linux-kernel - Re: [PATCH] ipv6: ip6mr: Recalc UDP checksum before forwarding

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Date:   Wed, 13 Dec 2017 15:21:40 -0200
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     Brendan McGrath <redmcg@...mandi.dyndns.org>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ipv6: ip6mr: Recalc UDP checksum before forwarding

Hi,

On Wed, Dec 13, 2017 at 10:20:48PM +1100, Brendan McGrath wrote:
> Currently, when forwarding from a Virtual Interface to a Physical
> Interface, ip_summed is set to a value of CHECKSUM_UNNECESSARY and
> the UDP checksum has not been calculated.
> 
> When the packet is then forwarded by a Multicast Router, the checksum
> value is left as is and therefore rejected by the receiving
> machine(s).
> 
> This patch ensures the checksum is recalculated before forwarding.
> 
> Signed-off-by: Brendan McGrath <redmcg@...mandi.dyndns.org>
> ---
> 
> It's a bit ugly putting UDP specific code in this spot - but I'm not
> aware of any other protocols that are:
> a) multicast;
> b) forwarded; and
> c) checksummed
> 
>  net/ipv6/ip6mr.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
> index 890f9bda..ee4370a 100644
> --- a/net/ipv6/ip6mr.c
> +++ b/net/ipv6/ip6mr.c
> @@ -2077,6 +2077,13 @@ static int ip6mr_forward2(struct net *net, struct mr6_table *mrt,
>  	ipv6h = ipv6_hdr(skb);
>  	ipv6h->hop_limit--;
>  
> +	if (ipv6h->nexthdr == NEXTHDR_UDP &&
> +					skb->ip_summed != CHECKSUM_PARTIAL) {
	    ^

This indentation is wrong. The 2nd line should start right after the (
column in the 1st line, like:
+	if (ipv6h->nexthdr == NEXTHDR_UDP &&
+	    skb->ip_summed != CHECKSUM_PARTIAL) {
Adjust with spaces as needed.

Running the patch through scripts/checkpatch.pl before posting will
catch these.

> +		struct udphdr *uh = udp_hdr(skb);
> +		udp6_set_csum(false, skb, &ipv6_hdr(skb)->saddr,
> +					&ipv6_hdr(skb)->daddr, ntohs(uh->len));
                              ^  same deal here.

> +	}
> +
>  	IP6CB(skb)->flags |= IP6SKB_FORWARDED;
>  
>  	return NF_HOOK(NFPROTO_IPV6, NF_INET_FORWARD,
> -- 
> 2.7.4
>