irda: irttp: allow zero byte packets

Sending zero byte packets is not neccessarily an error (AF_INET accepts it,
too), so just apply a shortcut. This was discovered because of a non-working
software with WINE. See

http://bugs.winehq.org/show_bug.cgi?id=19397#c86
http://thread.gmane.org/gmane.linux.irda.general/1643

for very detailed debugging information and a testcase. Kudos to Wolfgang for
those!

Reported-by: Wolfgang Schwotzer <[e-mail]>
Signed-off-by: Wolfram Sang <[e-mail]>
Tested-by: Mike Evans <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/irda/irttp.c

···
  */
 int irttp_udata_request(struct tsap_cb *self, struct sk_buff *skb)
 {
+	int ret = -1;
+
 	IRDA_ASSERT(self != NULL, return -1;);
 	IRDA_ASSERT(self->magic == TTP_TSAP_MAGIC, return -1;);
 	IRDA_ASSERT(skb != NULL, return -1;);
 
 	IRDA_DEBUG(4, "%s()\n", __func__);
 
+	/* Take shortcut on zero byte packets */
+	if (skb->len == 0) {
+		ret = 0;
+		goto err;
+	}
+
 	/* Check that nothing bad happens */
-	if ((skb->len == 0) || (!self->connected)) {
-		IRDA_DEBUG(1, "%s(), No data, or not connected\n",
-			   __func__);
+	if (!self->connected) {
+		IRDA_DEBUG(1, "%s(), Not connected\n", __func__);
 		goto err;
 	}
 
···
 
 err:
 	dev_kfree_skb(skb);
-	return -1;
+	return ret;
 }
 EXPORT_SYMBOL(irttp_udata_request);
 
···
 	IRDA_DEBUG(2, "%s() : queue len = %d\n", __func__,
 		   skb_queue_len(&self->tx_queue));
 
+	/* Take shortcut on zero byte packets */
+	if (skb->len == 0) {
+		ret = 0;
+		goto err;
+	}
+
 	/* Check that nothing bad happens */
-	if ((skb->len == 0) || (!self->connected)) {
-		IRDA_WARNING("%s: No data, or not connected\n", __func__);
+	if (!self->connected) {
+		IRDA_WARNING("%s: Not connected\n", __func__);
 		ret = -ENOTCONN;
 		goto err;
 	}