Re: [RFC net-next] ipv6: Use destination address determined by IPVS

To: Hannes Frederic Sowa <hannes@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [RFC net-next] ipv6: Use destination address determined by IPVS
Cc: Simon Horman <horms@xxxxxxxxxxxx>, YOSHIFUJI Hideaki / 吉藤英明 <yoshfuji@xxxxxxxxxxxxxx>, lvs-devel@xxxxxxxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, Mark Brooks <mark@xxxxxxxxxxxxxxxx>, Phil Oester <kernel@xxxxxxxxxxxx>
From: Julian Anastasov <ja@xxxxxx>
Date: Sun, 20 Oct 2013 10:11:16 +0300 (EEST)

On Sun, 20 Oct 2013, Hannes Frederic Sowa wrote:

> > Hm, maybe. I don't have too much insight into netfilter stack and
> > what are the differences between OUTPUT and FORWARD path but plan to
> > investigate. ;)
> It seems tables are processed with bh disabled, so no preemption while
> recursing. So I guess the use of tee_active is safe for breaking the
> tie here.

        May be, I'll check it again, for now I see only
rcu_read_lock() in nf_hook_slow() which is preemptable.
Looking at rcu_preempt_note_context_switch, many levels of
RCU locks are preemptable too.

> The reason I exhaust stack space is that we can actually send out packets
> while looking up routes (rt6_probe). The nonreachability of the default
> gateway and the to-teed-to box does the rest.

        In my test I used link route to local subnet,
--gateway to IP that is not present. I'll try other

> We need to change the route lookup of the duplicated packet in xt_tee to not
> cause ndisc probes to be generated.
> The more I review the patch the more I think it is ok. But we could actually
> try to just always return rt6i_gateway, as we should always be handed a cloned
> rt6_info where the gateway is already filled in, no?

        Yes, this patch is ok and after spending the whole
saturday I'm preparing a new patch that will convert
rt6_nexthop() to return just rt6i_gateway, without daddr.
This can happen after filling rt6i_gateway in all places.

        For your concern for loopback, I don't see problem,
local/anycast route will have rt6i_gateway=IP, they are
simple DST_HOST routes. I'm preparing now the patches and
will post them in following hours.


Julian Anastasov <ja@xxxxxx>
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