Index: head/cddl/lib/libspl/Makefile
===================================================================
--- head/cddl/lib/libspl/Makefile
+++ head/cddl/lib/libspl/Makefile
@@ -14,13 +14,14 @@
         list.c \
         mkdirp.c \
         page.c \
-        strlcat.c \
-        strlcpy.c \
         timestamp.c \
         zone.c \
         include/sys/list.h \
         include/sys/list_impl.h
 
+# These functions are not required when bootstrapping and the atomic code
+# will not compile when building on macOS.
+.if !defined(BOOTSTRAPPING)
 SRCS += \
         getexecname.c \
         gethostid.c \
@@ -37,6 +38,7 @@
 .else
 .PATH: ${SRCTOP}/sys/contrib/openzfs/lib/libspl/asm-generic
 SRCS += atomic.c
+.endif
 .endif
 
 
Index: head/tools/build/cross-build/include/common/string.h
===================================================================
--- head/tools/build/cross-build/include/common/string.h
+++ head/tools/build/cross-build/include/common/string.h
@@ -37,6 +37,10 @@
  */
 #pragma once
 
+/* Avoid incompatible opensolaris redeclarations (without _FORTIFY_SOURCE). */
+#define HAVE_STRLCAT 1
+#define HAVE_STRLCPY 1
+
 #include_next <string.h>
 /*
  * FreeBSD string.h #includes strings.h and all libmd code depends on
Index: head/tools/build/cross-build/include/common/sys/cdefs.h
===================================================================
--- head/tools/build/cross-build/include/common/sys/cdefs.h
+++ head/tools/build/cross-build/include/common/sys/cdefs.h
@@ -276,3 +276,11 @@
 #ifndef __DEQUALIFY
 #define __DEQUALIFY(type, var) ((type)(__uintptr_t)(const volatile void *)(var))
 #endif
+
+
+/* Expose all declarations when using FreeBSD headers */
+#define	__POSIX_VISIBLE		200809
+#define	__XSI_VISIBLE		700
+#define	__BSD_VISIBLE		1
+#define	__ISO_C_VISIBLE		2011
+#define	__EXT1_VISIBLE		1
Index: head/tools/build/cross-build/include/common/sys/uio.h
===================================================================
--- head/tools/build/cross-build/include/common/sys/uio.h
+++ head/tools/build/cross-build/include/common/sys/uio.h
@@ -1,51 +0,0 @@
-/*-
- * 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.
- *
- * $FreeBSD$
- */
-#pragma once
-#include_next <sys/uio.h>
-
-/* needed by opensolaris: */
-#ifdef __linux__
-enum uio_rw { UIO_READ, UIO_WRITE };
-
-/* Segment flag values. */
-enum uio_seg {
-	UIO_USERSPACE, /* from user data space */
-	UIO_SYSSPACE, /* from system space */
-	UIO_NOCOPY /* don't copy, already in object */
-};
-#endif
Index: head/tools/build/cross-build/include/linux/sys/stat.h
===================================================================
--- head/tools/build/cross-build/include/linux/sys/stat.h
+++ head/tools/build/cross-build/include/linux/sys/stat.h
@@ -59,3 +59,6 @@
 #ifndef S_ISTXT
 #define S_ISTXT S_ISVTX
 #endif
+
+/* This include is needed for OpenZFS bootstrap */
+#include <sys/mount.h>
Index: head/tools/build/cross-build/include/mac/sys/_types.h
===================================================================
--- head/tools/build/cross-build/include/mac/sys/_types.h
+++ head/tools/build/cross-build/include/mac/sys/_types.h
@@ -43,3 +43,5 @@
  * __darwin_ct_rune_t exists.
  */
 typedef __darwin_ct_rune_t __ct_rune_t;
+/* Needed for opensolaris compat. */
+typedef	__int64_t	off64_t;
Index: head/tools/build/mk/Makefile.boot
===================================================================
--- head/tools/build/mk/Makefile.boot
+++ head/tools/build/mk/Makefile.boot
@@ -44,7 +44,6 @@
 CFLAGS+=	-Werror=implicit-function-declaration -Werror=implicit-int \
 		-Werror=return-type -Wundef
 CFLAGS+=	-DHAVE_NBTOOL_CONFIG_H=1
-CFLAGS+=	-D__BSD_VISIBLE=1
 CFLAGS+=	-I${SRCTOP}/tools/build/cross-build/include/common
 
 # b64_pton and b64_ntop is in libresolv on MacOS and Linux: