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

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Message-Id: <200908041739.33796.tvrtko.ursulin@sophos.com>
Date:	Tue, 4 Aug 2009 17:39:33 +0100
From:	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>
To:	Eric Paris <eparis@...hat.com>
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Subject: Re: fanotify - overall design before I start sending patches

On Tuesday 04 August 2009 17:27:48 Eric Paris wrote:
> 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.

Ok, keep it open then for a while and I guess it is trivial to change this bit 
of behaviour.

> > > 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.

This is probably good enough. Subtree and mount point exclusions would be even 
better (in addition to superblock magic exclusions - I would not get rid of 
them) but I have no idea how realistic this requirement is, or whether it is 
possible to do it more efficiently in kernel space at all.

Tvrtko
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