drivers/net/wan/hdlc: Set skb->protocol before transmitting

This patch sets skb->protocol before transmitting frames on the HDLC
device, so that a user listening on the HDLC device with an AF_PACKET
socket will see outgoing frames' sll_protocol field correctly set and
consistent with that of incoming frames.

1. Control frames in hdlc_cisco and hdlc_ppp

When these drivers send control frames, skb->protocol is not set.

This value should be set to htons(ETH_P_HDLC), because when receiving
control frames, their skb->protocol is set to htons(ETH_P_HDLC).

When receiving, hdlc_type_trans in hdlc.h is called, which then calls
cisco_type_trans or ppp_type_trans. The skb->protocol of control frames
is set to htons(ETH_P_HDLC) so that the control frames can be received
by hdlc_rcv in hdlc.c, which calls cisco_rx or ppp_rx to process the
control frames.

2. hdlc_fr

When this driver sends control frames, skb->protocol is set to internal
values used in this driver.

When this driver sends data frames (from upper stacked PVC devices),
skb->protocol is the same as that of the user data packet being sent on
the upper PVC device (for normal PVC devices), or is htons(ETH_P_802_3)
(for Ethernet-emulating PVC devices).

However, skb->protocol for both control frames and data frames should be
set to htons(ETH_P_HDLC), because when receiving, all frames received on
the HDLC device will have their skb->protocol set to htons(ETH_P_HDLC).

When receiving, hdlc_type_trans in hdlc.h is called, and because this
driver doesn't provide a type_trans function in struct hdlc_proto,
all frames will have their skb->protocol set to htons(ETH_P_HDLC).
The frames are then received by hdlc_rcv in hdlc.c, which calls fr_rx
to process the frames (control frames are consumed and data frames
are re-received on upper PVC devices).

Cc: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wan/hdlc_cisco.c

@@ -118,6 +118,7 @@ static void cisco_keepalive_send(struct net_device *dev, u32 type,
 	skb_put(skb, sizeof(struct cisco_packet));
 	skb->priority = TC_PRIO_CONTROL;
 	skb->dev = dev;
+	skb->protocol = htons(ETH_P_HDLC);
 	skb_reset_network_header(skb);
 
 	dev_queue_xmit(skb);

drivers/net/wan/hdlc_fr.c

@@ -433,6 +433,8 @@ static netdev_tx_t pvc_xmit(struct sk_buff *skb, struct net_device *dev)
 			if (pvc->state.fecn) /* TX Congestion counter */
 				dev->stats.tx_compressed++;
 			skb->dev = pvc->frad;
+			skb->protocol = htons(ETH_P_HDLC);
+			skb_reset_network_header(skb);
 			dev_queue_xmit(skb);
 			return NETDEV_TX_OK;
 		}
@@ -555,6 +557,7 @@ static void fr_lmi_send(struct net_device *dev, int fullrep)
 	skb_put(skb, i);
 	skb->priority = TC_PRIO_CONTROL;
 	skb->dev = dev;
+	skb->protocol = htons(ETH_P_HDLC);
 	skb_reset_network_header(skb);
 
 	dev_queue_xmit(skb);

drivers/net/wan/hdlc_ppp.c

@@ -251,6 +251,7 @@ static void ppp_tx_cp(struct net_device *dev, u16 pid, u8 code,
 
 	skb->priority = TC_PRIO_CONTROL;
 	skb->dev = dev;
+	skb->protocol = htons(ETH_P_HDLC);
 	skb_reset_network_header(skb);
 	skb_queue_tail(&tx_queue, skb);
 }