netdev - Re: [Patch net] ip_gre: set dev->hard_header

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Date:   Thu, 8 Oct 2020 16:31:48 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Xie He <xie.he.0141@...il.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        syzbot <syzbot+4a2c52677a8a1aa283cb@...kaller.appspotmail.com>,
        William Tu <u9012063@...il.com>
Subject: Re: [Patch net] ip_gre: set dev->hard_header_len properly

On Thu, Oct 8, 2020 at 4:11 PM Xie He <xie.he.0141@...il.com> wrote:
>
> On Thu, Oct 8, 2020 at 12:20 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > On Thu, Oct 8, 2020 at 3:17 PM Xie He <xie.he.0141@...il.com> wrote:
> > >
> > > However, there's something I don't understand in the GRE code. The
> > > ipgre_header function only creates an IP header (20 bytes) + a GRE
> > > base header (4 bytes), but pushes and returns "t->hlen +
> > > sizeof(*iph)". What is t->hlen?
> >
> > GRE is variable length depending on flags:
> >
> >         tunnel->tun_hlen = gre_calc_hlen(tunnel->parms.o_flags);
> >
> >
> > > It seems to me it is the sum of
> > > t->tun_hlen and t->encap_hlen. What are these two?
>
> OK. I understand that t->tun_hlen is the GRE header length. What is
> t->encap_hlen?

I've looked at that closely either.

Appears to be to account for additional FOU/GUE encap:

"
commit 56328486539ddd07cbaafec7a542a2c8a3043623
Author: Tom Herbert <therbert@...gle.com>
Date:   Wed Sep 17 12:25:58 2014 -0700
    net: Changes to ip_tunnel to support foo-over-udp encapsulation

    This patch changes IP tunnel to support (secondary) encapsulation,
    Foo-over-UDP. Changes include:

    1) Adding tun_hlen as the tunnel header length, encap_hlen as the
       encapsulation header length, and hlen becomes the grand total
       of these.
    2) Added common netlink define to support FOU encapsulation.
    3) Routines to perform FOU encapsulation.

    Signed-off-by: Tom Herbert <therbert@...gle.com>
    Signed-off-by: David S. Miller <davem@...emloft.net>
"