Date:	Fri, 20 Aug 2010 13:25:03 +0200
From:	Andreas Gruenbacher <agruen@...e.de>
To:	Eric Paris <eparis@...hat.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Matt Helsley <matthltc@...ibm.com>,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
	Michael Kerrisk <michael.kerrisk@...il.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [GIT PULL] notification tree: directory events

On Friday 20 August 2010 05:38:17 Eric Paris wrote:
> So the actual bugs you have reported, I see two.
> 
> The (nearly) unbounded number of potential outstanding notifications
> events is a known situation, pointed out in previous discussions well
> before this commit and is one of the (numerous) reasons why fanotify is
> at this time CAP_SYS_ADMIN only.  It is something that is difficult to
> address while still making fanotify useful for permissions gating.  But
> the issue is clearly noted.

Clearly noting and blissfully ignoring the problem is not enough
unfortunately; this needs to be addressed now.  I have already pointed out 
(quoted below) that permission gating is not the worst problem here; the worst 
problem are listeners whose fanotify event queue just grows and grows.  There 
is no throttling, and no guarantee that even a listener which simply reads and 
completely ignores all events will manage to keep up.  The system will run out 
of memory eventually.

Here is a quote from a previous message about this problem that only went to 
linux-kernel:

On Tuesday 17 August 2010 05:39:47 Eric Paris wrote:
> On Mon, 2010-08-16 at 22:32 +0200, Andreas Gruenbacher wrote:
> > Q: What prevents the system from going out of memory when a listener
> > decides to stop reading events or simply can't keep up?  There doesn't
> > seem to be a limit on the queue depth.  Listeners currently need
> > CAP_SYS_ADMIN, but somehow limiting the queue depth and throttling when
> > things start to go bad still sounds like a reasonable thing to do,
> > right?
> 
> It's an interesting question and obviously one that I've thought about.
> You remember when we talked previously I said the hardest part left was
> allowing non-root users to use the interface.  It gets especially
> difficult when thinking about perm-events.  I was specifically told not
> to timeout or drop those.  But when dealing with non-root users using
> perm events?   As for pure notification we can do something like inotify
> does quite easily.
> 
> I'm not certain exactly what the best semantics are for non trusted
> users, so I didn't push any patches that way.  Suggestions welcome   :)

The system will happily go OOM for trusted users and non-perm events if the 
listener doesn't keep up, so some throttling, dropping, or both needs to 
happen for non-perm events.  This is the critical case.  Doing what inotify 
does (queue an overflow event and drop further events) seems to make sense 
here.

The situation with perm-events is less severe because the number of 
outstanding perm events is bounded by the number of running processes.  This 
may be enough of a limit.

I don't think we need to worry about perm-events for untrusted users.  We can 
start supporting some kinds of non-perm-events for untrusted users later; this 
won't change the existing interface.

Andreas
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