netdev - Re: [PATCH net 1/2] ip_tunnel: restore binding to ifaces with a large mtu

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Date:   Wed, 30 May 2018 23:29:04 +0300
From:   Ido Schimmel <idosch@...sch.org>
To:     Nicolas Dichtel <nicolas.dichtel@...nd.com>
Cc:     davem@...emloft.net, petrm@...lanox.com, idosch@...lanox.com,
        netdev@...r.kernel.org
Subject: Re: [PATCH net 1/2] ip_tunnel: restore binding to ifaces with a
 large mtu

On Wed, May 30, 2018 at 10:28:42AM +0200, Nicolas Dichtel wrote:
> After commit f6cc9c054e77, the following conf is broken (note that the
> default loopback mtu is 65536, ie IP_MAX_MTU + 1):
> 
> $ ip tunnel add gre1 mode gre local 10.125.0.1 remote 10.125.0.2 dev lo
> add tunnel "gre0" failed: Invalid argument
> $ ip l a type dummy
> $ ip l s dummy1 up
> $ ip l s dummy1 mtu 65535
> $ ip tunnel add gre1 mode gre local 10.125.0.1 remote 10.125.0.2 dev dummy1
> add tunnel "gre0" failed: Invalid argument
> 
> dev_set_mtu() doesn't allow to set a mtu which is too large.
> First, let's cap the mtu returned by ip_tunnel_bind_dev(). Second, remove
> the magic value 0xFFF8 and use IP_MAX_MTU instead.
> 0xFFF8 seems to be there for ages, I don't know why this value was used.
> 
> With a recent kernel, it's also possible to set a mtu > IP_MAX_MTU:
> $ ip l s dummy1 mtu 66000
> After that patch, it's also possible to bind an ip tunnel on that kind of
> interface.
> 
> CC: Petr Machata <petrm@...lanox.com>
> CC: Ido Schimmel <idosch@...lanox.com>
> Link: https://git.kernel.org/pub/scm/linux/kernel/git/davem/netdev-vger-cvs.git/commit/?id=e5afd356a411a
> Fixes: f6cc9c054e77 ("ip_tunnel: Emit events for post-register MTU changes")
> Signed-off-by: Nicolas Dichtel <nicolas.dichtel@...nd.com>

Reviewed-by: Ido Schimmel <idosch@...lanox.com>

There is another instance of this magic number in the file, but it's
written in lower case so you might have missed it - see
ip_tunnel_newlink(). Can you please take care of it in v2?

Thanks for the fix, Nicolas!