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

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Message-ID: <CAM_iQpU1hU0Wg9sdTwFAG17Gk4-85+=xvZdQeb3oswhBKtAsPA@mail.gmail.com>
Date:   Fri, 9 Oct 2020 13:38:52 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Xie He <xie.he.0141@...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 12:51 PM Xie He <xie.he.0141@...il.com> wrote:
>
> 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.

Interesting point. I think needed_headroom is 0 until we call
ipgre_changelink(), but needed_headroom is already being used
in multiple places for skb_cow_head() in the same file, I guess
they should be replaced with hard_head_len because for GRE tunnel
those are its link-layer header. What makes it more complicated
is that header_ops is actually set conditionally, so should be
hard_header_len/needed_head_room accordingly.

Thanks.