netdev - Re: [PATCH net] ip6_tunnel: set inner ipproto before ip6_tnl

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Message-Id: <25F92C95-1D1C-4C86-BAFC-08BD47F747EF@yandex-team.ru>
Date:   Wed, 28 Oct 2020 00:45:32 +0300
From:   Alexander Ovechkin <ovov@...dex-team.ru>
To:     Vadim Fedorenko <vfedorenko@...ek.ru>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net] ip6_tunnel: set inner ipproto before ip6_tnl_encap.

But ip6_tnl_encap assigns to proto number of outer protocol (i.e. =
protocol that encapsulates our original packet). Setting inner_ipproto =
to this value makes no sense.=20

For example in case of ipv6 inside MPLS inside fou6 encapsulation we =
have following packet structure:
+--------------+ <---+
|    ipv6      |     |
+--------------+     +- fou6-encap
|     udp      |     |
+--------------+ <---+
|     mpls     | <---   mpls-enacp
+--------------+ <---+
|  inner-ipv6  |     |
+--------------+     +- original packet
|     ...      |     |
+--------------+ <---+
After ip6_tnl_encap proto will be equal to IPPROTO_UDP, that is =
obviously is not inner ipproto.

Actually if pproto =3D=3D IPPROTO_MPLS than we have two layers of =
encapsulation and it is meaningless to set inner ipproto, cause =
currently there is no support for segmentation of packets with two =
layers of encapsulation.


> On 17 Oct 2020, at 03:59, Vadim Fedorenko <vfedorenko@...ek.ru> wrote:
> 
> 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);
>