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

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Date:	Tue, 04 Aug 2009 12:27:48 -0400
From:	Eric Paris <eparis@...hat.com>
To:	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
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	Douglas Leeder <douglas.leeder@...hos.com>,
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	"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 17:09 +0100, Tvrtko Ursulin wrote:
> Hi Eric, all,
> 
> On Friday 24 July 2009 21:13:49 Eric Paris wrote:
> > If a FAN_ACCESS_PERM or FAN_OPEN_PERM event is received the listener
> > must send a response before the 5 second timeout.  If no response is
> > sent before the 5 second timeout the original operation is allowed.  If
> > this happens too many times (10 in a row) the fanotify group is evicted
> > from the kernel and will not get any new events.  Sending a response is
> 
> 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.

> > The only other current interface is the ability to ignore events by
> > superblock magic number.  This makes it easy to ignore all events
> > in /proc which can be difficult to accomplish firing FANOTIFY_SET_MARK
> > with ignored_masks over and over as processes are created and destroyed.
> 
> Just to double-check, that would also work for any other filesystem and is 
> controllable from userspace?

Yes you set these in userspace using setsockopt().  It is based on
superblock magic number as found in linux/magic.h.  So one could
exclude, procfs, sysfs, selinuxfs, etc.  It does not provide a way to
say 'this ext3 filesystem but not that ext3 filesystem' as ext3 has a
single magic number.

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