net: add sock_init_data_uid()

Add sock_init_data_uid() to explicitly initialize the socket uid.
To initialise the socket uid, sock_init_data() assumes a the struct
socket* sock is always embedded in a struct socket_alloc, used to
access the corresponding inode uid. This may not be true.
Examples are sockets created in tun_chr_open() and tap_open().

Fixes: 86741ec2 ("net: core: Add a UID field to struct sock.")
Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -1956,7 +1956,12 @@ void sk_common_release(struct sock *sk);
  *	Default socket callbacks and setup code
  */
 
-/* Initialise core socket variables */
+/* Initialise core socket variables using an explicit uid. */
+void sock_init_data_uid(struct socket *sock, struct sock *sk, kuid_t uid);
+
+/* Initialise core socket variables.
+ * Assumes struct socket *sock is embedded in a struct socket_alloc.
+ */
 void sock_init_data(struct socket *sock, struct sock *sk);
 
 /*

net/core/sock.c

@@ -3383,7 +3383,7 @@ void sk_stop_timer_sync(struct sock *sk, struct timer_list *timer)
 }
 EXPORT_SYMBOL(sk_stop_timer_sync);
 
-void sock_init_data(struct socket *sock, struct sock *sk)
+void sock_init_data_uid(struct socket *sock, struct sock *sk, kuid_t uid)
 {
 	sk_init_common(sk);
 	sk->sk_send_head	=	NULL;
@@ -3403,11 +3403,10 @@ void sock_init_data(struct socket *sock, struct sock *sk)
 		sk->sk_type	=	sock->type;
 		RCU_INIT_POINTER(sk->sk_wq, &sock->wq);
 		sock->sk	=	sk;
-		sk->sk_uid	=	SOCK_INODE(sock)->i_uid;
 	} else {
 		RCU_INIT_POINTER(sk->sk_wq, NULL);
-		sk->sk_uid	=	make_kuid(sock_net(sk)->user_ns, 0);
 	}
+	sk->sk_uid	=	uid;
 
 	rwlock_init(&sk->sk_callback_lock);
 	if (sk->sk_kern_sock)
@@ -3466,6 +3465,16 @@ void sock_init_data(struct socket *sock, struct sock *sk)
 	refcount_set(&sk->sk_refcnt, 1);
 	atomic_set(&sk->sk_drops, 0);
 }
+EXPORT_SYMBOL(sock_init_data_uid);
+
+void sock_init_data(struct socket *sock, struct sock *sk)
+{
+	kuid_t uid = sock ?
+		SOCK_INODE(sock)->i_uid :
+		make_kuid(sock_net(sk)->user_ns, 0);
+
+	sock_init_data_uid(sock, sk, uid);
+}
 EXPORT_SYMBOL(sock_init_data);
 
 void lock_sock_nested(struct sock *sk, int subclass)