When an fdb entry is configured as static and sticky it should never
roam. However there are times where it would be useful to know when
this happens so a user application can act on it. For this reason,
extend the fdb notification mechanism to send a notification when the
bridge detects a host that is attempting to roam when it has been
configured not to.

This is achieved by temporarily updating the fdb entry with the new
port, setting a new notify roaming bit, firing off a notification, and
restoring the original port immediately afterwards. The port remains
unchanged, respecting the sticky flag, but userspace is now notified
of the new port the host was seen on.

The roaming bit is cleared if the entry becomes inactive or if it is
replaced by a user entry.

Signed-off-by: Elliot Ayrey <elliot.ayrey@alliedtelesis.co.nz>
---
 include/uapi/linux/neighbour.h |  4 ++-
 net/bridge/br_fdb.c            | 64 +++++++++++++++++++++++-----------
 net/bridge/br_input.c          | 10 ++++--
 net/bridge/br_private.h        |  3 ++
 4 files changed, 58 insertions(+), 23 deletions(-)

diff --git a/include/uapi/linux/neighbour.h b/include/uapi/linux/neighbour.h
index 5e67a7eaf4a7..e1c686268808 100644
--- a/include/uapi/linux/neighbour.h
+++ b/include/uapi/linux/neighbour.h
@@ -201,10 +201,12 @@ enum {
  /* FDB activity notification bits used in NFEA_ACTIVITY_NOTIFY:
   * - FDB_NOTIFY_BIT - notify on activity/expire for any entry
   * - FDB_NOTIFY_INACTIVE_BIT - mark as inactive to avoid multiple notifications
+  * - FDB_NOTIFY_ROAMING_BIT - mark as attempting to roam
   */
 enum {
 	FDB_NOTIFY_BIT		= (1 << 0),
-	FDB_NOTIFY_INACTIVE_BIT	= (1 << 1)
+	FDB_NOTIFY_INACTIVE_BIT	= (1 << 1),
+	FDB_NOTIFY_ROAMING_BIT	= (1 << 2)
 };
 
 /* embedded into NDA_FDB_EXT_ATTRS:
diff --git a/net/bridge/br_fdb.c b/net/bridge/br_fdb.c
index 72663ca824d3..a8b841e74e15 100644
--- a/net/bridge/br_fdb.c
+++ b/net/bridge/br_fdb.c
@@ -145,6 +145,8 @@ static int fdb_fill_info(struct sk_buff *skb, const struct net_bridge *br,
 			goto nla_put_failure;
 		if (test_bit(BR_FDB_NOTIFY_INACTIVE, &fdb->flags))
 			notify_bits |= FDB_NOTIFY_INACTIVE_BIT;
+		if (test_bit(BR_FDB_NOTIFY_ROAMING, &fdb->flags))
+			notify_bits |= FDB_NOTIFY_ROAMING_BIT;
 
 		if (nla_put_u8(skb, NFEA_ACTIVITY_NOTIFY, notify_bits)) {
 			nla_nest_cancel(skb, nest);
@@ -554,8 +556,10 @@ void br_fdb_cleanup(struct work_struct *work)
 					work_delay = min(work_delay,
 							 this_timer - now);
 				else if (!test_and_set_bit(BR_FDB_NOTIFY_INACTIVE,
-							   &f->flags))
+							   &f->flags)) {
+					clear_bit(BR_FDB_NOTIFY_ROAMING, &f->flags);
 					fdb_notify(br, f, RTM_NEWNEIGH, false);
+				}
 			}
 			continue;
 		}
@@ -880,6 +884,19 @@ static bool __fdb_mark_active(struct net_bridge_fdb_entry *fdb)
 		  test_and_clear_bit(BR_FDB_NOTIFY_INACTIVE, &fdb->flags));
 }
 
+void br_fdb_notify_roaming(struct net_bridge *br, struct net_bridge_port *p,
+			   struct net_bridge_fdb_entry *fdb)
+{
+	struct net_bridge_port *old_p = READ_ONCE(fdb->dst);
+
+	if (test_bit(BR_FDB_NOTIFY, &fdb->flags) &&
+	    !test_and_set_bit(BR_FDB_NOTIFY_ROAMING, &fdb->flags)) {
+		WRITE_ONCE(fdb->dst, p);
+		fdb_notify(br, fdb, RTM_NEWNEIGH, false);
+		WRITE_ONCE(fdb->dst, old_p);
+	}
+}
+
 void br_fdb_update(struct net_bridge *br, struct net_bridge_port *source,
 		   const unsigned char *addr, u16 vid, unsigned long flags)
 {
@@ -906,21 +923,24 @@ void br_fdb_update(struct net_bridge *br, struct net_bridge_port *source,
 			}
 
