netdev - [PATCH net-next] ip6_vti: adjust vti mtu according to mtu of output device

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Date:   Wed,  6 Dec 2017 19:38:19 +0300
From:   Alexey Kodanev <alexey.kodanev@...cle.com>
To:     netdev@...r.kernel.org
Cc:     Steffen Klassert <steffen.klassert@...unet.com>,
        David Miller <davem@...emloft.net>,
        Petr Vorel <pvorel@...e.cz>,
        Alexey Kodanev <alexey.kodanev@...cle.com>
Subject: [PATCH net-next] ip6_vti: adjust vti mtu according to mtu of output device

LTP/udp6_ipsec_vti tests fail when sending large UDP datagrams
that require fragmentation and underlying device MTU <= 1500.
This happens because ip6_vti sets mtu to ETH_DATA_LEN and not
updating it depending on a destiantion address.

Futhure attempts to send UDP packets may succeed because pmtu
get updated on ICMPV6_PKT_TOOBIG in vti6_err().

Here is the example when output device MTU set to 9000:

  # ip a sh ltp_ns_veth2
    ltp_ns_veth2@if7: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 ...
      inet 10.0.0.2/24 scope global ltp_ns_veth2
      inet6 fd00::2/64 scope global
      ...
  # ip li add vti6 type vti6 local fd00::2 remote fd00::1
  # ip li show vti6
    vti6@...E: <POINTOPOINT,NOARP> mtu 1500 ...
      link/tunnel6 fd00::2 peer fd00::1

After the patch:

  # ip li add vti6 type vti6 local fd00::2 remote fd00::1
  # ip li show vti6
    vti6@...E: <POINTOPOINT,NOARP> mtu 8832 ...
      link/tunnel6 fd00::2 peer fd00::1

Regarding ip_vti, it already tunes mtu with ip_tunnel_bind_dev():

  # ip li add vti4 type vti local 10.0.0.2 remote 10.0.0.1
  # ip li sh vti4
    vti4@...E: <POINTOPOINT,NOARP> mtu 8832 ...
      link/ipip 10.0.0.2 peer 10.0.0.1

Reported-by: Petr Vorel <pvorel@...e.cz>
Signed-off-by: Alexey Kodanev <alexey.kodanev@...cle.com>
---

ip6_vti mtu offset is the same (168) as in ip_vti because ip_vti
offset includes two sizes of struct iphdr: in dev->hard_header_len
and in t_hlen in ip_tunnel_bind_dev(). I'm not sure if it's correct.

 net/ipv6/ip6_vti.c |   18 ++++++++++++++++++
 1 files changed, 18 insertions(+), 0 deletions(-)

diff --git a/net/ipv6/ip6_vti.c b/net/ipv6/ip6_vti.c
index dbb74f3..47e6464 100644
--- a/net/ipv6/ip6_vti.c
+++ b/net/ipv6/ip6_vti.c
@@ -638,6 +638,24 @@ static void vti6_link_config(struct ip6_tnl *t)
 		dev->flags |= IFF_POINTOPOINT;
 	else
 		dev->flags &= ~IFF_POINTOPOINT;
+
+	if (p->flags & IP6_TNL_F_CAP_XMIT) {
+		int strict = (ipv6_addr_type(&p->raddr) &
+			      (IPV6_ADDR_MULTICAST | IPV6_ADDR_LINKLOCAL));
+
+		struct rt6_info *rt = rt6_lookup(t->net,
+						 &p->raddr, &p->laddr,
+						 p->link, strict);
+
+		if (!rt)
+			return;
+
+		if (rt->dst.dev) {
+			dev->mtu = max(rt->dst.dev->mtu - dev->hard_header_len,
+				       IPV6_MIN_MTU);
+		}
+		ip6_rt_put(rt);
+	}
 }
 
 /**
-- 
1.7.1