Included <asm/types.h> so __u64 is defined.
diff --git a/arch/um/drivers/ubd_user.c b/arch/um/drivers/ubd_user.c
index 95bc20d..e2bf2a9 100644
--- a/arch/um/drivers/ubd_user.c
+++ b/arch/um/drivers/ubd_user.c
@@ -17,6 +17,7 @@
 #include <string.h>
 #include <sys/mman.h>
 #include <sys/param.h>
+#include "asm/types.h"
 #include "user_util.h"
 #include "kern_util.h"
 #include "user.h"