linux-kernel - Re: [PATCH 2/2] wan/hdlc

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Date:   Mon, 13 Jan 2020 05:44:21 -0800
From:   Jakub Kicinski <kubakici@...pl>
To:     Martin Schiller <ms@....tdt.de>
Cc:     khc@...waw.pl, davem@...emloft.net, linux-x25@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] wan/hdlc_x25: fix skb handling

On Mon, 13 Jan 2020 13:45:51 +0100, Martin Schiller wrote:
>  o call skb_reset_network_header() before hdlc->xmit()
>  o change skb proto to HDLC (0x0019) before hdlc->xmit()
>  o call dev_queue_xmit_nit() before hdlc->xmit()
> 
> This changes make it possible to trace (tcpdump) outgoing layer2
> (ETH_P_HDLC) packets
> 
>  o use a copy of the skb for lapb_data_request() in x25_xmit()

It's not clear to me why

> This fixes the problem, that tracing layer3 (ETH_P_X25) packets
> results in a malformed first byte of the packets.
> 
> Signed-off-by: Martin Schiller <ms@....tdt.de>
> ---
>  drivers/net/wan/hdlc_x25.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/wan/hdlc_x25.c b/drivers/net/wan/hdlc_x25.c
> index b28051eba736..434e5263eddf 100644
> --- a/drivers/net/wan/hdlc_x25.c
> +++ b/drivers/net/wan/hdlc_x25.c
> @@ -72,6 +72,7 @@ static int x25_data_indication(struct net_device *dev, struct sk_buff *skb)
>  	unsigned char *ptr;
>  
>  	skb_push(skb, 1);
> +	skb_reset_network_header(skb);
>  
>  	if (skb_cow(skb, 1))

This skb_cow() here is for the next handler down to have a 1 byte of
headroom guaranteed? It'd seem more natural to have skb_cow before the
push.. not that it's related to your patch.

>  		return NET_RX_DROP;
> @@ -88,6 +89,9 @@ static int x25_data_indication(struct net_device *dev, struct sk_buff *skb)
>  static void x25_data_transmit(struct net_device *dev, struct sk_buff *skb)
>  {
>  	hdlc_device *hdlc = dev_to_hdlc(dev);

Please insert a new line after the variable declaration since you're
touching this one.

> +	skb_reset_network_header(skb);
> +	skb->protocol = hdlc_type_trans(skb, dev);
> +	dev_queue_xmit_nit(skb, dev);
>  	hdlc->xmit(skb, dev); /* Ignore return value :-( */
>  }
>