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From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
CC:     Alexey Kodanev <alexey.kodanev@oracle.com>,
        "David S . Miller" <davem@davemloft.net>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.14 53/67] ip6_vti: adjust vti mtu according to
 mtu of lower device
Thread-Topic: [PATCH AUTOSEL for 4.14 53/67] ip6_vti: adjust vti mtu according
 to mtu of lower device
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Date:   Thu, 8 Mar 2018 04:57:54 +0000
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From: Alexey Kodanev <alexey.kodanev@oracle.com>

[ Upstream commit 53c81e95df1793933f87748d36070a721f6cb287 ]

LTP/udp6_ipsec_vti tests fail when sending large UDP datagrams over
ip6_vti that require fragmentation and the underlying device has an
MTU smaller than 1500 plus some extra space for headers. This happens
because ip6_vti, by default, sets MTU to ETH_DATA_LEN and not updating
it depending on a destination address or link parameter. Further
attempts to send UDP packets may succeed because pmtu gets updated on
ICMPV6_PKT_TOOBIG in vti6_err().

In case the lower device has larger MTU size, e.g. 9000, ip6_vti works
but not using the possible maximum size, output packets have 1500 limit.

The above cases require manual MTU setup after ip6_vti creation. However
ip_vti already updates MTU based on lower device with ip_tunnel_bind_dev().

Here is the example when the lower device MTU is 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@NONE: <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@NONE: <POINTOPOINT,NOARP> mtu 8832 ...
        link/tunnel6 fd00::2 peer fd00::1

Reported-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 net/ipv6/ip6_vti.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/net/ipv6/ip6_vti.c b/net/ipv6/ip6_vti.c
index bcdc2d557de1..413f01be0c9b 100644
--- a/net/ipv6/ip6_vti.c
+++ b/net/ipv6/ip6_vti.c
@@ -626,6 +626,7 @@ static void vti6_link_config(struct ip6_tnl *t)
 {
 	struct net_device *dev =3D t->dev;
 	struct __ip6_tnl_parm *p =3D &t->parms;
+	struct net_device *tdev =3D NULL;
=20
 	memcpy(dev->dev_addr, &p->laddr, sizeof(struct in6_addr));
 	memcpy(dev->broadcast, &p->raddr, sizeof(struct in6_addr));
@@ -638,6 +639,25 @@ static void vti6_link_config(struct ip6_tnl *t)
 		dev->flags |=3D IFF_POINTOPOINT;
 	else
 		dev->flags &=3D ~IFF_POINTOPOINT;
+
+	if (p->flags & IP6_TNL_F_CAP_XMIT) {
+		int strict =3D (ipv6_addr_type(&p->raddr) &
+			      (IPV6_ADDR_MULTICAST | IPV6_ADDR_LINKLOCAL));
+		struct rt6_info *rt =3D rt6_lookup(t->net,
+						 &p->raddr, &p->laddr,
+						 p->link, strict);
+
+		if (rt)
+			tdev =3D rt->dst.dev;
+		ip6_rt_put(rt);
+	}
+
+	if (!tdev && p->link)
+		tdev =3D __dev_get_by_index(t->net, p->link);
+
+	if (tdev)
+		dev->mtu =3D max_t(int, tdev->mtu - dev->hard_header_len,
+				 IPV6_MIN_MTU);
 }
=20
 /**
--=20
2.14.1