netdev - [PATCH net v2 1/2] ipv6: Honor route mtu if it is within limit of dev mtu

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Date:   Mon, 13 Jun 2022 23:01:54 -0600
From:   Subash Abhinov Kasiviswanathan <quic_subashab@...cinc.com>
To:     <davem@...emloft.net>, <dsahern@...nel.org>,
        <yoshfuji@...ux-ipv6.org>, <kuba@...nel.org>,
        <netdev@...r.kernel.org>, <sbrivio@...hat.com>
CC:     Kaustubh Pandey <quic_kapandey@...cinc.com>,
        Sean Tranchetti <quic_stranche@...cinc.com>,
        Subash Abhinov Kasiviswanathan <quic_subashab@...cinc.com>
Subject: [PATCH net v2 1/2] ipv6: Honor route mtu if it is within limit of dev mtu

From: Kaustubh Pandey <quic_kapandey@...cinc.com>

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

Fixes: e9fa1495d738 ("ipv6: Reflect MTU changes on PMTU of exceptions for MTU-less routes")
Signed-off-by: Kaustubh Pandey <quic_kapandey@...cinc.com>
Signed-off-by: Sean Tranchetti <quic_stranche@...cinc.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <quic_subashab@...cinc.com>
---
v1 -> v2: Update the exception route logic as mentioned by David Ahern.
Also add fixes tag.

 net/ipv6/route.c | 11 +----------
 1 file changed, 1 insertion(+), 10 deletions(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index d25dc83..6f7e8c5 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1991,19 +1991,11 @@ static bool rt6_mtu_change_route_allowed(struct inet6_dev *idev,
 	/* If the new MTU is lower than the route PMTU, this new MTU will be the
 	 * lowest MTU in the path: always allow updating the route PMTU to
 	 * reflect PMTU decreases.
-	 *
-	 * If the new MTU is higher, and the route PMTU is equal to the local
-	 * MTU, this means the old MTU is the lowest in the path, so allow
-	 * updating it: if other nodes now have lower MTUs, PMTU discovery will
-	 * handle this.
 	 */
 
 	if (dst_mtu(&rt->dst) >= mtu)
 		return true;
 
-	if (dst_mtu(&rt->dst) == idev->cnf.mtu6)
-		return true;
-
 	return false;
 }
 
@@ -4914,8 +4906,7 @@ 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);
-- 
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