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

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Date:   Mon, 01 Feb 2021 13:47:54 +0100
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 X25 <linux-x25@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] net: lapb: Copy the skb before sending a packet

On 2021-02-01 11:49, Xie He wrote:
> On Mon, Feb 1, 2021 at 2:05 AM Martin Schiller <ms@....tdt.de> wrote:
>> 
>> What kind of packages do you mean are corrupted?
>> ETH_P_X25 or ETH_P_HDLC?
> 
> I mean ETH_P_X25. I was using "lapbether.c" to test so there was no 
> ETH_P_HDLC.
> 
>> I have also sent a patch here in the past that addressed corrupted
>> ETH_P_X25 frames on an AF_PACKET socket:
>> 
>> https://lkml.org/lkml/2020/1/13/388
>> 
>> Unfortunately I could not track and describe exactly where the problem
>> was.
> 
> Ah... Looks like we had the same problem.
> 
>> I just wonder when/where is the logically correct place to copy the 
>> skb.
>> Shouldn't it be copied before removing the pseudo header (as I did in 
>> my
>> patch)?
> 
> I think it's not necessary to copy it before removing the pseudo
> header, because "skb_pull" will not change any data in the data
> buffer. "skb_pull" will only change the values of "skb->data" and
> "skb->len". These values are not shared between clones of skbs, so
> it's safe to change them without affecting other clones of the skb.
> 
> I also choose to copy the skb in the LAPB module (rather than in the
> drivers) because I see all drivers have this problem (including the
> recently deleted x25_asy.c driver), so it'd be better to fix this
> issue in the LAPB module, for all drivers.

OK.

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