On Thu, Jan 28, 2021 at 8:02 PM Vadim Fedorenko <vfedore...@novek.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 <m...@slava.cc>
> Signed-off-by: Vadim Fedorenko <vfedore...@novek.ru>
Acked-by: Willem de Bruijn <will...@google.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")
