support: Add <support/next_to_fault.h>

Based on the implementation in resolv/tst-inet_pton.c.

3 files changed, 101 insertions, 0 deletions

diff --git a/support/Makefile b/support/Makefile
index dafb1737a4..f7a878b950 100644
--- a/support/Makefile
+++ b/support/Makefile
@@ -32,6 +32,7 @@ libsupport-routines = \
   check_netent \
   delayed_exit \
   ignore_stderr \
+  next_to_fault \
   oom_error \
   resolv_test \
   set_fortify_handler \
diff --git a/support/next_to_fault.c b/support/next_to_fault.c
new file mode 100644
index 0000000000..7c6b077898
--- /dev/null
+++ b/support/next_to_fault.c
@@ -0,0 +1,52 @@
+/* Memory allocation next to an unmapped page.
+   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/next_to_fault.h>
+#include <support/xunistd.h>
+#include <sys/mman.h>
+#include <sys/param.h>
+
+struct support_next_to_fault
+support_next_to_fault_allocate (size_t size)
+{
+  long page_size = sysconf (_SC_PAGE_SIZE);
+  TEST_VERIFY_EXIT (page_size > 0);
+  struct support_next_to_fault result;
+  result.region_size = roundup (size, page_size) + page_size;
+  if (size + page_size <= size || result.region_size <= size)
+    FAIL_EXIT1 ("support_next_to_fault_allocate (%zu): overflow", size);
+  result.region_start
+    = xmmap (NULL, result.region_size, PROT_READ | PROT_WRITE,
+             MAP_PRIVATE | MAP_ANONYMOUS, -1);
+  /* Unmap the page after the allocation.  */
+  xmprotect (result.region_start + (result.region_size - page_size),
+             page_size, PROT_NONE);
+  /* Align the allocation within the region so that it ends just
+     before the PROT_NONE page.  */
+  result.buffer = result.region_start + result.region_size - page_size - size;
+  result.length = size;
+  return result;
+}
+
+void
+support_next_to_fault_free (struct support_next_to_fault *ntf)
+{
+  xmunmap (ntf->region_start, ntf->region_size);
+  *ntf = (struct support_next_to_fault) { NULL, };
+}
diff --git a/support/next_to_fault.h b/support/next_to_fault.h
new file mode 100644
index 0000000000..dd71c28ac0
--- /dev/null
+++ b/support/next_to_fault.h
@@ -0,0 +1,48 @@
+/* Memory allocation next to an unmapped page.
+   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/>.  */
+
+#ifndef SUPPORT_NEXT_TO_FAULT_H
+#define SUPPORT_NEXT_TO_FAULT_H
+
+#include <sys/cdefs.h>
+#include <sys/types.h>
+
+__BEGIN_DECLS
+
+/* The memory region created by next_to_fault_allocate.  */
+struct support_next_to_fault
+{
+  /* The user data.  */
+  char *buffer;
+  size_t length;
+
+  /* The entire allocated region.  */
+  void *region_start;
+  size_t region_size;
+};
+
+/* Allocate a buffer of SIZE bytes just before a page which is mapped
+   with PROT_NONE (so that overrunning the buffer will cause a
+   fault).  */
+struct support_next_to_fault support_next_to_fault_allocate (size_t size);
+
+/* Deallocate the memory region allocated by
+   next_to_fault_allocate.  */
+void support_next_to_fault_free (struct support_next_to_fault *);
+
+#endif /* SUPPORT_NEXT_TO_FAULT_H */