Subject: Re: [PATCH net] Revert "ipv6: Allow for partial checksums on non-ufo packets"

From: Vlad Yasevich <[email-masked]>
Date: Tue, 12 May 2015 14:05:40 -0400

On 05/08/2015 06:22 PM, Eric Dumazet wrote:
> From: Eric Dumazet <[email-masked]>
> 
> Wolfgang Walter reported crashes using traceroute6, root caused to this
> commit.
> 
> UDP packets can be sent from RAW sockets.
> 
> Note that the faulty commit addressed a non problem, as normal
> UDP v6 packets use udp_v6_send_skb() which correctly setup
> CHECKSUM_PARTIAL properly if device has the correct feature.

Actually, they didn't.  udp_v6_send_skb() uses ip_summed from
the skb, which is populated by __ip6_append_data.  That in turn
always set it CHECKSUM_NONE.

The code in question looks identically to that of __ip_append_data().
The problem is similar in both places.

-vlad
> 
> It seems not worth trying to 'fix' the raw path.
> 
> Fixes: 32dce968dd98 ("ipv6: Allow for partial checksums on non-ufo packets")
> Signed-off-by: Eric Dumazet <[email-masked]>
> Reported-by: Wolfgang Walter <[email-masked]>
> Cc: Vlad Yasevich <[email-masked]>
> ---
>  net/ipv6/ip6_output.c |   11 +----------
>  1 file changed, 1 insertion(+), 10 deletions(-)
> 
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index 7fde1f265c90..1fe5c687328e 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -1218,7 +1218,6 @@ static int __ip6_append_data(struct sock *sk,
>  	u32 tskey = 0;
>  	struct rt6_info *rt = (struct rt6_info *)cork->dst;
>  	struct ipv6_txoptions *opt = v6_cork->opt;
> -	int csummode = CHECKSUM_NONE;
>  
>  	skb = skb_peek_tail(queue);
>  	if (!skb) {
> @@ -1275,14 +1274,6 @@ emsgsize:
>  			tskey = sk->sk_tskey++;
>  	}
>  
> -	/* If this is the first and only packet and device
> -	 * supports checksum offloading, let's use it.
> -	 */
> -	if (!skb && sk->sk_protocol == IPPROTO_UDP &&
> -	    length + fragheaderlen < mtu &&
> -	    rt->dst.dev->features & NETIF_F_V6_CSUM &&
> -	    !exthdrlen)
> -		csummode = CHECKSUM_PARTIAL;
>  	/*
>  	 * Let's try using as much space as possible.
>  	 * Use MTU if total length of the message fits into the MTU.
> @@ -1396,7 +1387,7 @@ alloc_new_skb:
>  			 *	Fill in the control structures
>  			 */
>  			skb->protocol = htons(ETH_P_IPV6);
> -			skb->ip_summed = csummode;
> +			skb->ip_summed = CHECKSUM_NONE;
>  			skb->csum = 0;
>  			/* reserve for fragmentation and ipsec header */
>  			skb_reserve(skb, hh_len + sizeof(struct frag_hdr) +
> 
>