netdev - Re: Passive OS fingerprint xtables match.

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Message-ID: <20090212204115.GL6759@linux.vnet.ibm.com>
Date:	Thu, 12 Feb 2009 12:41:15 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Evgeniy Polyakov <zbr@...emap.net>
Cc:	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Netfilter Development Mailinglist 
	<netfilter-devel@...r.kernel.org>
Subject: Re: Passive OS fingerprint xtables match.

On Thu, Feb 12, 2009 at 08:51:30PM +0300, Evgeniy Polyakov wrote:
> Hi.
> 
> On Thu, Feb 12, 2009 at 09:42:53AM -0800, Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> > > Passive OS fingerprint homepage (archives, examples):
> > > http://www.ioremap.net/projects/osf
> > 
> > I advocate using this to get more accurate censuses of machines
> > accessing given web servers, given the tendency of browsers to lie
> > about themselves in order to avoid being shut out of certain web sites.
> > 
> > One question about the module-unload sequence below.
> > 
> > Given a reasonable answer to that question, I am OK with this from
> > an RCU viewpoint.
> 
> Thanks a lot Paul, but are they actually questions? You answered all of them :)

The last one was.  Your answer was that you prevented any concurrent
access, but I frankly don't understand the netfilter/connector code
well enough to do more than simply believe you.

							Thanx, Paul

> > > +static void __devexit ipt_osf_fini(void)
> > > +{
> > > +	struct ipt_osf_finger *f;
> > > +	int i;
> > > +
> > > +	cn_del_callback(&cn_osf_id);
> > > +	xt_unregister_match(&ipt_osf_match);
> > > +
> > > +	rcu_read_lock();
> > > +	for (i=0; i<ARRAY_SIZE(ipt_osf_fingers); ++i) {
> > > +		struct ipt_osf_finger_storage *st = &ipt_osf_fingers[i];
> > > +
> > > +		list_for_each_entry_rcu(f, &st->finger_list, finger_entry) {
> > > +			list_del_rcu(&f->finger_entry);
> > 
> > For the above to be safe:
> > 
> > o	Any remaining RCU callbacks cannot reference the list
> > 	(and your callbacks do in fact meet this constraint).
> 
> They do not access that list.
> 
> > o	Any timers have to have fired or been cancelled (but you
> > 	don't seem to have any timers, so not a problem).
> 
> No timers, tasklets, work queues or whatever else postponing the work.
> 
> > o	All pathways to the data structure have to have been
> > 	shut down.  This is the tough one -- or is it simply
> > 	a requirement that the guy removing the module has shut
> > 	down all requests?
> 
> They can be accessed via connector configuration, but it is stopped
> above (and it waits for currently active users to run away);
> and netfilter path, which should be prevented after match was also
> unregistered above.
> 
> -- 
> 	Evgeniy Polyakov
> --
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