 			/* fastpath: update of existing entry */
-			if (unlikely(source != READ_ONCE(fdb->dst) &&
-				     !test_bit(BR_FDB_STICKY, &fdb->flags))) {
-				br_switchdev_fdb_notify(br, fdb, RTM_DELNEIGH);
-				WRITE_ONCE(fdb->dst, source);
-				fdb_modified = true;
-				/* Take over HW learned entry */
-				if (unlikely(test_bit(BR_FDB_ADDED_BY_EXT_LEARN,
-						      &fdb->flags)))
-					clear_bit(BR_FDB_ADDED_BY_EXT_LEARN,
-						  &fdb->flags);
-				/* Clear locked flag when roaming to an
-				 * unlocked port.
-				 */
-				if (unlikely(test_bit(BR_FDB_LOCKED, &fdb->flags)))
-					clear_bit(BR_FDB_LOCKED, &fdb->flags);
+			if (unlikely(source != READ_ONCE(fdb->dst))) {
+				if (unlikely(test_bit(BR_FDB_STICKY, &fdb->flags))) {
+					br_fdb_notify_roaming(br, source, fdb);
+				} else {
+					br_switchdev_fdb_notify(br, fdb, RTM_DELNEIGH);
+					WRITE_ONCE(fdb->dst, source);
+					fdb_modified = true;
+					/* Take over HW learned entry */
+					if (unlikely(test_bit(BR_FDB_ADDED_BY_EXT_LEARN,
+							      &fdb->flags)))
+						clear_bit(BR_FDB_ADDED_BY_EXT_LEARN,
+							  &fdb->flags);
+					/* Clear locked flag when roaming to an
+					 * unlocked port.
+					 */
+					if (unlikely(test_bit(BR_FDB_LOCKED, &fdb->flags)))
+						clear_bit(BR_FDB_LOCKED, &fdb->flags);
+				}
 			}
 
 			if (unlikely(test_bit(BR_FDB_ADDED_BY_USER, &flags))) {
@@ -1045,6 +1065,7 @@ static bool fdb_handle_notify(struct net_bridge_fdb_entry *fdb, u8 notify)
 		   test_and_clear_bit(BR_FDB_NOTIFY, &fdb->flags)) {
 		/* disabled activity tracking, clear notify state */
 		clear_bit(BR_FDB_NOTIFY_INACTIVE, &fdb->flags);
+		clear_bit(BR_FDB_NOTIFY_ROAMING, &fdb->flags);
 		modified = true;
 	}
 
@@ -1457,10 +1478,13 @@ int br_fdb_external_learn_add(struct net_bridge *br, struct net_bridge_port *p,
 
 		fdb->updated = jiffies;
 
-		if (READ_ONCE(fdb->dst) != p &&
-		    !test_bit(BR_FDB_STICKY, &fdb->flags)) {
-			WRITE_ONCE(fdb->dst, p);
-			modified = true;
+		if (READ_ONCE(fdb->dst) != p) {
+			if (test_bit(BR_FDB_STICKY, &fdb->flags)) {
+				br_fdb_notify_roaming(br, p, fdb);
+			} else {
+				WRITE_ONCE(fdb->dst, p);
+				modified = true;
+			}
 		}
 
 		if (test_and_set_bit(BR_FDB_ADDED_BY_EXT_LEARN, &fdb->flags)) {
diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
index ceaa5a89b947..512ffab16f5d 100644
--- a/net/bridge/br_input.c
+++ b/net/bridge/br_input.c
@@ -120,8 +120,14 @@ int br_handle_frame_finish(struct net *net, struct sock *sk, struct sk_buff *skb
 				br_fdb_update(br, p, eth_hdr(skb)->h_source,
 					      vid, BIT(BR_FDB_LOCKED));
 			goto drop;
-		} else if (READ_ONCE(fdb_src->dst) != p ||
-			   test_bit(BR_FDB_LOCAL, &fdb_src->flags)) {
+		} else if (READ_ONCE(fdb_src->dst) != p) {
+			/* FDB is trying to roam. Notify userspace and drop
+			 * the packet
+			 */
+			if (test_bit(BR_FDB_STICKY, &fdb_src->flags))
+				br_fdb_notify_roaming(br, p, fdb_src);
+			goto drop;
+		} else if (test_bit(BR_FDB_LOCAL, &fdb_src->flags)) {
 			/* FDB mismatch. Drop the packet without roaming. */
 			goto drop;
 		} else if (test_bit(BR_FDB_LOCKED, &fdb_src->flags)) {
diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
index 041f6e571a20..18d3cb5fec0e 100644
--- a/net/bridge/br_private.h
+++ b/net/bridge/br_private.h
@@ -277,6 +277,7 @@ enum {
 	BR_FDB_NOTIFY_INACTIVE,
 	BR_FDB_LOCKED,
 	BR_FDB_DYNAMIC_LEARNED,
+	BR_FDB_NOTIFY_ROAMING,
 };
 
 struct net_bridge_fdb_key {
@@ -874,6 +875,8 @@ int br_fdb_external_learn_del(struct net_bridge *br, struct net_bridge_port *p,
 			      bool swdev_notify);
 void br_fdb_offloaded_set(struct net_bridge *br, struct net_bridge_port *p,
 			  const unsigned char *addr, u16 vid, bool offloaded);
+void br_fdb_notify_roaming(struct net_bridge *br, struct net_bridge_port *p,
+			   struct net_bridge_fdb_entry *fdb);
 
 /* br_forward.c */
 enum br_pkt_type {

> This is achieved by temporarily updating the fdb entry with the new
> port, setting a new notify roaming bit, firing off a notification, and
> restoring the original port immediately afterwards. The port remains
> unchanged, respecting the sticky flag, but userspace is now notified
> of the new port the host was seen on.

This sounds a bit hacky. Could you add a new optional attribute to the
netlink message indicating the roam destination, so there is no need
to play games with the actual port?

I'm not too deep into how these all works, but i also wounder about
backwards compatibility. Old code which does not look for
FDB_NOTIFY_ROAMING_BIT is going to think it really has moved, with
your code. By using a new attribute, and not changing the port, old
code just sees a notification it is on the port it always was on,
which is less likely to cause issues?

And do we want to differentiate between it wants to roam, but the
sticky bit has stopped that, and it really has roamed?

	Andrew