netdev - Re: [PATCH v2] vxlan: restore dev->mtu setting based on lower device

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Message-Id: <20171216.230545.886181859330965948.davem@davemloft.net>
Date:   Sat, 16 Dec 2017 23:05:45 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     alexey.kodanev@...cle.com
Cc:     netdev@...r.kernel.org, sbrivio@...hat.com,
        mschiffer@...verse-factory.net, juyan@...hat.com, jbenc@...hat.com,
        haliu@...hat.com
Subject: Re: [PATCH v2] vxlan: restore dev->mtu setting based on lower
 device

From: Alexey Kodanev <alexey.kodanev@...cle.com>
Date: Thu, 14 Dec 2017 20:20:00 +0300

> 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@...hat.com>
> Fixes: a985343ba906 ("vxlan: refactor verification and application of configuration")
> Signed-off-by: Alexey Kodanev <alexey.kodanev@...cle.com>

Applied and queued up for -stable, thanks.