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

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Date:   Sat, 10 Oct 2020 20:55:41 -0700
From:   Xie He <xie.he.0141@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...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 Sat, Oct 10, 2020 at 2:49 PM Xie He <xie.he.0141@...il.com> wrote:
>
> Another reason why tunnel devices usually don't provide
> header_ops->create might be to keep consistency with TAP devices. TAP
> devices are just like tunnel (TUN) devices except that they use an
> additional Ethernet header after the tunnel headers. I guess that TAP
> devices would usually use Ethernet's header_ops to expose only the
> Ethernet header to the user, but hide the outer tunnel headers from
> the user and let them be constructed on the transmission path (so that
> TAP devices would appear exactly like Ethernet devices). If we want to
> keep TUN devices consistent with TAP devices, we should hide the
> tunnel headers from the user for TUN devices, too.

Actually there's a "Universal TUN/TAP driver" in the kernel, which
passes L3 packets or Ethernet frames (that are being sent) back to the
user space and lets a user space program add the tunnel headers.

https://www.kernel.org/doc/Documentation/networking/tuntap.rst

In this case, we are not able to construct the tunnel headers until we
pass the packets / Ethernet frames back to the user space to the user
space program. We can only hide the tunnel headers.

To keep other TUN/TAP devices in the kernel consistent with the
"Universal TUN/TAP driver", we should hide the tunnel headers from the
user for those devices, too.

> Actually, a lot of devices expose a fake L2 header in header_ops,
> instead of their real L2 header anyway. Wi-Fi devices usually pretend
> to be Ethernet devices and expose an Ethernet header in header_ops. So
> I think it is acceptable to not expose the real L2 header in
> header_ops. (This is also what needed_headroom is created for - to
> request additional header space for headers not exposed in
> header_ops.)