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Date:   Sat, 17 Oct 2020 01:59:31 +0100
From:   Vadim Fedorenko <vfedorenko@...ek.ru>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Alexander Ovechkin <ovov@...dex-team.ru>
Cc:     Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net] ip6_tunnel: set inner ipproto before ip6_tnl_encap.

On 16.10.2020 18:55, Willem de Bruijn wrote:
> On Fri, Oct 16, 2020 at 7:14 AM Alexander Ovechkin <ovov@...dex-team.ru> wrote:
>> ip6_tnl_encap assigns to proto transport protocol which
>> encapsulates inner packet, but we must pass to set_inner_ipproto
>> protocol of that inner packet.
>>
>> Calling set_inner_ipproto after ip6_tnl_encap might break gso.
>> For example, in case of encapsulating ipv6 packet in fou6 packet, inner_ipproto
>> would be set to IPPROTO_UDP instead of IPPROTO_IPV6. This would lead to
>> incorrect calling sequence of gso functions:
>> ipv6_gso_segment -> udp6_ufo_fragment -> skb_udp_tunnel_segment -> udp6_ufo_fragment
>> instead of:
>> ipv6_gso_segment -> udp6_ufo_fragment -> skb_udp_tunnel_segment -> ip6ip6_gso_segment
>>
>> Signed-off-by: Alexander Ovechkin <ovov@...dex-team.ru>
> Commit 6c11fbf97e69 ("ip6_tunnel: add MPLS transmit support") moved
> the call from ip6_tnl_encap's caller to inside ip6_tnl_encap.
>
> It makes sense that that likely broke this behavior for UDP (L4) tunnels.
>
> But it was moved on purpose to avoid setting the inner protocol to
> IPPROTO_MPLS. That needs to use skb->inner_protocol to further
> segment.
>
> I suspect we need to set this before or after conditionally to avoid
> breaking that use case.
I hope it could be fixed with something like this:

diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index a0217e5..87368b0 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -1121,6 +1121,7 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device 
*dev, __u8 dsfield,
         bool use_cache = false;
         u8 hop_limit;
         int err = -1;
+       __u8 pproto = proto;

         if (t->parms.collect_md) {
                 hop_limit = skb_tunnel_info(skb)->key.ttl;
@@ -1280,7 +1281,7 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device 
*dev, __u8 dsfield,
                 ipv6_push_frag_opts(skb, &opt.ops, &proto);
         }

-       skb_set_inner_ipproto(skb, proto);
+       skb_set_inner_ipproto(skb, pproto == IPPROTO_MPLS ? proto : pproto);

         skb_push(skb, sizeof(struct ipv6hdr));
         skb_reset_network_header(skb);

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