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

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Message-ID: <CAJht_EN2f=3fwjsW5GcXEAZJuJ934HFVAwxBFff-FAT17a=64w@mail.gmail.com>
Date:   Wed, 14 Oct 2020 19:24:46 -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 Wed, Oct 14, 2020 at 6:38 PM Xie He <xie.he.0141@...il.com> wrote:
>
> On Wed, Oct 14, 2020 at 1:19 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > On Wed, Oct 14, 2020 at 3:48 PM Xie He <xie.he.0141@...il.com> wrote:
> > >
> > > I thought we agreed that ideally GRE devices would not have
> > > header_ops. Currently GRE devices (in normal situations) indeed do not
> > > use header_ops (and use ARHPHRD_IPGRE as dev->type). I think we should
> > > keep this behavior.
> > >
> > > To solve the problem of the same dev->type having different
> > > hard_header_len values which you mentioned. I think we should create a
> > > new dev->type (ARPHRD_IPGRE_SPECIAL) for GRE devices that use
> > > header_ops.
> > >
> > > Also, for collect_md, I think we should use ARHPHRD_IPGRE. I see no
> > > reason to use ARPHRD_NONE.
> >
> > What does ARPHRD_IPGRE define beyond ARPHRD_NONE? And same for
> > ARPHRD_TUNNEL variants. If they are indistinguishable, they are the
> > same and might as well have the same label.
>
> It is indeed reasonable to keep devices indistinguishable to each
> other having the same dev->type label. But I see a lot of devices in
> the kernel without header_ops having different dev->type labels. For
> example, ARPHRD_SLIP should be the same as ARPHRD_RAWIP. One feature
> distinguishing these devices might be their dev->mtu.
>
> GRE devices may have their special dev->mtu determined by the maximum
> IP packet size and the GRE header length.
>
> For ARPHRD_TUNNEL, it may also have its own dev->mtu. I also see it
> has header_ops->parse_protocol (but it doesn't have
> header_ops->create).

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.