can: dev: __can_get_echo_skb(): print error message, if trying to echo non existing skb

Prior to echoing a successfully transmitted CAN frame (by calling
can_get_echo_skb()), CAN drivers have to put the CAN frame (by calling
can_put_echo_skb() in the transmit function). These put and get function
take an index as parameter, which is used to identify the CAN frame.

A driver calling can_get_echo_skb() with a index not pointing to a skb
is a BUG, so add an appropriate error message.

Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/dev.c

@@ -480,6 +480,8 @@ EXPORT_SYMBOL_GPL(can_put_echo_skb);
 struct sk_buff *__can_get_echo_skb(struct net_device *dev, unsigned int idx, u8 *len_ptr)
 {
 	struct can_priv *priv = netdev_priv(dev);
+	struct sk_buff *skb = priv->echo_skb[idx];
+	struct canfd_frame *cf;
 
 	if (idx >= priv->echo_skb_max) {
 		netdev_err(dev, "%s: BUG! Trying to access can_priv::echo_skb out of bounds (%u/max %u)\n",
@@ -487,21 +489,20 @@ struct sk_buff *__can_get_echo_skb(struct net_device *dev, unsigned int idx, u8
 		return NULL;
 	}
 
-	if (priv->echo_skb[idx]) {
-		/* Using "struct canfd_frame::len" for the frame
-		 * length is supported on both CAN and CANFD frames.
-		 */
-		struct sk_buff *skb = priv->echo_skb[idx];
-		struct canfd_frame *cf = (struct canfd_frame *)skb->data;
-		u8 len = cf->len;
-
-		*len_ptr = len;
-		priv->echo_skb[idx] = NULL;
-
-		return skb;
+	if (!skb) {
+		netdev_err(dev, "%s: BUG! Trying to echo non existing skb: can_priv::echo_skb[%u]\n",
+			   __func__, idx);
+		return NULL;
 	}
 
-	return NULL;
+	/* Using "struct canfd_frame::len" for the frame
+	 * length is supported on both CAN and CANFD frames.
+	 */
+	cf = (struct canfd_frame *)skb->data;
+	*len_ptr = cf->len;
+	priv->echo_skb[idx] = NULL;
+
+	return skb;
 }
 
 /*