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

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Date:   Fri, 9 Oct 2020 12:51:21 -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 12:41 PM Xie He <xie.he.0141@...il.com> wrote:
>
> 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?

It seems to me that the original code includes t->encap_hlen,
t->tun_hlen, and the IP header length in needed_headroom. (Right?) If
we are including these in hard_header_len, we need to move them out of
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?