netdev - Re: [net v3] drivers/net/wan/lapbether: Use needed_headroom instead of hard_header

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Date:   Wed, 05 Aug 2020 07:23:55 +0200
From:   Martin Schiller <ms@....tdt.de>
To:     Xie He <xie.he.0141@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux X25 <linux-x25@...r.kernel.org>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Brian Norris <briannorris@...omium.org>,
        netdev-owner@...r.kernel.org
Subject: Re: [net v3] drivers/net/wan/lapbether: Use needed_headroom instead
 of hard_header_len

On 2020-08-04 21:20, Xie He wrote:
> On Tue, Aug 4, 2020 at 5:43 AM Martin Schiller <ms@....tdt.de> wrote:
>> 
>> I'm not an expert in the field, but after reading the commit message 
>> and
>> the previous comments, I'd say that makes sense.
> 
> Thanks!
> 
>> Shouldn't this kernel panic be intercepted by a skb_cow() before the
>> skb_push() in lapbeth_data_transmit()?
> 
> When a skb is passing down a protocol stack for transmission, there
> might be several different skb_push calls to prepend different
> headers. It would be the best (in terms of performance) if we can
> allocate the needed header space in advance, so that we don't need to
> reallocate the skb every time a new header needs to be prepended.

Yes, I agree.

> Adding skb_cow before these skb_push calls would indeed help
> preventing kernel panics, but that might not be the essential issue
> here, and it might also prevent us from discovering the real issue. (I
> guess this is also the reason skb_cow is not included in skb_push
> itself.)

Well, you are right that the panic is "useful" to discover the real
problem. But on the other hand, if it is possible to prevent a panic, I
think we should do so. Maybe with adding a warning, when skb_cow() needs
to reallocate memory.

But this is getting a little bit off topic. For this patch I can say:

LGTM.

Reviewed-by: Martin Schiller <ms@....tdt.de>