[PATCH net-next 4/4] net, neigh: Add NTF_MANAGED flag for managed neighbor entries

[Daniel Borkmann <daniel@iogearbox.net>]

Allow a user space control plane to insert entries with a new NTF_EXT_MANAGED
flag. The flag then indicates to the kernel that the neighbor entry should be
periodically probed for keeping the entry in NUD_REACHABLE state iff possible.

The use case for this is targeting XDP or tc BPF load-balancers which use
the bpf_fib_lookup() BPF helper in order to piggyback on neighbor resolution
for their backends. Given they cannot be resolved in fast-path, a control
plane inserts the L3 (without L2) entries manually into the neighbor table
and lets the kernel do the neighbor resolution either on the gateway or on
the backend directly in case the latter resides in the same L2. This avoids
to deal with L2 in the control plane and to rebuild what the kernel already
does best anyway.

NTF_EXT_MANAGED can be combined with NTF_EXT_LEARNED in order to avoid GC
eviction. The kernel then adds NTF_MANAGED flagged entries to a per-neighbor
table which gets triggered by the system work queue to periodically call
neigh_event_send() for performing the resolution. The implementation allows
migration from/to NTF_MANAGED neighbor entries, so that already existing
entries can be converted by the control plane if needed. Potentially, we could
make the interval for periodically calling neigh_event_send() configurable;
right now it's set to DELAY_PROBE_TIME which is also in line with mlxsw which
has similar driver-internal infrastructure c723c735fa6b ("mlxsw: spectrum_router:
Periodically update the kernel's neigh table"). In future, the latter could
possibly reuse the NTF_MANAGED neighbors as well.

Example:

  # ./ip/ip n replace 192.168.178.30 dev enp5s0 managed extern_learn
  # ./ip/ip n
  192.168.178.30 dev enp5s0 lladdr f4:8c:50:5e:71:9a managed extern_learn REACHABLE
  [...]

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Roopa Prabhu <roopa@nvidia.com>
Link: https://linuxplumbersconf.org/event/11/contributions/953/
---
 include/net/neighbour.h        |  21 ++++--
 include/uapi/linux/neighbour.h |  34 ++++++----
 net/core/neighbour.c           | 113 ++++++++++++++++++++++++---------
 3 files changed, 120 insertions(+), 48 deletions(-)

diff --git a/include/net/neighbour.h b/include/net/neighbour.h
index 26d4ada0aea9..e8e48be66755 100644
--- a/include/net/neighbour.h
+++ b/include/net/neighbour.h
@@ -155,6 +155,7 @@ struct neighbour {
 	int			(*output)(struct neighbour *, struct sk_buff *);
 	const struct neigh_ops	*ops;
 	struct list_head	gc_list;
+	struct list_head	managed_list;
 	struct rcu_head		rcu;
 	struct net_device	*dev;
 	u8			primary_key[0];
@@ -216,11 +217,13 @@ struct neigh_table {
 	int			gc_thresh3;
 	unsigned long		last_flush;
 	struct delayed_work	gc_work;
+	struct delayed_work	managed_work;
 	struct timer_list 	proxy_timer;
 	struct sk_buff_head	proxy_queue;
 	atomic_t		entries;
 	atomic_t		gc_entries;
 	struct list_head	gc_list;
+	struct list_head	managed_list;
 	rwlock_t		lock;
 	unsigned long		last_rand;
 	struct neigh_statistics	__percpu *stats;
@@ -250,17 +253,21 @@ static inline void *neighbour_priv(const struct neighbour *n)
 }
 
