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

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Message-Id: <1249411815.2361.26.camel@dhcp231-106.rdu.redhat.com>
Date:	Tue, 04 Aug 2009 14:50:15 -0400
From:	Eric Paris <eparis@...hat.com>
To:	John Stoffel <john@...ffel.org>
Cc:	Valdis.Kletnieks@...edu,
	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
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	"greg@...ah.com" <greg@...ah.com>,
	"jcm@...hat.com" <jcm@...hat.com>,
	Douglas Leeder <douglas.leeder@...hos.com>,
	"tytso@....edu" <tytso@....edu>,
	"arjan@...radead.org" <arjan@...radead.org>,
	"david@...g.hm" <david@...g.hm>,
	"jengelh@...ozas.de" <jengelh@...ozas.de>,
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	"mrkafk@...il.com" <mrkafk@...il.com>,
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	"jack@...e.cz" <jack@...e.cz>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
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	"alan@...rguk.ukuu.org.uk" <alan@...rguk.ukuu.org.uk>,
	"mmorley@....in" <mmorley@....in>, "pavel@...e.cz" <pavel@...e.cz>
Subject: Re: fanotify - overall design before I start sending patches

On Tue, 2009-08-04 at 14:20 -0400, John Stoffel wrote:
> >>>>> "Valdis" == Valdis Kletnieks <Valdis.Kletnieks@...edu> writes:
> 
> Valdis> On Tue, 04 Aug 2009 12:27:48 EDT, Eric Paris said:
> >> On Tue, 2009-08-04 at 17:09 +0100, Tvrtko Ursulin wrote:
> >> > Would it make more sense to deny on timeouts and then evict? I am thinking it
> >> > would be more secure with no significant drawbacks. Also for usages like HSM
> >> > allowing it without data being in place might present wrong content to the
> >> > user.
> >> 
> >> I'd be willing to go that route as long as noone else complains.
> 
> Valdis> Yes, in my world, "deny on timeout and evict" is the better
> Valdis> design decision.  For an HSM, you'd rather have a
> Valdis> quick-and-ugly death on a failed file open than an app
> Valdis> accidentally reading the HSM's stub data thinking it's the
> Valdis> original data.
> 
> Speaking as somone who is working slowly to deploy an HSM service, one
> thing to note is that when you *do* see the stub file contents, you
> know that your HSM is busted somehow. 
> 
> How will fanotify deal with this issue?  Sorry, I haven't paid enough
> attention to this thread though I know I should since it's up my $WORK
> alley.

fanotify doesn't explicitly deal with it at all.  If the HSM implemented
as a fanotify listener starts to misbehave and not respond, processes
will start to get EACCES.  If it misses 10 in a row it'll be evicted and
processes will start to see the stubs.

-Eric

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