[PATCH] ipv6: Honor route mtu if it is within limit of dev mtu

[Kaustubh Pandey]

When netdevice MTU is increased via sysfs, NETDEV_CHANGEMTU is raised.

addrconf_notify -> rt6_mtu_change -> rt6_mtu_change_route ->
fib6_nh_mtu_change

As part of handling NETDEV_CHANGEMTU notification we land up on a
condition where if route mtu is less than dev mtu and route mtu equals
ipv6_devconf mtu, route mtu gets updated.

Due to this v6 traffic end up using wrong MTU then configured earlier.
This commit fixes this by removing comparison with ipv6_devconf
and updating route mtu only when it is greater than incoming dev mtu.

This can be easily reproduced with below script:
pre-condition:
device up(mtu = 1500) and route mtu for both v4 and v6 is 1500

test-script:
ip route change 192.168.0.0/24 dev eth0 src 192.168.0.1 mtu 1400
ip -6 route change 2001::/64 dev eth0 metric 256 mtu 1400
echo 1400 > /sys/class/net/eth0/mtu
ip route change 192.168.0.0/24 dev eth0 src 192.168.0.1 mtu 1500
echo 1500 > /sys/class/net/eth0/mtu

Signed-off-by: Kaustubh Pandey <kapandey@xxxxxxxxxxxxxx>
---
 net/ipv6/route.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 1536f49..653b6c7 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -4813,8 +4813,7 @@ static int fib6_nh_mtu_change(struct fib6_nh *nh, void *_arg)
 		struct inet6_dev *idev = __in6_dev_get(arg->dev);
 		u32 mtu = f6i->fib6_pmtu;
 
-		if (mtu >= arg->mtu ||
-		    (mtu < arg->mtu && mtu == idev->cnf.mtu6))
+		if (mtu >= arg->mtu)
 			fib6_metric_set(f6i, RTAX_MTU, arg->mtu);
 
 		spin_lock_bh(&rt6_exception_lock);
-- 
2.7.4