[PATCH 4.14 106/146] vxlan: restore dev->mtu setting based on lower device

Greg Kroah-Hartman
2018-01-01T14:48:12Z

From: Alexey Kodanev <alexey.kodanev@oracle.com>

[ Upstream commit f870c1ff65a6d1f3a083f277280802ee09a5b44d ]

Stefano Brivio says:
Commit a985343ba906 ("vxlan: refactor verification and
application of configuration") introduced a change in the
behaviour of initial MTU setting: earlier, the MTU for a link
created on top of a given lower device, without an initial MTU
specification, was set to the MTU of the lower device minus
headroom as a result of this path in vxlan_dev_configure():

if (!conf->mtu)
dev->mtu = lowerdev->mtu -
(use_ipv6 ? VXLAN6_HEADROOM : VXLAN_HEADROOM);

which is now gone. Now, the initial MTU, in absence of a
configured value, is simply set by ether_setup() to ETH_DATA_LEN
(1500 bytes).

This breaks userspace expectations in case the MTU of
the lower device is higher than 1500 bytes minus headroom.

This patch restores the previous behaviour on newlink operation. Since
max_mtu can be negative and we update dev->mtu directly, also check it
for valid minimum.

Reported-by: Junhan Yan <juyan@redhat.com>
Fixes: a985343ba906 ("vxlan: refactor verification and application of configuration")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Acked-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/net/vxlan.c | 5 +++++
1 file changed, 5 insertions(+)

--- a/drivers/net/vxlan.c
+++ b/drivers/net/vxlan.c
@@ -3105,6 +3105,11 @@ static void vxlan_config_apply(struct ne

max_mtu = lowerdev->mtu - (use_ipv6 ? VXLAN6_HEADROOM :
VXLAN_HEADROOM);
+ if (max_mtu < ETH_MIN_MTU)
+ max_mtu = ETH_MIN_MTU;
+
+ if (!changelink && !conf->mtu)
+ dev->mtu = max_mtu;
}

if (dev->mtu > max_mtu)