netdev - IPv6 UFO for VMs

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Message-ID: <1413935045.5994.2.camel@decadent.org.uk>
Date:	Wed, 22 Oct 2014 00:44:05 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	netdev <netdev@...r.kernel.org>
Cc:	virtualization@...ts.linux-foundation.org,
	Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: IPv6 UFO for VMs

There are several ways that VMs can take advantage of UFO and get the
host to do fragmentation for them:

drivers/net/macvtap.c:                  gso_type = SKB_GSO_UDP;
drivers/net/tun.c:                      skb_shinfo(skb)->gso_type = SKB_GSO_UDP;
drivers/net/virtio_net.c:                       skb_shinfo(skb)->gso_type = SKB_GSO_UDP;

Our implementation of UFO for IPv6 does:

		fptr = (struct frag_hdr *)(skb_network_header(skb) + unfrag_ip6hlen);
		fptr->nexthdr = nexthdr;
		fptr->reserved = 0;
		fptr->identification = skb_shinfo(skb)->ip6_frag_id;

which assumes ip6_frag_id has been set.  That's only true if the local
stack constructed the skb; otherwise it appears we get zero.

This seems to be a regression as a result of:

commit 916e4cf46d0204806c062c8c6c4d1f633852c5b6
Author: Hannes Frederic Sowa <hannes@...essinduktion.org>
Date:   Fri Feb 21 02:55:35 2014 +0100

    ipv6: reuse ip6_frag_id from ip6_ufo_append_data

However, that change seems reasonable - we *shouldn't* be choosing IDs
for any other stack.  Any paravirt net driver that can use IPv6 UFO
needs to have some way of passing a fragmentation ID to put in
skb_shared_info::ip6_frag_id.

Ben.

-- 
Ben Hutchings
For every action, there is an equal and opposite criticism. - Harrison

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