netdev - Re: [net v2] net: ip_tunnel: fix mtu calculation

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Date:   Thu, 28 Jan 2021 21:21:16 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Vadim Fedorenko <vfedorenko@...ek.ru>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "Willem de Bruijn <willemdebruijn.kernel@...il.com>--to=Slava Bacherikov" 
        <mail@...va.cc>, Network Development <netdev@...r.kernel.org>
Subject: Re: [net v2] net: ip_tunnel: fix mtu calculation

On Thu, Jan 28, 2021 at 8:02 PM Vadim Fedorenko <vfedorenko@...ek.ru> wrote:
>
> dev->hard_header_len for tunnel interface is set only when header_ops
> are set too and already contains full overhead of any tunnel encapsulation.
> That's why there is not need to use this overhead twice in mtu calc.
>
> Fixes: fdafed459998 ("ip_gre: set dev->hard_header_len and dev->needed_headroom properly")
> Reported-by: Slava Bacherikov <mail@...va.cc>
> Signed-off-by: Vadim Fedorenko <vfedorenko@...ek.ru>

Acked-by: Willem de Bruijn <willemb@...gle.com>

It is easy to verify that if hard_header_len is zero the calculation
does not change. And as discussed, ip_gre is the only ip_tunnel
user that sometimes has it non-zero (for legacy reasons that
we cannot revert now). In that case it is equivalent to tun->hlen +
sizeof(struct iphdr). LGTM. Thanks!

Btw, ip6_gre might need the same after commit 832ba596494b
("net: ip6_gre: set dev->hard_header_len when using header_ops")