:07:12AM -0800, Greg KH wrote:
> > > On Wed, Jan 12, 2005 at 06:51:57PM -0800, Paul E. McKenney wrote:
> > > > 
> > > > The current hope is that adding (a) shared and asymmetrically shared
> > > > subtrees between namespaces/locations in the same namespace, (b) stackable
> > > > LSM modules, and (c) dynamic recursive union mount would enable Linux
> > > > to provide this in a technically sound manner.  [But this is not clear
> > > > to me yet.]
> > > 
> > > I don't see how (b) has anything to do with this.  Anyone care to
> > > explain that?
> > 
> > It would allow tracking the processes that are using a given view,
> > so that state associated with that view could be cleaned up when the
> > last process exits.  One case that motivates this approach:
> > 
> > 1.	one process creates a view (e.g,. "setview" so that
> > 	"/vob/foo/bar.c" references version 1.2, just as
> > 	"/views/v1.2/vob/foo/bar.c" would),
> > 
> > 2.	this process forks off several descendants, then exits, and
> > 
> > 3.	the descendant processes eventually exit.
> > 
> > The underlying filesystem could use stackable LSM modules to track fork()s
> > and exit()s, allowing it to work out when all processes using a given
> > view had terminated.
> 
> But why the "stackable" requirement?  Why not just use the LSM interface
> and be done with it?
> 
> Or do you want to combine this LSM with SELinux or any other LSM
> (root-plug, seclvl, etc.)?  If so, that's asking for trouble (see the
> stackable LSM problems that have been posted multiple times here, and on
> the linux-security-modules mailing list.)

I believe that combining with SELinux is indeed one of the motivations.
And, yes, stackable LSM modules are a bit controversial.

But any way of tracking processes retaining a reference to a view would
meet the need.


							Thanx, Paul
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