 /* flags for neigh_update() */
-#define NEIGH_UPDATE_F_OVERRIDE			0x00000001
-#define NEIGH_UPDATE_F_WEAK_OVERRIDE		0x00000002
-#define NEIGH_UPDATE_F_OVERRIDE_ISROUTER	0x00000004
-#define NEIGH_UPDATE_F_USE			0x10000000
-#define NEIGH_UPDATE_F_EXT_LEARNED		0x20000000
-#define NEIGH_UPDATE_F_ISROUTER			0x40000000
-#define NEIGH_UPDATE_F_ADMIN			0x80000000
+#define NEIGH_UPDATE_F_OVERRIDE			BIT(0)
+#define NEIGH_UPDATE_F_WEAK_OVERRIDE		BIT(1)
+#define NEIGH_UPDATE_F_OVERRIDE_ISROUTER	BIT(2)
+#define NEIGH_UPDATE_F_USE			BIT(3)
+#define NEIGH_UPDATE_F_MANAGED			BIT(4)
+#define NEIGH_UPDATE_F_EXT_LEARNED		BIT(5)
+#define NEIGH_UPDATE_F_ISROUTER			BIT(6)
+#define NEIGH_UPDATE_F_ADMIN			BIT(7)
 
 /* In-kernel representation for NDA_FLAGS_EXT flags: */
 #define NTF_OLD_MASK		0xff
 #define NTF_EXT_SHIFT		8
+#define NTF_EXT_MASK		(NTF_EXT_MANAGED)
+
+#define NTF_MANAGED		(NTF_EXT_MANAGED << NTF_EXT_SHIFT)
 
 extern const struct nla_policy nda_policy[];
 
diff --git a/include/uapi/linux/neighbour.h b/include/uapi/linux/neighbour.h
index a80cca141855..db05fb55055e 100644
--- a/include/uapi/linux/neighbour.h
+++ b/include/uapi/linux/neighbour.h
@@ -41,14 +41,16 @@ enum {
  *	Neighbor Cache Entry Flags
  */
 
-#define NTF_USE		0x01
-#define NTF_SELF	0x02
-#define NTF_MASTER	0x04
-#define NTF_PROXY	0x08	/* == ATF_PUBL */
-#define NTF_EXT_LEARNED	0x10
-#define NTF_OFFLOADED   0x20
-#define NTF_STICKY	0x40
-#define NTF_ROUTER	0x80
+#define NTF_USE		(1 << 0)
+#define NTF_SELF	(1 << 1)
+#define NTF_MASTER	(1 << 2)
+#define NTF_PROXY	(1 << 3)	/* == ATF_PUBL */
+#define NTF_EXT_LEARNED	(1 << 4)
+#define NTF_OFFLOADED   (1 << 5)
+#define NTF_STICKY	(1 << 6)
+#define NTF_ROUTER	(1 << 7)
+/* Extended flags under NDA_FLAGS_EXT: */
+#define NTF_EXT_MANAGED	(1 << 0)
 
 /*
  *	Neighbor Cache Entry States.
@@ -66,12 +68,22 @@ enum {
 #define NUD_PERMANENT	0x80
 #define NUD_NONE	0x00
 
-/* NUD_NOARP & NUD_PERMANENT are pseudostates, they never change
- * and make no address resolution or NUD.
- * NUD_PERMANENT also cannot be deleted by garbage collectors.
+/* NUD_NOARP & NUD_PERMANENT are pseudostates, they never change and make no
+ * address resolution or NUD.
+ *
+ * NUD_PERMANENT also cannot be deleted by garbage collectors. This holds true
+ * for dynamic entries with NTF_EXT_LEARNED flag as well. However, upon carrier
+ * down event, NUD_PERMANENT entries are not flushed whereas NTF_EXT_LEARNED
+ * flagged entries explicitly are (which is also consistent with the routing
+ * subsystem).
+ *
  * When NTF_EXT_LEARNED is set for a bridge fdb entry the different cache entry
  * states don't make sense and thus are ignored. Such entries don't age and
  * can roam.
+ *
+ * NTF_EXT_MANAGED flagged neigbor entries are managed by the kernel on behalf
+ * of a user space control plane, and automatically refreshed so that (if
+ * possible) they remain in NUD_REACHABLE state.
  */
 
 struct nda_cacheinfo {
diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 5245e888c981..eae73efa9245 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -122,6 +122,8 @@ static void neigh_mark_dead(struct neighbour *n)
 		list_del_init(&n->gc_list);
 		atomic_dec(&n->tbl->gc_entries);
 	}
+	if (!list_empty(&n->managed_list))
+		list_del_init(&n->managed_list);
 }
 
 static void neigh_update_gc_list(struct neighbour *n)
@@ -130,7 +132,6 @@ static void neigh_update_gc_list(struct neighbour *n)
 
 	write_lock_bh(&n->tbl->lock);
 	write_lock(&n->lock);
-
 	if (n->dead)
 		goto out;
 
