drbd: new-connection and new-minor succeed, if the object already exists

Follow O_CREAT semantics when creating connection or minor device/volume
objects.  If we need O_CREAT|O_EXCL semantics some time down the road,
we can add NLM_F_EXCL to the netlink message flags.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_nl.c

@@ -2590,8 +2590,11 @@ int drbd_adm_create_connection(struct sk_buff *skb, struct genl_info *info)
 		goto out;
 
 	if (adm_ctx.tconn) {
+		if (info->nlhdr->nlmsg_flags & NLM_F_EXCL) {
 			retcode = ERR_INVALID_REQUEST;
 			drbd_msg_put_info("connection exists");
+		}
+		/* else: still NO_ERROR */
 		goto out;
 	}
 
@@ -2626,6 +2629,15 @@ int drbd_adm_add_minor(struct sk_buff *skb, struct genl_info *info)
 		goto out;
 	}
 
+	/* drbd_adm_prepare made sure already
+	 * that mdev->tconn and mdev->vnr match the request. */
+	if (adm_ctx.mdev) {
+		if (info->nlhdr->nlmsg_flags & NLM_F_EXCL)
+			retcode = ERR_MINOR_EXISTS;
+		/* else: still NO_ERROR */
+		goto out;
+	}
+
 	retcode = conn_new_minor(adm_ctx.tconn, dh->minor, adm_ctx.volume);
 out:
 	drbd_adm_finish(info, retcode);