can: export a safe netdev_priv wrapper for candev

In net_device notifier calls, it was impossible to determine
if a CAN device is based on candev in a safe way.
This patch adds such test in order to access candev storage
from within those notifiers.

Signed-off-by: Kurt Van Dijck <[e-mail]>
Acked-by: Oliver Hartkopp <[e-mail]>
Signed-off-by: Fabio Baltieri <[e-mail]>
Signed-off-by: Marc Kleine-Budde <[e-mail]>

drivers/net/can/dev.c

···
 }
 EXPORT_SYMBOL_GPL(unregister_candev);
 
+/*
+ * Test if a network device is a candev based device
+ * and return the can_priv* if so.
+ */
+struct can_priv *safe_candev_priv(struct net_device *dev)
+{
+	if ((dev->type != ARPHRD_CAN) || (dev->rtnl_link_ops != &can_link_ops))
+		return NULL;
+
+	return netdev_priv(dev);
+}
+EXPORT_SYMBOL_GPL(safe_candev_priv);
+
 static __init int can_dev_init(void)
 {
 	int err;

include/linux/can/dev.h

···
 struct net_device *alloc_candev(int sizeof_priv, unsigned int echo_skb_max);
 void free_candev(struct net_device *dev);
 
+/* a candev safe wrapper around netdev_priv */
+struct can_priv *safe_candev_priv(struct net_device *dev);
+
 int open_candev(struct net_device *dev);
 void close_candev(struct net_device *dev);