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

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Date:   Thu, 8 Oct 2020 14:54:34 -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 2:48 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> > 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?
>
> The L4 tunnel infra uses the two callbacks encap_hlen (e.g.,
> fou_encap_hlen) and build_header (fou_build_header) in struct
> ip_tunnel_encap_ops to first allocate more space and later call back
> into the specific implementation to fill that data. build_header is
> called from __gre6_xmit -> ip6_tnl_xmit (or its ipv4 equivalent).
> This happens after gre has pushed its header, so the headers
> will come before that.

OK. If the t->encap_hlen header needs to be placed before the GRE
header, then I think the ipgre_header function should leave some space
before the GRE header to place the t->encap_hlen header, rather than
leaving space after the GRE header.