@@ -149,32 +150,59 @@ static void neigh_update_gc_list(struct neighbour *n)
 		list_add_tail(&n->gc_list, &n->tbl->gc_list);
 		atomic_inc(&n->tbl->gc_entries);
 	}
+out:
+	write_unlock(&n->lock);
+	write_unlock_bh(&n->tbl->lock);
+}
+
+static void neigh_update_managed_list(struct neighbour *n)
+{
+	bool on_managed_list, add_to_managed;
+
+	write_lock_bh(&n->tbl->lock);
+	write_lock(&n->lock);
+	if (n->dead)
+		goto out;
+
+	add_to_managed = n->flags & NTF_MANAGED;
+	on_managed_list = !list_empty(&n->managed_list);
 
+	if (!add_to_managed && on_managed_list)
+		list_del_init(&n->managed_list);
+	else if (add_to_managed && !on_managed_list)
+		list_add_tail(&n->managed_list, &n->tbl->managed_list);
 out:
 	write_unlock(&n->lock);
 	write_unlock_bh(&n->tbl->lock);
 }
 
-static bool neigh_update_ext_learned(struct neighbour *neigh, u32 flags,
-				     int *notify)
+static void neigh_update_flags(struct neighbour *neigh, u32 flags, int *notify,
+			       bool *gc_update, bool *managed_update)
 {
-	bool rc = false;
-	u32 ndm_flags;
+	u32 ndm_flags, old_flags = neigh->flags;
 
 	if (!(flags & NEIGH_UPDATE_F_ADMIN))
-		return rc;
+		return;
+
+	ndm_flags  = (flags & NEIGH_UPDATE_F_EXT_LEARNED) ? NTF_EXT_LEARNED : 0;
+	ndm_flags |= (flags & NEIGH_UPDATE_F_MANAGED) ? NTF_MANAGED : 0;
 
-	ndm_flags = (flags & NEIGH_UPDATE_F_EXT_LEARNED) ? NTF_EXT_LEARNED : 0;
-	if ((neigh->flags ^ ndm_flags) & NTF_EXT_LEARNED) {
+	if ((old_flags ^ ndm_flags) & NTF_EXT_LEARNED) {
 		if (ndm_flags & NTF_EXT_LEARNED)
 			neigh->flags |= NTF_EXT_LEARNED;
 		else
 			neigh->flags &= ~NTF_EXT_LEARNED;
-		rc = true;
 		*notify = 1;
+		*gc_update = true;
+	}
+	if ((old_flags ^ ndm_flags) & NTF_MANAGED) {
+		if (ndm_flags & NTF_MANAGED)
+			neigh->flags |= NTF_MANAGED;
+		else
+			neigh->flags &= ~NTF_MANAGED;
+		*notify = 1;
+		*managed_update = true;
 	}
-
-	return rc;
 }
 
 static bool neigh_del(struct neighbour *n, struct neighbour __rcu **np,
@@ -422,6 +450,7 @@ static struct neighbour *neigh_alloc(struct neigh_table *tbl,
 	refcount_set(&n->refcnt, 1);
 	n->dead		  = 1;
 	INIT_LIST_HEAD(&n->gc_list);
+	INIT_LIST_HEAD(&n->managed_list);
 
 	atomic_inc(&tbl->entries);
 out:
@@ -650,7 +679,8 @@ ___neigh_create(struct neigh_table *tbl, const void *pkey,
 	n->dead = 0;
 	if (!exempt_from_gc)
 		list_add_tail(&n->gc_list, &n->tbl->gc_list);
-
+	if (n->flags & NTF_MANAGED)
+		list_add_tail(&n->managed_list, &n->tbl->managed_list);
 	if (want_ref)
 		neigh_hold(n);
 	rcu_assign_pointer(n->next,
@@ -1205,8 +1235,6 @@ static void neigh_update_hhs(struct neighbour *neigh)
 	}
 }
 
