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

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Date:   Fri, 9 Oct 2020 12:41:55 -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 Fri, Oct 9, 2020 at 10:44 AM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> On Thu, Oct 8, 2020 at 4:40 PM Xie He <xie.he.0141@...il.com> wrote:
> >
> > I found another possible issue. Shouldn't we update hard_header_len
> > every time t->tun_hlen and t->hlen are updated in ipgre_link_update?
>
> Good catch. It should be updated there like ->needed_headroom.
> I will update my patch.

Thanks. But there is still something that I don't understand. What is
needed_headroom used for? If we are requesting space for t->encap_hlen
and t->tun_hlen in hard_header_len. Do we still need to use
needed_headroom?

Also, if we update hard_header_len or needed_headroom in
ipgre_link_update, would there be racing issues if they are updated
while skbs are being sent?

If these are indeed issues, it might not be easy to fix this driver.
Willem, do you have any thoughts?