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

From: Ido Schimmel <[email-masked]>
Date: Wed, 30 May 2018 23:29:04 +0300

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 <[email-masked]>
> CC: Ido Schimmel <[email-masked]>
> 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 <[email-masked]>

Reviewed-by: Ido Schimmel <[email-masked]>

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!