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

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Date:   Thu, 8 Oct 2020 13:10:42 -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 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?