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

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Date:	Sun, 20 Oct 2013 00:36:57 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Julian Anastasov <ja@....bg>, Simon Horman <horms@...ge.net.au>,
	YOSHIFUJI Hideaki / 吉藤英明 
	<yoshfuji@...ux-ipv6.org>, lvs-devel@...r.kernel.org,
	netdev@...r.kernel.org, Mark Brooks <mark@...dbalancer.org>,
	Phil Oester <kernel@...uxace.com>
Subject: Re: [RFC net-next] ipv6: Use destination address determined by IPVS

On Sat, Oct 19, 2013 at 08:34:33PM +0200, Hannes Frederic Sowa wrote:
> On Sat, Oct 19, 2013 at 07:37:10PM +0300, Julian Anastasov wrote:
> > 
> > 	Hello,
> > 
> > On Fri, 18 Oct 2013, Hannes Frederic Sowa wrote:
> > 
> > > I played around with your patch and tested xt_TEE. I added a TEE rule to
> > > mangle/OUTPUT and pinged. This happend, I have not yet analyzed it:
> > > 
> > > [  101.126649] ------------[ cut here ]------------
> > > [  101.128436] BUG: unable to handle kernel paging request at fffffffb8a2fda88
> > > [  101.129421] IP: [<ffffffff810c9737>] cpuacct_charge+0x97/0x200
> > > [  101.129421] PGD 1c0f067 PUD 0
> > > [  101.129421] Thread overran stack, or stack corrupted
> > 
> > 	Problem with process stack? May be some packet loop
> > happens? Because I can not reproduce such problem in my
> > virtual setup, I tested TEE too, with careful packet
> > matching and 1 CPU. Should I assume that you don't have such
> > oops when the patch is not applied, with the same TEE rule?
> 
> Oh, sorry, you are right. It happens with an unpatched net-next  kernel, too.
> 
> I inserted the TEE rule in mangel/OUTGOING and had only one route, ip -6 r a
> default via fe80::1 dev eth0 which at the time of the panic was actually not
> reachable.
> 
> > > [  101.129421] Oops: 0000 [#1] SMP
> > 
> > 	You don't appear to have PREEMPT in above line.
> > I'm not sure when preemption is enabled if tee_tg6() does
> > not have a problem with its anti-loop measures (tee_active).
> > Is preemption possible in OUTPUT hook, i.e. can we change
> > the CPU while playing with tee_active and as result change
> > different flag?
> 
> 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.

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.

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?

Greetings,

  Hannes

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