diff --git a/tools/build/cross-build/include/linux/sys/types.h b/tools/build/cross-build/include/linux/sys/types.h
index 770b3057f8a8..222560167a71 100644
--- a/tools/build/cross-build/include/linux/sys/types.h
+++ b/tools/build/cross-build/include/linux/sys/types.h
@@ -1,42 +1,49 @@
 /*-
  * SPDX-License-Identifier: BSD-2-Clause
  *
  * Copyright 2018-2020 Alex Richardson <arichardson@FreeBSD.org>
  *
  * This software was developed by SRI International and the University of
  * Cambridge Computer Laboratory (Department of Computer Science and
  * Technology) under DARPA contract HR0011-18-C-0016 ("ECATS"), as part of the
  * DARPA SSITH research programme.
  *
  * This software was developed by SRI International and the University of
  * Cambridge Computer Laboratory under DARPA/AFRL contract (FA8750-10-C-0237)
  * ("CTSRD"), as part of the DARPA CRASH research programme.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
 #pragma once
 
 #include_next <sys/types.h>
 
+/*
+ * stddef.h for both gcc and clang will define __size_t when size_t has
+ * been defined (except on *BSD where it doesn't touch __size_t). So if
+ * we're building on Linux, we know that if that's not defined, we have
+ * to typedef __size_t for FreeBSD's use of __size_t in places to work
+ * during bootstrapping.
+ */
 #ifndef __size_t
 typedef __SIZE_TYPE__ __size_t;
 #endif