netdev - Re: [Patch net v2] ip_gre: set dev->hard_header_len and dev->needed

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Date:   Thu, 15 Oct 2020 12:19:07 -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 v2] ip_gre: set dev->hard_header_len and
 dev->needed_headroom properly

On Thu, Oct 15, 2020 at 6:42 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Wed, Oct 14, 2020 at 10:25 PM Xie He <xie.he.0141@...il.com> wrote:
> >
> > Actually I think dev->type can be seen from user space. For example,
> > when you type "ip link", it will display the link type for you. So I
> > think it is useful to keep different dev->type labels without merging
> > them even if they appear to have no difference.
>
> Ah, indeed. These constants are matched in iproute2 in lib/ll_types.c
> to string representations.
>
> Good catch. Yes, then they have to stay as is.

So in this case it may be better to keep dev->type as ARHPHRD_IPGRE
for GRE devices with or without header_ops. This way we can avoid the
need to update iproute2.

We can still consider changing GRE devices in collect_md mode from
ARPHRD_NONE to ARHPHRD_IPGRE. The original author who changed it to
ARPHRD_NONE might assume ARHPHRD_IPGRE is for GRE devices with
header_ops, so he felt he needed to distinguish GRE devices without
header_ops by dev->type. However, ARHPHRD_IPGRE is actually already
used for GRE devices with header_ops AND without header_ops. So it
doesn't hurt to use ARHPHRD_IPGRE for GRE devices in collect_md mode,
too. This would also make iproute2 to correctly display GRE devices in
collect_md mode as GRE devices.