Index: head/include/libgen.h
===================================================================
--- head/include/libgen.h	(revision 304859)
+++ head/include/libgen.h	(revision 304860)
@@ -1,42 +1,61 @@
 /*	$OpenBSD: libgen.h,v 1.4 1999/05/28 22:00:22 espie Exp $	*/
 /*	$FreeBSD$	*/
 
 /*
  * Copyright (c) 1997 Todd C. Miller <Todd.Miller@courtesan.com>
  * All rights reserved.
  *
  * 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.
  * 3. The name of the author may not be used to endorse or promote products
  *    derived from this software without specific prior written permission.
  *
  * THIS SOFTWARE IS PROVIDED ``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 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.
  */
 
 #ifndef _LIBGEN_H_
 #define	_LIBGEN_H_
 
 #include <sys/cdefs.h>
 
 __BEGIN_DECLS
 char	*basename(char *);
 char	*basename_r(const char *, char *);
 char	*dirname(char *);
 __END_DECLS
 
+/*
+ * In FreeBSD 12, the prototype of dirname() was modified to comply to
+ * POSIX. This function may now modify its input. Unfortunately, our
+ * copy of xinstall(8) shipped with previous versions of FreeBSD is
+ * built using the host headers and libc during the bootstrapping phase
+ * and depends on the old behavior.
+ *
+ * Apply a workaround where we explicitly link against dirname@FBSD_1.0
+ * in case this function is called on constant strings, instead of
+ * making the build fail.
+ */
+#if defined(__generic) && !defined(__cplusplus)
+__BEGIN_DECLS
+char	*__old_dirname(const char *);
+__END_DECLS
+__sym_compat(dirname, __old_dirname, FBSD_1.0);
+#define	dirname(x)	__generic(x, const char *, __old_dirname, dirname)(x)
+#endif
+
 #endif /* !_LIBGEN_H_ */
Index: head/lib/libc/gen/dirname.c
===================================================================
--- head/lib/libc/gen/dirname.c	(revision 304859)
+++ head/lib/libc/gen/dirname.c	(revision 304860)
@@ -1,90 +1,90 @@
 /*-
  * Copyright (c) 2015-2016 Nuxi, https://nuxi.nl/
  *
  * 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.
  */
 
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
 #include <libgen.h>
 #include <stdbool.h>
 #include <string.h>
 
 char *
-dirname(char *path)
+(dirname)(char *path)
 {
 	const char *in, *prev, *begin, *end;
 	char *out;
 	size_t prevlen;
 	bool skipslash;
 
 	/*
 	 * If path is a null pointer or points to an empty string,
 	 * dirname() shall return a pointer to the string ".".
 	 */
 	if (path == NULL || *path == '\0')
 		return ((char *)".");
 
 	/* Retain at least one leading slash character. */
 	in = out = *path == '/' ? path + 1 : path;
 
 	skipslash = true;
 	prev = ".";
 	prevlen = 1;
 	for (;;) {
 		/* Extract the next pathname component. */
 		while (*in == '/')
 			++in;
 		begin = in;
 		while (*in != '/' && *in != '\0')
 			++in;
 		end = in;
 		if (begin == end)
 			break;
 
 		/*
 		 * Copy over the previous pathname component, except if
 		 * it's dot. There is no point in retaining those.
 		 */
 		if (prevlen != 1 || *prev != '.') {
 			if (!skipslash)
 				*out++ = '/';
 			skipslash = false;
 			memmove(out, prev, prevlen);
 			out += prevlen;
 		}
 
 		/* Preserve the pathname component for the next iteration. */
 		prev = begin;
 		prevlen = end - begin;
 	}
 
 	/*
 	 * If path does not contain a '/', then dirname() shall return a
 	 * pointer to the string ".".
 	 */
 	if (out == path)
 		*out++ = '.';
 	*out = '\0';
 	return (path);
 }