__inet_pton_length: Implement new internal helper function

support/Makefile

@@ -74,6 +74,7 @@ libsupport-routines = \
   xmemstream \
   xmkdir \
   xmmap \
+  xmprotect \
   xmunmap \
   xopen \
   xpipe \

support/xmprotect.c

@@ -0,0 +1,28 @@
+/* mprotect with error checking.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/check.h>
+#include <support/xunistd.h>
+#include <sys/mman.h>
+
+void
+xmprotect (void *addr, size_t length, int prot)
+{
+  if (mprotect (addr, length, prot) != 0)
+    FAIL_EXIT1 ("mprotect (%p, %zu, 0x%x): %m", addr, length, prot);
+}

support/xunistd.h

@@ -48,7 +48,7 @@ void xwrite (int, const void *, size_t);
 
 /* Invoke mmap with a zero file offset.  */
 void *xmmap (void *addr, size_t length, int prot, int flags, int fd);
-
+void xmprotect (void *addr, size_t length, int prot);
 void xmunmap (void *addr, size_t length);
 
 __END_DECLS