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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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