Function allocates a memory region with two sub-regions. The first one
contains addresses from range [0:valid_size[ with access allowed. The
second region contains addresses from range [valid_size:overflow_size[
and access to this region triggers SIGSEGV.
This mechanism is useful for checks which fail very rarely and need
to be as fast as possible.
Signed-off-by: Tomek Grabiec <tgrab...@gmail.com>
---
include/vm/guard-page.h | 1 +
vm/guard-page.c | 35 +++++++++++++++++++++++++++++++++++
2 files changed, 36 insertions(+), 0 deletions(-)
diff --git a/include/vm/guard-page.h b/include/vm/guard-page.h
index 7f8ac58..a779b02 100644
--- a/include/vm/guard-page.h
+++ b/include/vm/guard-page.h
@@ -2,6 +2,7 @@
#define _VM_GUARD_PAGE_
void *alloc_guard_page(void);
+void *alloc_offset_guard(unsigned long valid_size, unsigned long
overflow_size);
int hide_guard_page(void *page_ptr);
int unhide_guard_page(void *page_ptr);
diff --git a/vm/guard-page.c b/vm/guard-page.c
index 5404787..420230a 100644
--- a/vm/guard-page.c
+++ b/vm/guard-page.c
@@ -30,6 +30,41 @@
#include <sys/mman.h>
#include <unistd.h>
+#include <assert.h>
+#include <stdio.h>
+
+/**
+ * alloc_offset_guard - allocates a memory region with two
+ * sub-regions. The first one contains addresses from [0:valid_size[
+ * with access allowed. The second region contains addresses
+ * [valid_size:overflow_size[ and access to this region triggers
+ * SIGSEGV.
+ */
+void *alloc_offset_guard(unsigned long valid_size, unsigned long overflow_size)
+{
+ int valid_pages, overflow_pages;
+ int page_size;
+ void *result;
+
+ assert(overflow_size > 0);
+
+ page_size = getpagesize();
+
+ valid_pages = (valid_size + page_size - 1) / page_size;
+ overflow_pages = (overflow_size + page_size - 1) / page_size;
+
+ result = alloc_pages(valid_pages + overflow_pages);
+
+ mprotect(result + valid_pages * page_size,
+ overflow_pages * page_size,
+ PROT_NONE);
+
+ unsigned long offset = page_size - valid_size % page_size;
+ if (offset == (unsigned long) page_size)
+ offset = 0;
+
+ return result + offset;
+}
void *alloc_guard_page(void)
{
--
1.6.0.6
------------------------------------------------------------------------------
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