skmsg: Update saved hooks only once

Only update psock->saved_* if psock->sk_proto has not been initialized
yet. This allows us to get rid of tcp_bpf_reinit_sk_prot.
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200309111243.6982-3-lmb@cloudflare.com

include/linux/skmsg.h

@@ -347,11 +347,23 @@ static inline void sk_psock_update_proto(struct sock *sk,
 					 struct sk_psock *psock,
 					 struct proto *ops)
 {
-	psock->saved_unhash = sk->sk_prot->unhash;
-	psock->saved_close = sk->sk_prot->close;
+	/* Initialize saved callbacks and original proto only once, since this
+	 * function may be called multiple times for a psock, e.g. when
+	 * psock->progs.msg_parser is updated.
+	 *
+	 * Since we've not installed the new proto, psock is not yet in use and
+	 * we can initialize it without synchronization.
+	 */
+	if (!psock->sk_proto) {
+		struct proto *orig = READ_ONCE(sk->sk_prot);
+
+		psock->saved_unhash = orig->unhash;
+		psock->saved_close = orig->close;
 		psock->saved_write_space = sk->sk_write_space;
 
-	psock->sk_proto = sk->sk_prot;
+		psock->sk_proto = orig;
+	}
+
 	/* Pairs with lockless read in sk_clone_lock() */
 	WRITE_ONCE(sk->sk_prot, ops);
 }

net/ipv4/tcp_bpf.c

@@ -637,20 +637,6 @@ static void tcp_bpf_update_sk_prot(struct sock *sk, struct sk_psock *psock)
 	sk_psock_update_proto(sk, psock, &tcp_bpf_prots[family][config]);
 }
 
-static void tcp_bpf_reinit_sk_prot(struct sock *sk, struct sk_psock *psock)
-{
-	int family = sk->sk_family == AF_INET6 ? TCP_BPF_IPV6 : TCP_BPF_IPV4;
-	int config = psock->progs.msg_parser   ? TCP_BPF_TX   : TCP_BPF_BASE;
-
-	/* Reinit occurs when program types change e.g. TCP_BPF_TX is removed
-	 * or added requiring sk_prot hook updates. We keep original saved
-	 * hooks in this case.
-	 *
-	 * Pairs with lockless read in sk_clone_lock().
-	 */
-	WRITE_ONCE(sk->sk_prot, &tcp_bpf_prots[family][config]);
-}
-
 static int tcp_bpf_assert_proto_ops(struct proto *ops)
 {
 	/* In order to avoid retpoline, we make assumptions when we call
@@ -670,7 +656,7 @@ void tcp_bpf_reinit(struct sock *sk)
 
 	rcu_read_lock();
 	psock = sk_psock(sk);
-	tcp_bpf_reinit_sk_prot(sk, psock);
+	tcp_bpf_update_sk_prot(sk, psock);
 	rcu_read_unlock();
 }