-
-
 /* Generic update routine.
    -- lladdr is new lladdr or NULL, if it is not supplied.
    -- new    is new state.
@@ -1218,6 +1246,7 @@ static void neigh_update_hhs(struct neighbour *neigh)
 				if it is different.
 	NEIGH_UPDATE_F_ADMIN	means that the change is administrative.
 	NEIGH_UPDATE_F_USE	means that the entry is user triggered.
+	NEIGH_UPDATE_F_MANAGED	means that the entry will be auto-refreshed.
 	NEIGH_UPDATE_F_OVERRIDE_ISROUTER allows to override existing
 				NTF_ROUTER flag.
 	NEIGH_UPDATE_F_ISROUTER	indicates if the neighbour is known as
@@ -1225,17 +1254,15 @@ static void neigh_update_hhs(struct neighbour *neigh)
 
    Caller MUST hold reference count on the entry.
  */
-
 static int __neigh_update(struct neighbour *neigh, const u8 *lladdr,
 			  u8 new, u32 flags, u32 nlmsg_pid,
 			  struct netlink_ext_ack *extack)
 {
-	bool ext_learn_change = false;
-	u8 old;
-	int err;
-	int notify = 0;
-	struct net_device *dev;
+	bool gc_update = false, managed_update = false;
 	int update_isrouter = 0;
+	struct net_device *dev;
+	int err, notify = 0;
+	u8 old;
 
 	trace_neigh_update(neigh, lladdr, new, flags, nlmsg_pid);
 
@@ -1254,8 +1281,8 @@ static int __neigh_update(struct neighbour *neigh, const u8 *lladdr,
 	    (old & (NUD_NOARP | NUD_PERMANENT)))
 		goto out;
 
-	ext_learn_change = neigh_update_ext_learned(neigh, flags, &notify);
-	if (flags & NEIGH_UPDATE_F_USE) {
+	neigh_update_flags(neigh, flags, &notify, &gc_update, &managed_update);
+	if (flags & (NEIGH_UPDATE_F_USE | NEIGH_UPDATE_F_MANAGED)) {
 		new = old & ~NUD_PERMANENT;
 		neigh->nud_state = new;
 		err = 0;
@@ -1405,15 +1432,13 @@ static int __neigh_update(struct neighbour *neigh, const u8 *lladdr,
 	if (update_isrouter)
 		neigh_update_is_router(neigh, flags, &notify);
 	write_unlock_bh(&neigh->lock);
-
-	if (((new ^ old) & NUD_PERMANENT) || ext_learn_change)
+	if (((new ^ old) & NUD_PERMANENT) || gc_update)
 		neigh_update_gc_list(neigh);
-
+	if (managed_update)
+		neigh_update_managed_list(neigh);
 	if (notify)
 		neigh_update_notify(neigh, nlmsg_pid);
-
 	trace_neigh_update_done(neigh, err);
-
 	return err;
 }
 
@@ -1539,6 +1564,20 @@ int neigh_direct_output(struct neighbour *neigh, struct sk_buff *skb)
 }
 EXPORT_SYMBOL(neigh_direct_output);
 
+static void neigh_managed_work(struct work_struct *work)
+{
+	struct neigh_table *tbl = container_of(work, struct neigh_table,
+					       managed_work.work);
+	struct neighbour *neigh;
+
+	write_lock_bh(&tbl->lock);
+	list_for_each_entry(neigh, &tbl->managed_list, managed_list)
+		neigh_event_send(neigh, NULL);
+	queue_delayed_work(system_power_efficient_wq, &tbl->managed_work,
+			   NEIGH_VAR(&tbl->parms, DELAY_PROBE_TIME));
+	write_unlock_bh(&tbl->lock);
+}
+
 static void neigh_proxy_process(struct timer_list *t)
 {
 	struct neigh_table *tbl = from_timer(tbl, t, proxy_timer);
@@ -1685,6 +1724,8 @@ void neigh_table_init(int index, struct neigh_table *tbl)
 
