netdev - Re: [PATCH] gso: do not skip outer ip header in case of ipip and net

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Date:   Sun, 13 Feb 2022 23:27:57 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Tao Liu <thomas.liu@...oud.cn>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        davem@...emloft.net, yoshfuji@...ux-ipv6.org, dsahern@...nel.org,
        kuba@...nel.org, edumazet@...gle.com, sridhar.samudrala@...el.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] gso: do not skip outer ip header in case of ipip and net_failover

On Sun, Feb 13, 2022 at 11:03 PM Tao Liu <thomas.liu@...oud.cn> wrote:
>
> Sorry for bothering, just repost it.
>
> > 2022年2月14日 09:28，Willem de Bruijn <willemdebruijn.kernel@...il.com> 写道：
> >
> > On Sun, Feb 13, 2022 at 10:10 AM Tao Liu <thomas.liu@...oud.cn> wrote:
> >>
> >> We encouter a tcp drop issue in our cloud environment. Packet GROed in host
> >> forwards to a VM virtio_net nic with net_failover enabled. VM acts as a
> >> IPVS LB with ipip encapsulation. The full path like:
> >> host gro -> vm virtio_net rx -> net_failover rx -> ipvs fullnat
> >> -> ipip encap -> net_failover tx -> virtio_net tx
> >>
> >> When net_failover transmits a ipip pkt (gso_type = 0x0103), there is no gso
> >> performed because it supports TSO and GSO_IPXIP4. But network_header has
> >> been pointing to inner ip header.
> >
> > If the packet is configured correctly, and net_failover advertises
> > that it can handle TSO packets with IPIP encap, then still virtio_net
> > should not advertise it and software GSO be applied on its
> > dev_queue_xmit call.
> >
> > This is assuming that the packet not only has SKB_GSO_IPXIP4 correctly
> > set, but also tunneling fields like skb->encapsulated and
> > skb_inner_network_header.
> Thanks very much for your comment!
>
> Yes, the packet is correct. Another thing i have not pointed directly is
> that the pkt has SKB_GSO_DODGY. net_failover do not advertises GSO_ROBUST
> but virtio_net do.

If net_failover does not advertise NETIF_F_GSO_ROBUST, then
tcp_gso_segment will pass a packet with SKB_GSO_DODGY to the
software gso stack, not taking the branch

        if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {

> >> ---
> >> net/ipv4/af_inet.c | 10 +++++++++-
> >> 1 file changed, 9 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
> >> index 9c465ba..f8b3f8a 100644
> >> --- a/net/ipv4/af_inet.c
> >> +++ b/net/ipv4/af_inet.c
> >> @@ -1425,10 +1425,18 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
> >> static struct sk_buff *ipip_gso_segment(struct sk_buff *skb,
> >>                                        netdev_features_t features)
> >> {
> >> +       struct sk_buff *segs;
> >> +       int nhoff;
> >> +
> >>        if (!(skb_shinfo(skb)->gso_type & SKB_GSO_IPXIP4))
> >>                return ERR_PTR(-EINVAL);
> >>
> >> -       return inet_gso_segment(skb, features);
> >> +       nhoff = skb_network_header(skb) - skb_mac_header(skb);
> >> +       segs = inet_gso_segment(skb, features);
> >> +       if (!segs)
> >> +               skb->network_header = skb_mac_header(skb) + nhoff - skb->head;
> >> +
> >> +       return segs;
> >> }
> >
> > If this would be needed for IPIP, then the same would be needed for SIT, etc.
> >
> > Is the skb_network_header
> >
> > 1. correctly pointing to the outer header of the TSO packet before the
> > call to inet_gso_segment
> > 2. incorrectly pointing to the inner header of the (still) TSO packet
> > after the call to inet_gso_segment
> >
> > inet_gso_segment already does the same operation: save nhoff, pull
> > network header, call callbacks.gso_segment (which can be
> > ipip_gso_segment->inet_gso_segment), then place the network header
> > back at nhoff.
> >
> values print in skb_mac_gso_segment() before callbacks.gso_segment:
> ipip:               vlan_depth=0 mac_len=0 skb->network_header=206
> net_failover:  vlan_depth=14 mac_len=14 skb->network_header=186
> virtio_net:      vlan_depth=34 mac_len=34 skb->network_header=206
>
> agree to add sit/ip4ip6/ip6ip6, and patch can be simplified as:

If IPIP GSO was so broken, I think we would have found it long before.

As said, inet_gso_segment should already do the right thing for ipip:
it will be called twice.