Re: [PATCH net-next v15 4/7] sch_cake: Add NAT awareness to packet classifier

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Tue, May 22, 2018 at 04:11:06PM +0200, Toke Høiland-Jørgensen wrote:
> Pablo Neira Ayuso <pablo@netfilter.org> writes:
> 
> > Hi Toke,
> >
> > On Tue, May 22, 2018 at 03:57:38PM +0200, Toke Høiland-Jørgensen wrote:
> >> When CAKE is deployed on a gateway that also performs NAT (which is a
> >> common deployment mode), the host fairness mechanism cannot distinguish
> >> internal hosts from each other, and so fails to work correctly.
> >> 
> >> To fix this, we add an optional NAT awareness mode, which will query the
> >> kernel conntrack mechanism to obtain the pre-NAT addresses for each packet
> >> and use that in the flow and host hashing.
> >> 
> >> When the shaper is enabled and the host is already performing NAT, the cost
> >> of this lookup is negligible. However, in unlimited mode with no NAT being
> >> performed, there is a significant CPU cost at higher bandwidths. For this
> >> reason, the feature is turned off by default.
> >> 
> >> Cc: netfilter-devel@vger.kernel.org
> >> Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
> >> ---
> >>  net/sched/sch_cake.c |   79 ++++++++++++++++++++++++++++++++++++++++++++++++++
> >>  1 file changed, 79 insertions(+)
> >> 
> >> diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
> >> index 68ac908470f1..6f7cae705c84 100644
> >> --- a/net/sched/sch_cake.c
> >> +++ b/net/sched/sch_cake.c
> >> @@ -71,6 +71,12 @@
> >>  #include <net/tcp.h>
> >>  #include <net/flow_dissector.h>
> >>  
> >> +#if IS_REACHABLE(CONFIG_NF_CONNTRACK)
> >> +#include <net/netfilter/nf_conntrack_core.h>
> >> +#include <net/netfilter/nf_conntrack_zones.h>
> >> +#include <net/netfilter/nf_conntrack.h>
> >> +#endif
> >> +
> >>  #define CAKE_SET_WAYS (8)
> >>  #define CAKE_MAX_TINS (8)
> >>  #define CAKE_QUEUES (1024)
> >> @@ -516,6 +522,60 @@ static bool cobalt_should_drop(struct cobalt_vars *vars,
> >>  	return drop;
> >>  }
> >>  
> >> +#if IS_REACHABLE(CONFIG_NF_CONNTRACK)
> >> +
> >> +static void cake_update_flowkeys(struct flow_keys *keys,
> >> +				 const struct sk_buff *skb)
> >> +{
> >> +	const struct nf_conntrack_tuple *tuple;
> >> +	enum ip_conntrack_info ctinfo;
> >> +	struct nf_conn *ct;
> >> +	bool rev = false;
> >> +
> >> +	if (tc_skb_protocol(skb) != htons(ETH_P_IP))
> >> +		return;
> >> +
> >> +	ct = nf_ct_get(skb, &ctinfo);
> >> +	if (ct) {
> >> +		tuple = nf_ct_tuple(ct, CTINFO2DIR(ctinfo));
> >> +	} else {
> >> +		const struct nf_conntrack_tuple_hash *hash;
> >> +		struct nf_conntrack_tuple srctuple;
> >> +
> >> +		if (!nf_ct_get_tuplepr(skb, skb_network_offset(skb),
> >> +				       NFPROTO_IPV4, dev_net(skb->dev),
> >> +				       &srctuple))
> >> +			return;
> >> +
> >> +		hash = nf_conntrack_find_get(dev_net(skb->dev),
> >> +					     &nf_ct_zone_dflt,
> >> +					     &srctuple);
> >> +		if (!hash)
> >> +			return;
> >> +
> >> +		rev = true;
> >> +		ct = nf_ct_tuplehash_to_ctrack(hash);
> >> +		tuple = nf_ct_tuple(ct, !hash->tuple.dst.dir);
> >> +	}
> >> +
> >> +	keys->addrs.v4addrs.src = rev ? tuple->dst.u3.ip : tuple->src.u3.ip;
> >> +	keys->addrs.v4addrs.dst = rev ? tuple->src.u3.ip : tuple->dst.u3.ip;
> >> +
> >> +	if (keys->ports.ports) {
> >> +		keys->ports.src = rev ? tuple->dst.u.all : tuple->src.u.all;
> >> +		keys->ports.dst = rev ? tuple->src.u.all : tuple->dst.u.all;
> >> +	}
> >> +	if (rev)
> >> +		nf_ct_put(ct);
> >> +}
> >
> > This is going to pull in the nf_conntrack module, even if you may not
> > want it, as soon as cake is in place.
> 
> Yeah, we are aware of that; we get a moddep on nf_conntrack. Our main
> deployment scenario has been home routers where conntrack is used
> anyway, so this has not been much of an issue. However, if there is a
> way to avoid this, and instead detect at runtime if conntrack is
> available, that would certainly be useful. Is there? :)

Yes, there is.

You place this function in net/netfilter/nf_conntrack_core.c, call it
nf_conntrack_get_tuple() which internally uses a rcu hook for this.
See nf_ct_attach() and ip_ct_attach() in net/netfilter/core.c for
instance.

This allows you to avoid the dependency with nf_conntrack (which would
be only called if the module has been explicitly loaded), which is
what you're searching for.