net: dsa: Give drivers the chance to veto certain upper devices

Some switches rely on unique pvids to ensure port separation in
standalone mode, because they don't have a port forwarding matrix
configurable in hardware. So, setups like a group of 2 uppers with the
same VLAN, swp0.100 and swp1.100, will cause traffic tagged with VLAN
100 to be autonomously forwarded between these switch ports, in spite
of there being no bridge between swp0 and swp1.

These drivers need to prevent this from happening. They need to have
VLAN filtering enabled in standalone mode (so they'll drop frames tagged
with unknown VLANs) and they can only accept an 8021q upper on a port as
long as it isn't installed on any other port too. So give them the
chance to veto bad user requests.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
[Kurt: Pass info instead of ptr]
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/dsa.h

@@ -536,6 +536,12 @@ struct dsa_switch_ops {
 	void	(*get_regs)(struct dsa_switch *ds, int port,
 			    struct ethtool_regs *regs, void *p);
 
+	/*
+	 * Upper device tracking.
+	 */
+	int	(*port_prechangeupper)(struct dsa_switch *ds, int port,
+				       struct netdev_notifier_changeupper_info *info);
+
 	/*
 	 * Bridge integration
 	 */

net/dsa/slave.c

@@ -2032,10 +2032,22 @@ static int dsa_slave_netdevice_event(struct notifier_block *nb,
 	switch (event) {
 	case NETDEV_PRECHANGEUPPER: {
 		struct netdev_notifier_changeupper_info *info = ptr;
+		struct dsa_switch *ds;
+		struct dsa_port *dp;
+		int err;
 
 		if (!dsa_slave_dev_check(dev))
 			return dsa_prevent_bridging_8021q_upper(dev, ptr);
 
+		dp = dsa_slave_to_port(dev);
+		ds = dp->ds;
+
+		if (ds->ops->port_prechangeupper) {
+			err = ds->ops->port_prechangeupper(ds, dp->index, info);
+			if (err)
+				return notifier_from_errno(err);
+		}
+
 		if (is_vlan_dev(info->upper_dev))
 			return dsa_slave_check_8021q_upper(dev, ptr);
 		break;