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

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Message-ID: <CAJht_ENmrPbhfPaD5kkiDVWQsvA_LRndPiCMrS9zdje6sVPk=g@mail.gmail.com>
Date:   Thu, 8 Oct 2020 14:35:43 -0700
From:   Xie He <xie.he.0141@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...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 1:32 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Thu, Oct 8, 2020 at 4:11 PM Xie He <xie.he.0141@...il.com> wrote:
> >
> > 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>
> "

I see the ipgre_xmit function would pull the header our header_ops
creates, and then call __gre_xmit. __gre_xmit will call
gre_build_header to complete the GRE header. gre_build_header expects
to find the base GRE header after pushing tunnel->tun_hlen. However,
if tunnel->encap_hlen is not 0, it couldn't find the base GRE header
there. Is there a problem?

Where exactly should we put the tunnel->encap_hlen header? Before the
GRE header or after?