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

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Message-ID: <CAM_iQpVv+OKb4483TOvdVrVXonLOvw9NAw=WApU5MnHmMdHz7Q@mail.gmail.com>
Date:   Mon, 12 Oct 2020 15:26:41 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Xie He <xie.he.0141@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        syzbot <syzbot+4a2c52677a8a1aa283cb@...kaller.appspotmail.com>,
        William Tu <u9012063@...il.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>
Subject: Re: [Patch net v2] ip_gre: set dev->hard_header_len and
 dev->needed_headroom properly

On Sun, Oct 11, 2020 at 3:46 PM Xie He <xie.he.0141@...il.com> wrote:
>
> On Sun, Oct 11, 2020 at 12:11 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > @@ -626,8 +626,7 @@ static netdev_tx_t ipgre_xmit(struct sk_buff *skb,
> >
> >         if (dev->header_ops) {
> >                 /* Need space for new headers */
> > -               if (skb_cow_head(skb, dev->needed_headroom -
> > -                                     (tunnel->hlen + sizeof(struct iphdr))))
> > +               if (skb_cow_head(skb, dev->hard_header_len))
> >                         goto free_skb;
> >
> >                 tnl_params = (const struct iphdr *)skb->data;
>
> As I understand, the skb_cow functions are for ensuring enough header
> space before skb->data. (Right?) However, at this stage our skb->data
> is already at the outer IP header, I think we don't need to request
> additional header space before the outer IP header.

Good point, I thought skb_headroom() == dev->hard_header_len,
but skb->data already points to the tunnel header like you said, so
we should pass 0 to skb_cow_head() here.

Thanks.