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

Martin Schiller
2020-01-14T08:10:44Z

On 2020-01-13 14:44, Jakub Kicinski wrote:
> 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

Well, this patch is ported form an older environment which is based on
linux-3.4. I can't reproduce the misbehavior with actual version, so I
will drop this part of the patch.

>
>> 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.

Thanks for the hint. I will move the skb_cow() before the skb_push().

>
>> 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.

OK, will do.

>
>> + skb_reset_network_header(skb);
>> + skb->protocol = hdlc_type_trans(skb, dev);

I will also insert an "if (dev_nit_active(dev))" here.

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