From mboxrd@z Thu Jan  1 00:00:00 1970
From: Amin Azez <azez@ufomechanic.net>
Subject: Re: xt_gateway locks up
Date: Fri, 26 Oct 2007 15:08:55 +0100
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References: <Pine.LNX.4.64.0710220012060.1997@fbirervta.pbzchgretzou.qr>
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Cc: Netfilter Developer Mailing List <netfilter-devel@vger.kernel.org>
To: Jan Engelhardt <jengelh@computergmbh.de>
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* Jan Engelhardt wrote, On 21/10/07 23:14:
> Hi Sam,
> 
> 
> just for the fun of it, I tried:
> 
> 	iptables -t mangle -A PREROUTING -m gateway --nexthop 10.10.96.1
> 
> and it locked up hard (not even sysrq-b was possible).
> I am not sure why that happened - it does work properly in -t filter -A 
> OUTPUT, though. I'll experiment a bit.
> The codebase I am on is at 
> http://dev.computergmbh.de/wsvn/misc_kernel/xt_gateway/

Yeah, my match function was very verbose:

{
        const struct ipt_gateway_info *info = matchinfo;
        const struct iphdr *iph = skb->nh.iph;

        return (  !!(info->flags & IPT_GATEWAY_INV) ^
                ( skb && skb->dst && skb->dst->neighbour &&
...

It assumes that IF skb->dst is set, that it is a neighbour table entry.

Your code is:

	dst = skb->dst;
	if ((neigh = dst->neighbour) == NULL)
		return false;


It doesn't consider that skb->dst may be NULL, which it will in PRE_ROUTING.

Sam