net: remove one stac/clac pair from move_addr_to_user()

Convert the get_user() and __put_user() code to the
fast masked_user_access_begin()/unsafe_{get|put}_user()
variant.

This patch increases the performance of an UDP recvfrom()
receiver (netserver) on 120 bytes messages by 7 %
on an AMD EPYC 7B12 64-Core Processor platform.

Presence of audit_sockaddr() makes difficult
to avoid the stac/clac pair in the copy_to_user() call,
this is left for a future patch.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250925230929.3727873-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/socket.c

@@ -276,28 +276,41 @@ int move_addr_to_kernel(void __user *uaddr, int ulen, struct sockaddr_storage *k
 static int move_addr_to_user(struct sockaddr_storage *kaddr, int klen,
 			     void __user *uaddr, int __user *ulen)
 {
-	int err;
 	int len;
 
 	BUG_ON(klen > sizeof(struct sockaddr_storage));
-	err = get_user(len, ulen);
-	if (err)
-		return err;
+
+	if (can_do_masked_user_access())
+		ulen = masked_user_access_begin(ulen);
+	else if (!user_access_begin(ulen, 4))
+		return -EFAULT;
+
+	unsafe_get_user(len, ulen, efault_end);
+
 	if (len > klen)
 		len = klen;
-	if (len < 0)
-		return -EINVAL;
+	/*
+	 *      "fromlen shall refer to the value before truncation.."
+	 *                      1003.1g
+	 */
+	if (len >= 0)
+		unsafe_put_user(klen, ulen, efault_end);
+
+	user_access_end();
+
 	if (len) {
+		if (len < 0)
+			return -EINVAL;
 		if (audit_sockaddr(klen, kaddr))
 			return -ENOMEM;
 		if (copy_to_user(uaddr, kaddr, len))
 			return -EFAULT;
 	}
-	/*
-	 *      "fromlen shall refer to the value before truncation.."
-	 *                      1003.1g
-	 */
-	return __put_user(klen, ulen);
+	return 0;
+
+efault_end:
+	user_access_end();
+	return -EFAULT;
 }
 
 static struct kmem_cache *sock_inode_cachep __ro_after_init;