[RFC] Faster libvirtd restart with nwfilter rules, one more time

[Eric Garver]

On Thu, Apr 09, 2020 at 11:53:46AM +0100, Daniel P. Berrangé wrote:
> Copying Eric Garver as a knowledgeable maintainer of firewalld to
> confirm a question I have....
> 
> On Fri, Mar 20, 2020 at 12:25:49PM +0300, nshirokovskiy wrote:
> > Hi, all.
> >
> > Some time ago I posted RFC [1] concerning an issue of unresponsive
> > libvird during restart if there is large number of VMs that have network
> > filters on their interfaces. It was identified that in most cases we
> > don't need actually to reinstall network filter rules on daemon restart.
> > Thus I proposed patches [2] that check whether we need to reapply rules
> > or not.
> >
> > The first version has a drawback that daemon won't reapply rules if
> > someone mangled them between daemon stop and start (and this can be done
> > just by restarting firewalld). The second one is just ugly :)
> >
> > Around that time Florian Westphal in a letter off the mailing list
> > suggested to use {iptables|ebtables}-restore to apply rules in one
> > binary call. These binaries has --noflush option so that we won't reset
> > current state of tables.

Firewalld does exactly this. It's a significant performance gain.
iptables updates are effectively: grab the _entire_ ruleset from kernel,
modify the ruleset in userspace, then push the _entire_ ruleset back to
the kernel. This is very costly. Using iptables-restore means there are
less round trips to the kernel.

> > We also need one more -L call for
> > iptables/ebtables to query current filter state to be able to construct
> > input for restore binaries.
> >
> > I wonder can we use this approach? I see currently only one issue - we
> > won't use firealld to spawn rules. But why we need to spawn rules
> > through firewalld if it present? We use passthrough mode anyway. I tried
> > to dig history for hints but didn't found anything. Patch [3] introduced
> > spawning rules thru firewalld-cmd.
> 
> For as long as libvirt has done firewall stuff we've have the issue
> with other apps on the system breaking / discarding our rules. Originally
> this was the "firewall" sysvinit script.
> 
> When firewalld came along, libvirt switched to creating rules using the
> firwalld passthrough mode API, in the belief that any time firewalld
> re-creates its rules, it would preserve any rules we'd created via the
> passthrough mode.

The confusion may stem from the fact that there are _three_ different
"direct rule" APIs:

    1) direct.addRule()
       These are tracked by firewalld and will survive firewalld
       reloads. It allows specifying a "priority", but rule indexing is
       handled by firewalld.

    2) direct.addPassthrough()
       These are tracked by firewalld and will survive firewalld
       reloads. The user must specify the "index" in the iptables
       ruleset.

    3) direct.passthrough()
       There are _NOT_ tracked by firewalld. They will _NOT_ survive a
       firewalld reload. This is a dumb passthrough to iptables.
       Firewalld has no memory of their execution immediately after they
       complete. I honestly don't understand why it exists as it
       provides no value.

Unfortunately, libvirt currently uses #3 [1]. If libvirt used #2 then
libvirt rules would persist firewalld reloads with libvirt taking zero
action. firewalld reinstalls the addPassthrough rules after reload.

The downside to #2 is that if a firewalld user executes "firewall-cmd
--runtime-to-permanent" the libvirt rules will be copied to firewalld's
_permanent_ configuration. This may be the reason libvirt is using
passthrough (#3).

[1] https://gitlab.com/libvirt/libvirt/-/blob/1e2ae2e3/src/util/virfirewalld.c#L293-303

> I vaguely recall some recentish discussions though where I think Eric
> Garver mentioned we were mistaken, and that firewalld does *nothing*
> to preserve passthrough mode rules.

See above.

> Eric, from firewalld's POV, is there any functional difference between
> an application directly creating rules by spawning "iptables", vs creating
> the same rules via the firewalld passthrough API ?

If using direct.passthrough (#3 above), then no.

> If there is no difference, then libvirt could stop using the firewalld
> passthrough API, and switch to the iptables bulk load tools.

Seem sensible to me.

[..]

Hope that was helpful.
Eric.