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

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Message-ID: <CAJht_EMf5i1qykEP6sZjLBcPAN9u9oQoZ34dfJ68Z5XL6rKuDQ@mail.gmail.com>
Date:   Wed, 5 Aug 2020 01:57:25 -0700
From:   Xie He <xie.he.0141@...il.com>
To:     Martin Schiller <ms@....tdt.de>
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 Tue, Aug 4, 2020 at 10:23 PM Martin Schiller <ms@....tdt.de> wrote:
>
> > 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>

Thank you so much!

Yes, it might be better to use skb_cow with a warning so that we can
prevent kernel panic while still being able to discover the problem.
If we want to do this, there are 2 more places in addition to
lapbeth_data_transmit that need to be guarded with skb_cow:
lapb_send_iframe and lapb_transmit_buffer in net/lapb/lapb_out.c.
Maybe we can address this in a separate patch.