drivers/net/wan/x25_asy: Added needed_headroom and a skb->len check

1. Added a skb->len check

This driver expects upper layers to include a pseudo header of 1 byte
when passing down a skb for transmission. This driver will read this
1-byte header. This patch added a skb->len check before reading the
header to make sure the header exists.

2. Added needed_headroom

When this driver transmits data,
  first this driver will remove a pseudo header of 1 byte,
  then the lapb module will prepend the LAPB header of 2 or 3 bytes.
So the value of needed_headroom in this driver should be 3 - 1.

Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Martin Schiller <ms@dev.tdt.de>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wan/x25_asy.c

@@ -307,6 +307,14 @@ static netdev_tx_t x25_asy_xmit(struct sk_buff *skb,
 		return NETDEV_TX_OK;
 	}
 
+	/* There should be a pseudo header of 1 byte added by upper layers.
+	 * Check to make sure it is there before reading it.
+	 */
+	if (skb->len < 1) {
+		kfree_skb(skb);
+		return NETDEV_TX_OK;
+	}
+
 	switch (skb->data[0]) {
 	case X25_IFACE_DATA:
 		break;
@@ -752,6 +760,12 @@ static void x25_asy_setup(struct net_device *dev)
 	dev->type		= ARPHRD_X25;
 	dev->tx_queue_len	= 10;
 
+	/* When transmitting data:
+	 * first this driver removes a pseudo header of 1 byte,
+	 * then the lapb module prepends an LAPB header of at most 3 bytes.
+	 */
+	dev->needed_headroom	= 3 - 1;
+
 	/* New-style flags. */
 	dev->flags		= IFF_NOARP;
 }