netdev - Re: [PATCH net] drivers/net/wan/lapbether: Added needed

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Date:   Mon, 10 Aug 2020 09:31:43 +0200
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Xie He <xie.he.0141@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux X25 <linux-x25@...r.kernel.org>,
        Martin Schiller <ms@....tdt.de>
Subject: Re: [PATCH net] drivers/net/wan/lapbether: Added needed_tailroom

On Sun, Aug 9, 2020 at 7:12 PM Xie He <xie.he.0141@...il.com> wrote:
>
> On Sun, Aug 9, 2020 at 1:48 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > Does this solve an actual observed bug?
> >
> > In many ways lapbeth is similar to tunnel devices. This is not common.
>
> Thank you for your comment!
>
> This doesn't solve a bug observed by me. But I think this should be
> necessary considering the logic of the code.
>
> Using "grep", I found that there were indeed Ethernet drivers that set
> needed_tailroom. I found it was set in these files:
>     drivers/net/ethernet/sun/sunvnet.c
>     drivers/net/ethernet/sun/ldmvsw.c
> Setting needed_tailroom may be necessary for this driver to run those
> Ethernet devices.

What happens when a tunnel device passes a packet to these devices?
That will also not have allocated the extra tailroom. Does that cause
a bug?