linux-kernel - [PATCH bpf-next v4 02/12] skmsg: update saved hooks only once

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Message-Id: <20200309111243.6982-3-lmb@cloudflare.com>
Date:   Mon,  9 Mar 2020 11:12:33 +0000
From:   Lorenz Bauer <lmb@...udflare.com>
To:     John Fastabend <john.fastabend@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>
Cc:     kernel-team@...udflare.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH bpf-next v4 02/12] 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@...udflare.com>
Reviewed-by: Jakub Sitnicki <jakub@...udflare.com>
Acked-by: John Fastabend <john.fastabend@...il.com>
---
 include/linux/skmsg.h | 20 ++++++++++++++++----
 net/ipv4/tcp_bpf.c    | 16 +---------------
 2 files changed, 17 insertions(+), 19 deletions(-)

diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
index 4d3d75d63066..2be51b7a5800 100644
--- a/include/linux/skmsg.h
+++ b/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;
-	psock->saved_write_space = sk->sk_write_space;
+	/* 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 = orig;
+	}
 
-	psock->sk_proto = sk->sk_prot;
 	/* Pairs with lockless read in sk_clone_lock() */
 	WRITE_ONCE(sk->sk_prot, ops);
 }
diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
index 7d6e1b75d4d4..3327afa05c3d 100644
--- a/net/ipv4/tcp_bpf.c
+++ b/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();
 }
 
-- 
2.20.1