Subject: Re: [PATCH net] ipv6: don't use CHECKSUM_PARTIAL on MSG_MORE/UDP_CORK sockets

From: Vlad Yasevich <[email-masked]>
Date: Tue, 20 Oct 2015 17:39:47 -0400

On 10/20/2015 10:38 AM, Hannes Frederic Sowa wrote:
> MSG_MORE might cause the packet to get fragmented in the end when
> passed down to the flush function and the transhdrlen check alone is
> not sufficient to protect against fragmentation. Instead check if the
> socket user intends to add more data to the socket on the first packet.
> 
> This broke checksum calculation for UDPv6 for NFS protocols.
> 
> Fixes: 32dce968dd987 ("ipv6: Allow for partial checksums on non-ufo packets")
> Cc: Vlad Yasevich <[email-masked]>
> Tested-by: Sabrina Dubroca <[email-masked]>
> Tested-by: Benjamin Coddington <[email-masked]>
> Signed-off-by: Hannes Frederic Sowa <[email-masked]>
> ---
>  net/ipv6/ip6_output.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index 61d403e..95c5780 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -1317,6 +1317,7 @@ emsgsize:
>  	 * sums only work when transhdrlen is set.
>  	 */
>  	if (transhdrlen && sk->sk_protocol == IPPROTO_UDP &&
> +	    !(flags & MSG_MORE) &&
>  	    length + fragheaderlen < mtu &&
>  	    rt->dst.dev->features & NETIF_F_V6_CSUM &&
>  	    !exthdrlen)
> 

Hmm... so while this solves this problem by simply avoiding the combination of
skb #1 having CHECKSUM_PARTIAL and others having CHECKSUM_NONE, I think the actual
problem is a bit deeper.
The above combination seems to work for me since udp6_hwcsum_outgoing() corrects
the checksum.  However, my testing so far has been on nics that have NETIF_F_V6_CSUM,
but without UFO support.

On such systems a simple test of using MSG_MORE an IPv6 udp socket sending 200 bytes
followed by 2000 bytes works correctly.

I am now wondering if this might be UFO related instead and looking for a nic that
has UFO support.

-vlad