Re: [PATCH net v2] ip6_gre: init dev->mtu and dev->hard_header_len correctly

[David Miller <davem@davemloft.net>]

From: Alexey Kodanev <alexey.kodanev@oracle.com>
Date: Thu, 18 Jan 2018 20:51:12 +0300

> Commit b05229f44228 ("gre6: Cleanup GREv6 transmit path,
> call common GRE functions") moved dev->mtu initialization
> from ip6gre_tunnel_setup() to ip6gre_tunnel_init(), as a
> result, the previously set values, before ndo_init(), are
> reset in the following cases:
> 
> * rtnl_create_link() can update dev->mtu from IFLA_MTU
>   parameter.
> 
> * ip6gre_tnl_link_config() is invoked before ndo_init() in
>   netlink and ioctl setup, so ndo_init() can reset MTU
>   adjustments with the lower device MTU as well, dev->mtu
>   and dev->hard_header_len.
> 
>   Not applicable for ip6gretap because it has one more call
>   to ip6gre_tnl_link_config(tunnel, 1) in ip6gre_tap_init().
> 
> Fix the first case by updating dev->mtu with 'tb[IFLA_MTU]'
> parameter if a user sets it manually on a device creation,
> and fix the second one by moving ip6gre_tnl_link_config()
> call after register_netdevice().
> 
> Fixes: b05229f44228 ("gre6: Cleanup GREv6 transmit path, call common GRE functions")
> Fixes: db2ec95d1ba4 ("ip6_gre: Fix MTU setting")
> Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
> ---
> 
> v2: Instead of checking whether dev->mtu equals zero or not
>     in ip6gre_tunnel_init_common(), update 'dev->mtu' once
>     more with 'IFLA_MTU' parameter after register_netdevice().

Applied and queued up for -stable, thanks Alexey.

I was almost tricked that this approach would have problems
because you do the MTU setting after register_netdevice(), but
we hold the RTNL semaphore so even if the device becomes
globally visible the user can't perform a change of the
device's MTU before we do it.

Thanks.