Re: iptables very slow after commit 784544739a25c30637397ace5489eeb6e15d7d49

Linus Torvalds
2009-04-11T19:05:55Z

On Fri, 10 Apr 2009, Paul E. McKenney wrote:
>
> 1. Assuming that the synchronize_net() is intended to guarantee
> that the new rules will be in effect before returning to
> user space:

Btw, I think that's a bad assumption.

The thing is, nobody can really care if the new rules are in effect or
not, because the thing you race with is not the "return to user space"
part, but the incoming packets.

And those incoming packets might have been incoming before the rules were
set up too.

So I seriously doubt you need to synchronize with any returning to user
space. What you want to synchronize with is then later actions that do
things like turning on the interface that the rules are attached to etc!

So I would suggest:

- remove the synchronize_net() entirely. Replace it with just freeing the
old rules using RCU.

- new packets will always end up seeing the new rules. That includes the
case of somebody doing "ifconfig eth0 up" that enables a new source of
packets, so there are no real security issues.

- if you enabled your network interfaces before you updated your packet
filtering rules, you already had a window where packets would come in
with the old rules, so doing a "synchronize_net()" in no way protects
against any race conditions anyway.

Am I missing something?

Linus