Re: [PATCH 2/2] wan/hdlc_x25: fix skb handling

Jakub Kicinski
2020-01-13T13:44:33Z

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@dev.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 :-( */
> }
>