Subject: Re: [PATCH net-next 6/6] net: Implmement RFC 6936 (zero RX csums for UDP/IPv6)

From: YOSHIFUJI Hideaki <[email-masked]>
Date: Tue, 08 Apr 2014 03:48:39 +0900

Tom Herbert wrote:
> RFC 6936 relaxes the requirement of RFC 2460 that UDP/IPv6 packets which
> are received with a zero UDP checksum value must be dropped. RFC 6936
> allow zero checksums to support tunnels over UDP.
> 
> This patch adds a new socket option UDP_CHECK6_ZERO_OKAY whcih can be
> set on a UDP socket to indicate that a zero checksum is acceptable
> (e.g. the socket is for a tunnel). The ip6 checksum and UDP receive
> functions were updated accordingly to deal with this.
> 
> Signed-off-by: Tom Herbert <[email-masked]>
> ---
>  include/linux/udp.h      |  3 ++-
>  include/uapi/linux/udp.h |  1 +
>  net/ipv4/udp.c           |  8 ++++++++
>  net/ipv6/ip6_checksum.c  | 19 ++++++++-----------
>  net/ipv6/udp.c           | 39 +++++++++++++++++++++++++++++++++++++--
>  5 files changed, 56 insertions(+), 14 deletions(-)
> 
> diff --git a/include/linux/udp.h b/include/linux/udp.h
> index 42278bb..647ffc9 100644
> --- a/include/linux/udp.h
> +++ b/include/linux/udp.h
> @@ -779,6 +791,19 @@ static int __udp6_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
>  	dif = inet6_iif(skb);
>  	sk = udp_v6_mcast_next(net, sk, uh->dest, daddr, uh->source, saddr, dif);
>  	while (sk) {
> +		if (!uh->check && !udp_sk(sk)->check6_zero_okay) {
> +			/*
> +			 * If checksum in packet is zero and not all the
> +			 * sockets accept a zero checksum then declare
> +			 * a checksum error.
> +			 */
> +			flush_stack(stack, count, skb, ~0);
> +			count = 0;
> +			udp6_csum_zero_error(skb);
> +			UDP6_INC_STATS_BH(net, UDP_MIB_CSUMERRORS, proto == IPPROTO_UDPLITE);
> +			UDP6_INC_STATS_BH(net, UDP_MIB_INERRORS, proto == IPPROTO_UDPLITE);
> +			break;
> +		}
>  		stack[count++] = sk;
>  		sk = udp_v6_mcast_next(net, sk_nulls_next(sk), uh->dest, daddr,
>  				       uh->source, saddr, dif);

This seems wrong; packets with zero-checksum will not be delivered to 
some sockets if some of sockets accept zero-checksums and others do not.

BTW, I have been thinking that we should introduce 4 options
(or bits) for IPv4/IPv6 checksumming for sender/receiver.

--yoshfuji