support: Add support_next_to_fault_before support function

Refactor the support_next_to_fault and add the
support_next_to_fault_before method returns a buffer with a protected
page before it, to be able to test buffer underflow accesses.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@redhat.com>

support/next_to_fault.c

@@ -1,4 +1,4 @@
-/* Memory allocation next to an unmapped page.
+/* Memory allocation either before or after an unmapped page.
    Copyright (C) 2017-2025 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
 
@@ -16,34 +16,58 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <stdbool.h>
 #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)
+static struct support_next_to_fault
+support_next_to_fault_allocate_any (size_t size, bool fault_after_alloc)
 {
   long page_size = sysconf (_SC_PAGE_SIZE);
+  long protect_offset = 0;
+  long buffer_offset = 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);
+    FAIL_EXIT1 ("%s (%zu): overflow", __func__, 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;
+
+  if (fault_after_alloc)
+    {
+      protect_offset = result.region_size - page_size;
+      buffer_offset = protect_offset - size;
+    }
+
+  /* Unmap the page before or after the allocation.  */
+  xmprotect (result.region_start + protect_offset, page_size, PROT_NONE);
+  /* Align the allocation within the region so that it starts after or ends
+     just before the PROT_NONE page.  */
+  result.buffer = result.region_start + buffer_offset;
   result.length = size;
   return result;
 }
 
+/* Unmapped a page after the buffer */
+struct support_next_to_fault
+support_next_to_fault_allocate (size_t size)
+{
+  return support_next_to_fault_allocate_any (size, true);
+}
+
+/* Unmapped a page before the buffer */
+struct support_next_to_fault
+support_next_to_fault_allocate_before (size_t size)
+{
+  return support_next_to_fault_allocate_any (size, false);
+}
+
 void
 support_next_to_fault_free (struct support_next_to_fault *ntf)
 {

support/next_to_fault.h

@@ -41,6 +41,11 @@ struct support_next_to_fault
    fault).  */
 struct support_next_to_fault support_next_to_fault_allocate (size_t size);
 
+/* Allocate a buffer of SIZE bytes just *after* a page which is mapped
+   with PROT_NONE (so that under-running the buffer will cause a
+   fault).  */
+struct support_next_to_fault support_next_to_fault_allocate_before (size_t size);
+
 /* Deallocate the memory region allocated by
    next_to_fault_allocate.  */
 void support_next_to_fault_free (struct support_next_to_fault *);