 	INIT_LIST_HEAD(&tbl->parms_list);
 	INIT_LIST_HEAD(&tbl->gc_list);
+	INIT_LIST_HEAD(&tbl->managed_list);
+
 	list_add(&tbl->parms.list, &tbl->parms_list);
 	write_pnet(&tbl->parms.net, &init_net);
 	refcount_set(&tbl->parms.refcnt, 1);
@@ -1716,9 +1757,13 @@ void neigh_table_init(int index, struct neigh_table *tbl)
 		WARN_ON(tbl->entry_size % NEIGH_PRIV_ALIGN);
 
 	rwlock_init(&tbl->lock);
+
 	INIT_DEFERRABLE_WORK(&tbl->gc_work, neigh_periodic_work);
 	queue_delayed_work(system_power_efficient_wq, &tbl->gc_work,
 			tbl->parms.reachable_time);
+	INIT_DEFERRABLE_WORK(&tbl->managed_work, neigh_managed_work);
+	queue_delayed_work(system_power_efficient_wq, &tbl->managed_work, 0);
+
 	timer_setup(&tbl->proxy_timer, neigh_proxy_process, 0);
 	skb_queue_head_init_class(&tbl->proxy_queue,
 			&neigh_table_proxy_queue_class);
@@ -1891,7 +1936,7 @@ static int neigh_add(struct sk_buff *skb, struct nlmsghdr *nlh,
 	if (tb[NDA_FLAGS_EXT]) {
 		u32 ext = nla_get_u32(tb[NDA_FLAGS_EXT]);
 
-		if (ext & ~0) {
+		if (ext & ~NTF_EXT_MASK) {
 			NL_SET_ERR_MSG(extack, "Invalid extended flags");
 			goto out;
 		}
@@ -1927,6 +1972,11 @@ static int neigh_add(struct sk_buff *skb, struct nlmsghdr *nlh,
 	if (ndm_flags & NTF_PROXY) {
 		struct pneigh_entry *pn;
 
+		if (ndm_flags & NTF_MANAGED) {
+			NL_SET_ERR_MSG(extack, "Invalid NTF_* flag combination");
+			goto out;
+		}
+
 		err = -ENOBUFS;
 		pn = pneigh_lookup(tbl, net, dst, dev, 1);
 		if (pn) {
@@ -1960,7 +2010,8 @@ static int neigh_add(struct sk_buff *skb, struct nlmsghdr *nlh,
 		exempt_from_gc = ndm->ndm_state & NUD_PERMANENT ||
 				 ndm_flags & NTF_EXT_LEARNED;
 		neigh = ___neigh_create(tbl, dst, dev,
-					ndm_flags & NTF_EXT_LEARNED,
+					ndm_flags &
+					(NTF_EXT_LEARNED | NTF_MANAGED),
 					exempt_from_gc, true);
 		if (IS_ERR(neigh)) {
 			err = PTR_ERR(neigh);
@@ -1984,12 +2035,14 @@ static int neigh_add(struct sk_buff *skb, struct nlmsghdr *nlh,
 		flags |= NEIGH_UPDATE_F_EXT_LEARNED;
 	if (ndm_flags & NTF_ROUTER)
 		flags |= NEIGH_UPDATE_F_ISROUTER;
+	if (ndm_flags & NTF_MANAGED)
+		flags |= NEIGH_UPDATE_F_MANAGED;
 	if (ndm_flags & NTF_USE)
 		flags |= NEIGH_UPDATE_F_USE;
 
 	err = __neigh_update(neigh, lladdr, ndm->ndm_state, flags,
 			     NETLINK_CB(skb).portid, extack);
-	if (!err && ndm_flags & NTF_USE) {
+	if (!err && ndm_flags & (NTF_USE | NTF_MANAGED)) {
 		neigh_event_send(neigh, NULL);
 		err = 0;
 	}
-- 
2.27.0