[PATCH RFC 1/5] net: Add distinct sk_psock field

[Chuck Lever <chuck.lever@oracle.com>]

The sk_psock facility populates the sk_user_data field with the
address of an extra bit of metadata. User space sockets never
populate the sk_user_data field, so this has worked out fine.

However, kernel consumers such as the RPC client and server do
populate the sk_user_data field. The sk_psock() function cannot tell
that the content of sk_user_data does not point to psock metadata,
so it will happily return a pointer to something else, cast to a
struct sk_psock.

Thus kernel consumers and psock currently cannot co-exist.

We could educate sk_psock() to return NULL if sk_user_data does
not point to a struct sk_psock. However, a more general solution
that enables full co-existence psock and other uses of sk_user_data
might be more interesting.

Move the struct sk_psock address to its own pointer field so that
the contents of the sk_user_data field is preserved.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 include/linux/skmsg.h |    2 +-
 include/net/sock.h    |    4 +++-
 net/core/skmsg.c      |    6 +++---
 3 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
index c5a2d6f50f25..5ef3a07c5b6c 100644
--- a/include/linux/skmsg.h
+++ b/include/linux/skmsg.h
@@ -277,7 +277,7 @@ static inline void sk_msg_sg_copy_clear(struct sk_msg *msg, u32 start)
 
 static inline struct sk_psock *sk_psock(const struct sock *sk)
 {
-	return rcu_dereference_sk_user_data(sk);
+	return rcu_dereference(sk->sk_psock);
 }
 
 static inline void sk_psock_set_state(struct sk_psock *psock,
diff --git a/include/net/sock.h b/include/net/sock.h
index c4b91fc19b9c..d2a513169527 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -327,7 +327,8 @@ struct sk_filter;
   *	@sk_tskey: counter to disambiguate concurrent tstamp requests
   *	@sk_zckey: counter to order MSG_ZEROCOPY notifications
   *	@sk_socket: Identd and reporting IO signals
-  *	@sk_user_data: RPC layer private data
+  *	@sk_user_data: Upper layer private data
+  *	@sk_psock: socket policy data (bpf)
   *	@sk_frag: cached page frag
   *	@sk_peek_off: current peek_offset value
   *	@sk_send_head: front of stuff to transmit
@@ -519,6 +520,7 @@ struct sock {
 
 	struct socket		*sk_socket;
 	void			*sk_user_data;
+	struct sk_psock	__rcu	*sk_psock;
 #ifdef CONFIG_SECURITY
 	void			*sk_security;
 #endif
diff --git a/net/core/skmsg.c b/net/core/skmsg.c
index cc381165ea08..2b3d01d92790 100644
--- a/net/core/skmsg.c
+++ b/net/core/skmsg.c
@@ -695,7 +695,7 @@ struct sk_psock *sk_psock_init(struct sock *sk, int node)
 
 	write_lock_bh(&sk->sk_callback_lock);
 
-	if (sk->sk_user_data) {
+	if (sk->sk_psock) {
 		psock = ERR_PTR(-EBUSY);
 		goto out;
 	}
@@ -726,7 +726,7 @@ struct sk_psock *sk_psock_init(struct sock *sk, int node)
 	sk_psock_set_state(psock, SK_PSOCK_TX_ENABLED);
 	refcount_set(&psock->refcnt, 1);
 
-	rcu_assign_sk_user_data_nocopy(sk, psock);
+	rcu_assign_pointer(sk->sk_psock, psock);
 	sock_hold(sk);
 
 out:
@@ -825,7 +825,7 @@ void sk_psock_drop(struct sock *sk, struct sk_psock *psock)
 {
 	write_lock_bh(&sk->sk_callback_lock);
 	sk_psock_restore_proto(sk, psock);
-	rcu_assign_sk_user_data(sk, NULL);
+	rcu_assign_pointer(sk->sk_psock, NULL);
 	if (psock->progs.stream_parser)
 		sk_psock_stop_strp(sk, psock);
 	else if (psock->progs.stream_verdict || psock->progs.skb_verdict)