diff --git a/tools/build/Makefile b/tools/build/Makefile
--- a/tools/build/Makefile
+++ b/tools/build/Makefile
@@ -190,6 +190,8 @@
 # On Linux, glibc does not provide strlcpy,strlcat or strmode.
 .PATH: ${LIBC_SRCTOP}/string
 SRCS+=	strlcpy.c strlcat.c strmode.c
+# On Linux, glibc provides ffs* but not fls*
+SRCS+=	fls.c flsl.c flsll.c
 # Compile the fgetln/fgetwln/closefrom fallback code from libbsd:
 SRCS+=	fgetln_fallback.c fgetwln_fallback.c closefrom.c
 CFLAGS.closefrom.c+=	-DSTDC_HEADERS -DHAVE_SYS_DIR_H -DHAVE_DIRENT_H \
diff --git a/tools/build/cross-build/include/linux/strings.h b/tools/build/cross-build/include/linux/strings.h
new file mode 100644
--- /dev/null
+++ b/tools/build/cross-build/include/linux/strings.h
@@ -0,0 +1,40 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright 2022 Jessica Clarke <jrtc27@FreeBSD.org>
+ *
+ * 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 <sys/cdefs.h>
+
+#include_next <strings.h>
+
+__BEGIN_DECLS
+
+int	fls(int) __pure2;
+int	flsl(long) __pure2;
+int	flsll(long long) __pure2;
+
+__END_DECLS