linux-kernel - Re: fanotify - overall design before I start sending patches

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Date:	Thu, 06 Aug 2009 14:18:10 -0400
From:	Eric Paris <eparis@...hat.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Douglas Leeder <douglas.leeder@...hos.com>,
	Pavel Machek <pavel@....cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"malware-list@...sg.printk.net" <malware-list@...sg.printk.net>,
	"Valdis.Kletnieks@...edu" <Valdis.Kletnieks@...edu>,
	"greg@...ah.com" <greg@...ah.com>,
	"jcm@...hat.com" <jcm@...hat.com>, "tytso@....edu" <tytso@....edu>,
	"arjan@...radead.org" <arjan@...radead.org>,
	"david@...g.hm" <david@...g.hm>,
	"jengelh@...ozas.de" <jengelh@...ozas.de>,
	"aviro@...hat.com" <aviro@...hat.com>,
	"mrkafk@...il.com" <mrkafk@...il.com>,
	"alexl@...hat.com" <alexl@...hat.com>,
	"hch@...radead.org" <hch@...radead.org>,
	"mmorley@....in" <mmorley@....in>
Subject: Re: fanotify - overall design before I start sending patches

On Thu, 2009-08-06 at 13:58 +0100, Alan Cox wrote:
> > > We are taking about the kind of fanotify client that says: No you cannot
> > > open/read/write/mmap/etc.. this file until I say you can, right?
> > 
> > Yes and no, it would be more accurate to say "this open takes long while we do 
> > something else in the background".
> 
> There are two or three ways to handle this
> 
> 1.	Block the open until the daemon dies or responds
> 2.	Have a timeout (which would need to be connection configurable)
> 3.	Require the daemon responds with "in progress" now and then.

I've taken option #3.  I don't see options #2 as viable, although off
list discussion from clamav people has said they believe they are
interested in #2 rather than #3.

> For a superuser managed service its no different to an NFS mount which
> can go wonky so the only real question is what should fanotify allow non
> privileged users to do. The answer would appear anyway to be: not use
> this aspect of such a facility.

That's the approach taken thus far.  Although non-blocking/access
notification will be opened up to normal users (currently even
notification is root only)

> For the superuser case the fact the daemon can be killed thus releasing
> anything stuff is analogous to umount -f of a stuck NFS mount which seems
> perfectly good for NFS.

It does work for NFS (which I would call case #1.)  I claim that it
doesn't work for this case since a global listener stuck would stop you
from running kill() since it owuldn't be able to get permission to open
it....

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