[PATCH net-next v1 01/11] net: flow_dissector: gate BPF program lookup behind a static key

[Dave Seddon <dave.seddon.ca@gmail.com>]

__skb_flow_dissect() takes rcu_read_lock(), resolves the netns from
skb->dev/skb->sk, and does two dependent rcu_dereference() loads of
the netns-BPF run_array (init_net's first - a shared cacheline - then
the packet netns') on every dissect, even though the overwhelmingly
common case is that no BPF flow dissector program is attached
anywhere on the system.

Add a static key that counts attached flow dissector programs and
skip the whole lookup block when it is zero, exactly as
bpf_sk_lookup_enabled does for the sibling NETNS_BPF_SK_LOOKUP attach
type (also see cgroup_bpf_enabled_key[] for the same pattern per
cgroup-BPF attach type).

The sk_lookup type is link-only, so its inc/dec sit only in
netns_bpf_attach_type_need()/unneed(), which the link attach/release
paths already call. The flow dissector type also supports legacy
BPF_PROG_ATTACH, so additionally inc on a fresh (non-replacement)
prog attach, dec in __netns_bpf_prog_detach(), and dec for a
remaining legacy prog in netns_bpf_pernet_pre_exit() (links are
already handled by the existing per-link unneed loop there). All
these paths run under netns_bpf_mutex.

As with sk_lookup, there is a brief window where a just-attached
program can miss a concurrent dissect (key flips after
rcu_assign_pointer) or a just-detached one can cause a wasted lookup;
both are benign and match the existing semantics of the sibling keys.

DEBUG_NET_WARN_ON_ONCE(!net) moves inside the gated block: it exists
to catch a missing netns for the BPF hook, which is only meaningful
when a program can actually be attached.

With the key disabled this removes the rcu_read_lock/unlock pair, the
netns resolution, and both run_array loads from the per-packet path.

Assisted-by: Claude:claude-fable-5 sparse smatch
Signed-off-by: Dave Seddon <dave.seddon.ca@gmail.com>
---
 include/linux/skbuff.h     |   1 +
 kernel/bpf/net_namespace.c |  17 +++++-
 net/core/flow_dissector.c  | 104 +++++++++++++++++++++----------------
 3 files changed, 75 insertions(+), 47 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 22eda1d54a0e..6448c59dc807 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1588,6 +1588,7 @@ void skb_flow_dissector_init(struct flow_dissector *flow_dissector,
 			     unsigned int key_count);
 
 struct bpf_flow_dissector;
+extern struct static_key_false netns_bpf_flow_dissector_enabled;
 u32 bpf_flow_dissect(struct bpf_prog *prog, struct bpf_flow_dissector *ctx,
 		     __be16 proto, int nhoff, int hlen, unsigned int flags);
 
diff --git a/kernel/bpf/net_namespace.c b/kernel/bpf/net_namespace.c
index 25f30f9edaef..0d48ea65697e 100644
--- a/kernel/bpf/net_namespace.c
+++ b/kernel/bpf/net_namespace.c
@@ -28,6 +28,9 @@ DEFINE_MUTEX(netns_bpf_mutex);
 static void netns_bpf_attach_type_unneed(enum netns_bpf_attach_type type)
 {
 	switch (type) {
+	case NETNS_BPF_FLOW_DISSECTOR:
+		static_branch_dec(&netns_bpf_flow_dissector_enabled);
+		break;
 #ifdef CONFIG_INET
 	case NETNS_BPF_SK_LOOKUP:
 		static_branch_dec(&bpf_sk_lookup_enabled);
@@ -41,6 +44,9 @@ static void netns_bpf_attach_type_unneed(enum netns_bpf_attach_type type)
 static void netns_bpf_attach_type_need(enum netns_bpf_attach_type type)
 {
 	switch (type) {
+	case NETNS_BPF_FLOW_DISSECTOR:
+		static_branch_inc(&netns_bpf_flow_dissector_enabled);
+		break;
 #ifdef CONFIG_INET
 	case NETNS_BPF_SK_LOOKUP:
 		static_branch_inc(&bpf_sk_lookup_enabled);
@@ -352,6 +358,11 @@ int netns_bpf_prog_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 	net->bpf.progs[type] = prog;
 	if (attached)
 		bpf_prog_put(attached);
+	else
+		/* Mark attach point as used on a fresh attach; a
+		 * replacement keeps the existing count.
+		 */
+		netns_bpf_attach_type_need(type);
 
 out_unlock:
 	mutex_unlock(&netns_bpf_mutex);
@@ -373,6 +384,8 @@ static int __netns_bpf_prog_detach(struct net *net,
 	attached = net->bpf.progs[type];
 	if (!attached || attached != old)
 		return -ENOENT;
+	/* Mark attach point as unused */
+	netns_bpf_attach_type_unneed(type);
 	netns_bpf_run_array_detach(net, type);
 	net->bpf.progs[type] = NULL;
 	bpf_prog_put(attached);
@@ -546,8 +559,10 @@ static void __net_exit netns_bpf_pernet_pre_exit(struct net *net)
 			net_link->net = NULL; /* auto-detach link */
 			netns_bpf_attach_type_unneed(type);
 		}
-		if (net->bpf.progs[type])
+		if (net->bpf.progs[type]) {
+			netns_bpf_attach_type_unneed(type);
 			bpf_prog_put(net->bpf.progs[type]);
+		}
 	}
 	mutex_unlock(&netns_bpf_mutex);
 }
diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 8aa4f9b4df81..b8c59209b460 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -73,6 +73,14 @@ void skb_flow_dissector_init(struct flow_dissector *flow_dissector,
 }
 EXPORT_SYMBOL(skb_flow_dissector_init);
 
