scm: optimize put_cmsg()

Calling two copy_to_user() for very small regions has very high overhead.

Switch to inlined unsafe_put_user() to save one stac/clac sequence,
and avoid copy_to_user().
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/scm.c

@@ -228,14 +228,16 @@ int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
 
 	if (msg->msg_control_is_user) {
 		struct cmsghdr __user *cm = msg->msg_control_user;
-		struct cmsghdr cmhdr;
-
-		cmhdr.cmsg_level = level;
-		cmhdr.cmsg_type = type;
-		cmhdr.cmsg_len = cmlen;
-		if (copy_to_user(cm, &cmhdr, sizeof cmhdr) ||
-		    copy_to_user(CMSG_USER_DATA(cm), data, cmlen - sizeof(*cm)))
-			return -EFAULT;
+
+		if (!user_write_access_begin(cm, cmlen))
+			goto efault;
+
+		unsafe_put_user(len, &cm->cmsg_len, efault_end);
+		unsafe_put_user(level, &cm->cmsg_level, efault_end);
+		unsafe_put_user(type, &cm->cmsg_type, efault_end);
+		unsafe_copy_to_user(CMSG_USER_DATA(cm), data,
+				    cmlen - sizeof(*cm), efault_end);
+		user_write_access_end();
 	} else {
 		struct cmsghdr *cm = msg->msg_control;
 
@@ -249,6 +251,11 @@ int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
 	msg->msg_control += cmlen;
 	msg->msg_controllen -= cmlen;
 	return 0;
+
+efault_end:
+	user_write_access_end();
+efault:
+	return -EFAULT;
 }
 EXPORT_SYMBOL(put_cmsg);