Re: [PATCH net] vxlan: Restore initial MTU setting based on lower device

On Wed, 13 Dec 2017 23:37:00 +0100
Stefano Brivio [off-list ref] wrote:

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.

Restore the previous behaviour by calculating, for a new link,
the MTU from the lower device, if present, and if no value is
explicitly configured.

Reported-by: Junhan Yan <redacted>
Fixes: a985343ba906 ("vxlan: refactor verification and application of configuration")
Signed-off-by: Stefano Brivio <redacted>
---

Good catch.
Acked-by: Stephen Hemminger <stephen@networkplumber.org>