netdev - Re: [PATCH net] net: lapb: Copy the skb before sending a packet

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Date:   Tue, 2 Feb 2021 08:41:40 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Xie He <xie.he.0141@...il.com>
Cc:     Julian Wiedmann <jwi@...ux.ibm.com>,
        "David S. Miller" <davem@...emloft.net>,
        Linux X25 <linux-x25@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Martin Schiller <ms@....tdt.de>
Subject: Re: [PATCH net] net: lapb: Copy the skb before sending a packet

On Mon, 1 Feb 2021 22:25:17 -0800 Xie He wrote:
> On Mon, Feb 1, 2021 at 8:42 PM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Mon, 1 Feb 2021 08:14:31 -0800 Xie He wrote:  
> > > On Mon, Feb 1, 2021 at 6:10 AM Julian Wiedmann <jwi@...ux.ibm.com> wrote:  
>  [...]  
> > >
> > > Calling "skb_cow_head" before we call "skb_clone" would indeed solve
> > > the problem of writes to our clones affecting clones in other parts of
> > > the system. But since we are still writing to the skb after
> > > "skb_clone", it'd still be better to replace "skb_clone" with
> > > "skb_copy" to avoid interference between our own clones.  
> >
> > Why call skb_cow_head() before skb_clone()? skb_cow_head should be
> > called before the data in skb head is modified. I'm assuming you're only
> > modifying "front" of the frame, right? skb_cow_head() should do nicely
> > in that case.  
> 
> The modification happens after skb_clone. If we call skb_cow_head
> after skb_clone (before the modification), then skb_cow_head would
> always see that the skb is a clone and would always copy it. Therefore
> skb_clone + skb_cow_head is equivalent to skb_copy.

You're right. I thought cow_head is a little more clever.