net: bridge: allow enslaving some DSA master network devices

Commit 8db0a2ee ("net: bridge: reject DSA-enabled master netdevices
as bridge members") added a special check in br_if.c in order to check
for a DSA master network device with a tagging protocol configured. This
was done because back then, such devices, once enslaved in a bridge
would become inoperative and would not pass DSA tagged traffic anymore
due to br_handle_frame returning RX_HANDLER_CONSUMED.

But right now we have valid use cases which do require bridging of DSA
masters. One such example is when the DSA master ports are DSA switch
ports themselves (in a disjoint tree setup). This should be completely
equivalent, functionally speaking, from having multiple DSA switches
hanging off of the ports of a switchdev driver. So we should allow the
enslaving of DSA tagged master network devices.

Instead of the regular br_handle_frame(), install a new function
br_handle_frame_dummy() on these DSA masters, which returns
RX_HANDLER_PASS in order to call into the DSA specific tagging protocol
handlers, and lift the restriction from br_add_if.
Suggested-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/dsa.h

@@ -651,7 +651,7 @@ struct dsa_switch_driver {
 struct net_device *dsa_dev_to_net_device(struct device *dev);
 
 /* Keep inline for faster access in hot path */
-static inline bool netdev_uses_dsa(struct net_device *dev)
+static inline bool netdev_uses_dsa(const struct net_device *dev)
 {
 #if IS_ENABLED(CONFIG_NET_DSA)
 	return dev->dsa_ptr && dev->dsa_ptr->rcv;

net/bridge/br_if.c

@@ -563,18 +563,32 @@ int br_add_if(struct net_bridge *br, struct net_device *dev,
 	unsigned br_hr, dev_hr;
 	bool changed_addr;
 
-	/* Don't allow bridging non-ethernet like devices, or DSA-enabled
-	 * master network devices since the bridge layer rx_handler prevents
-	 * the DSA fake ethertype handler to be invoked, so we do not strip off
-	 * the DSA switch tag protocol header and the bridge layer just return
-	 * RX_HANDLER_CONSUMED, stopping RX processing for these frames.
-	 */
+	/* Don't allow bridging non-ethernet like devices. */
 	if ((dev->flags & IFF_LOOPBACK) ||
 	    dev->type != ARPHRD_ETHER || dev->addr_len != ETH_ALEN ||
-	    !is_valid_ether_addr(dev->dev_addr) ||
-	    netdev_uses_dsa(dev))
+	    !is_valid_ether_addr(dev->dev_addr))
 		return -EINVAL;
 
+	/* Also don't allow bridging of net devices that are DSA masters, since
+	 * the bridge layer rx_handler prevents the DSA fake ethertype handler
+	 * to be invoked, so we don't get the chance to strip off and parse the
+	 * DSA switch tag protocol header (the bridge layer just returns
+	 * RX_HANDLER_CONSUMED, stopping RX processing for these frames).
+	 * The only case where that would not be an issue is when bridging can
+	 * already be offloaded, such as when the DSA master is itself a DSA
+	 * or plain switchdev port, and is bridged only with other ports from
+	 * the same hardware device.
+	 */
+	if (netdev_uses_dsa(dev)) {
+		list_for_each_entry(p, &br->port_list, list) {
+			if (!netdev_port_same_parent_id(dev, p->dev)) {
+				NL_SET_ERR_MSG(extack,
+					       "Cannot do software bridging with a DSA master");
+				return -EINVAL;
+			}
+		}
+	}
+
 	/* No bridging of bridges */
 	if (dev->netdev_ops->ndo_start_xmit == br_dev_xmit) {
 		NL_SET_ERR_MSG(extack,
@@ -618,7 +632,7 @@ int br_add_if(struct net_bridge *br, struct net_device *dev,
 	if (err)
 		goto err3;
 
-	err = netdev_rx_handler_register(dev, br_handle_frame, p);
+	err = netdev_rx_handler_register(dev, br_get_rx_handler(dev), p);
 	if (err)
 		goto err4;
 

net/bridge/br_input.c

@@ -17,6 +17,7 @@
 #endif
 #include <linux/neighbour.h>
 #include <net/arp.h>
+#include <net/dsa.h>
 #include <linux/export.h>
 #include <linux/rculist.h>
 #include "br_private.h"
@@ -257,7 +258,7 @@ static int nf_hook_bridge_pre(struct sk_buff *skb, struct sk_buff **pskb)
  * Return NULL if skb is handled
  * note: already called with rcu_read_lock
  */
-rx_handler_result_t br_handle_frame(struct sk_buff **pskb)
+static rx_handler_result_t br_handle_frame(struct sk_buff **pskb)
 {
 	struct net_bridge_port *p;
 	struct sk_buff *skb = *pskb;
@@ -359,3 +360,23 @@ rx_handler_result_t br_handle_frame(struct sk_buff **pskb)
 	}
 	return RX_HANDLER_CONSUMED;
 }
+
+/* This function has no purpose other than to appease the br_port_get_rcu/rtnl
+ * helpers which identify bridged ports according to the rx_handler installed
+ * on them (so there _needs_ to be a bridge rx_handler even if we don't need it
+ * to do anything useful). This bridge won't support traffic to/from the stack,
+ * but only hardware bridging. So return RX_HANDLER_PASS so we don't steal
+ * frames from the ETH_P_XDSA packet_type handler.
+ */
+static rx_handler_result_t br_handle_frame_dummy(struct sk_buff **pskb)
+{
+	return RX_HANDLER_PASS;
+}
+
+rx_handler_func_t *br_get_rx_handler(const struct net_device *dev)
+{
+	if (netdev_uses_dsa(dev))
+		return br_handle_frame_dummy;
+
+	return br_handle_frame;
+}

net/bridge/br_private.h

@@ -702,16 +702,16 @@ int nbp_backup_change(struct net_bridge_port *p, struct net_device *backup_dev);
 
 /* br_input.c */
 int br_handle_frame_finish(struct net *net, struct sock *sk, struct sk_buff *skb);
-rx_handler_result_t br_handle_frame(struct sk_buff **pskb);
+rx_handler_func_t *br_get_rx_handler(const struct net_device *dev);
 
 static inline bool br_rx_handler_check_rcu(const struct net_device *dev)
 {
-	return rcu_dereference(dev->rx_handler) == br_handle_frame;
+	return rcu_dereference(dev->rx_handler) == br_get_rx_handler(dev);
 }
 
 static inline bool br_rx_handler_check_rtnl(const struct net_device *dev)
 {
-	return rcu_dereference_rtnl(dev->rx_handler) == br_handle_frame;
+	return rcu_dereference_rtnl(dev->rx_handler) == br_get_rx_handler(dev);
 }
 
 static inline struct net_bridge_port *br_port_get_check_rcu(const struct net_device *dev)