Subject: [PATCH 40/43] ip6_gre: update mtu properly in ip6gre_err

From: Xin Long <[email-masked]>
Date: Tue, 5 Sep 2017 17:26:33 +0800

[ Upstream commit 5c25f30c93fdc5bf25e62101aeaae7a4f9b421b3 ]

Now when probessing ICMPV6_PKT_TOOBIG, ip6gre_err only subtracts the
offset of gre header from mtu info. The expected mtu of gre device
should also subtract gre header. Otherwise, the next packets still
can't be sent out.

Jianlin found this issue when using the topo:
  client(ip6gre)<---->(nic1)route(nic2)<----->(ip6gre)server

and reducing nic2's mtu, then both tcp and sctp's performance with
big size data became 0.

This patch is to fix it by also subtracting grehdr (tun->tun_hlen)
from mtu info when updating gre device's mtu in ip6gre_err(). It
also needs to subtract ETH_HLEN if gre dev'type is ARPHRD_ETHER.

Reported-by: Jianlin Shi <[email-masked]>
Signed-off-by: Xin Long <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/ipv6/ip6_gre.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index 64eea3962733..ca2a45134c6b 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -432,7 +432,9 @@ static void ip6gre_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 		}
 		break;
 	case ICMPV6_PKT_TOOBIG:
-		mtu = be32_to_cpu(info) - offset;
+		mtu = be32_to_cpu(info) - offset - t->tun_hlen;
+		if (t->dev->type == ARPHRD_ETHER)
+			mtu -= ETH_HLEN;
 		if (mtu < IPV6_MIN_MTU)
 			mtu = IPV6_MIN_MTU;
 		t->dev->mtu = mtu;
-- 
2.13.5