On Wed, Jan 27, 2021 at 11:14 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>
> ---
> net/ipv4/ip_tunnel.c | 18 +++++++++---------
> 1 file changed, 9 insertions(+), 9 deletions(-)
>
> diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
> index 64594aa..ad78825 100644
> --- a/net/ipv4/ip_tunnel.c
> +++ b/net/ipv4/ip_tunnel.c
> @@ -317,7 +317,7 @@ static int ip_tunnel_bind_dev(struct net_device *dev)
> }
>
> dev->needed_headroom = t_hlen + hlen;
> - mtu -= (dev->hard_header_len + t_hlen);
> + mtu -= dev->hard_header_len ? : t_hlen;
Safety of this change also depends on whether any other ip tunnels
might have non-zero hard_header_len.
I haven't fully checked yet, but at first scan I only see one other
instance of header_ops, and that ip_tunnel_header_ops does not have a
create implementation.
>
> if (mtu < IPV4_MIN_MTU)
> mtu = IPV4_MIN_MTU;
> @@ -347,7 +347,7 @@ static struct ip_tunnel *ip_tunnel_create(struct net *net,
> nt = netdev_priv(dev);
> t_hlen = nt->hlen + sizeof(struct iphdr);
> dev->min_mtu = ETH_MIN_MTU;
> - dev->max_mtu = IP_MAX_MTU - dev->hard_header_len - t_hlen;
> + dev->max_mtu = IP_MAX_MTU - dev->hard_header_len ? : t_hlen;
here and elsewhere: subtraction takes precedence over ternary
conditional, so (IP_MAX_MTU - ..) always true.