+/* Enabled while any netns has a BPF flow dissector program attached;
+ * lets __skb_flow_dissect() skip the program lookup entirely in the
+ * common no-program case. Maintained by the netns-BPF attach/detach
+ * paths in kernel/bpf/net_namespace.c.
+ */
+DEFINE_STATIC_KEY_FALSE(netns_bpf_flow_dissector_enabled);
+EXPORT_SYMBOL(netns_bpf_flow_dissector_enabled);
+
 #ifdef CONFIG_BPF_SYSCALL
 int flow_dissector_bpf_prog_attach_check(struct net *net,
 					 struct bpf_prog *prog)
@@ -1117,59 +1125,63 @@ bool __skb_flow_dissect(const struct net *net,
 					      FLOW_DISSECTOR_KEY_BASIC,
 					      target_container);
 
-	rcu_read_lock();
+	if (static_branch_unlikely(&netns_bpf_flow_dissector_enabled)) {
+		rcu_read_lock();
 
-	if (skb) {
-		if (!net) {
-			if (skb->dev)
-				net = dev_net_rcu(skb->dev);
-			else if (skb->sk)
-				net = sock_net(skb->sk);
+		if (skb) {
+			if (!net) {
+				if (skb->dev)
+					net = dev_net_rcu(skb->dev);
+				else if (skb->sk)
+					net = sock_net(skb->sk);
+			}
 		}
-	}
 
-	DEBUG_NET_WARN_ON_ONCE(!net);
-	if (net) {
-		enum netns_bpf_attach_type type = NETNS_BPF_FLOW_DISSECTOR;
-		struct bpf_prog_array *run_array;
-
-		run_array = rcu_dereference(init_net.bpf.run_array[type]);
-		if (!run_array)
-			run_array = rcu_dereference(net->bpf.run_array[type]);
-
-		if (run_array) {
-			struct bpf_flow_keys flow_keys;
-			struct bpf_flow_dissector ctx = {
-				.flow_keys = &flow_keys,
-				.data = data,
-				.data_end = data + hlen,
-			};
-			__be16 n_proto = proto;
-			struct bpf_prog *prog;
-			u32 result;
-
-			if (skb) {
-				ctx.skb = skb;
-				/* we can't use 'proto' in the skb case
-				 * because it might be set to skb->vlan_proto
-				 * which has been pulled from the data
-				 */
-				n_proto = skb->protocol;
-			}
+		DEBUG_NET_WARN_ON_ONCE(!net);
+		if (net) {
+			enum netns_bpf_attach_type type = NETNS_BPF_FLOW_DISSECTOR;
+			struct bpf_prog_array *run_array;
+
+			run_array = rcu_dereference(init_net.bpf.run_array[type]);
+			if (!run_array)
+				run_array = rcu_dereference(net->bpf.run_array[type]);
+
+			if (run_array) {
+				struct bpf_flow_keys flow_keys;
+				struct bpf_flow_dissector ctx = {
+					.flow_keys = &flow_keys,
+					.data = data,
+					.data_end = data + hlen,
+				};
+				__be16 n_proto = proto;
+				struct bpf_prog *prog;
+				u32 result;
+
+				if (skb) {
+					ctx.skb = skb;
+					/* we can't use 'proto' in the skb case
+					 * because it might be set to
+					 * skb->vlan_proto which has been pulled
+					 * from the data
+					 */
+					n_proto = skb->protocol;
+				}
 
-			prog = READ_ONCE(run_array->items[0].prog);
-			result = bpf_flow_dissect(prog, &ctx, n_proto, nhoff,
-						  hlen, flags);
-			if (result != BPF_FLOW_DISSECTOR_CONTINUE) {
-				__skb_flow_bpf_to_target(&flow_keys, flow_dissector,
-							 target_container);
-				rcu_read_unlock();
-				return result == BPF_OK;
+				prog = READ_ONCE(run_array->items[0].prog);
+				result = bpf_flow_dissect(prog, &ctx, n_proto,
+							  nhoff, hlen, flags);
+				if (result != BPF_FLOW_DISSECTOR_CONTINUE) {
+					__skb_flow_bpf_to_target(&flow_keys,
+								 flow_dissector,
+								 target_container);
+					rcu_read_unlock();
+					return result == BPF_OK;
+				}
 			}
 		}
-	}
 
-	rcu_read_unlock();
+		rcu_read_unlock();
+	}
 
 	if (dissector_uses_key(flow_dissector,
 			       FLOW_DISSECTOR_KEY_ETH_ADDRS)) {
-- 
2.54.0