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

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Message-ID: <CA+FuTSf2r1eFoFHov-cGDG=5iLAUGt+Jv7Aok7EFrVo8UqkJog@mail.gmail.com>
Date:   Wed, 5 Aug 2020 11:33:10 +0200
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Xie He <xie.he.0141@...il.com>
Cc:     Martin Schiller <ms@....tdt.de>,
        "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 Wed, Aug 5, 2020 at 10:57 AM Xie He <xie.he.0141@...il.com> wrote:
>
> 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.

Let's not add defenses to work around possibly buggy code. In the long
run that reduces code quality. Instead, fix bugs at the source.