[PATCH] mm: __nr_to_section - make it safe against overflow v2

Cyrill Gorcunov
2009-01-05T10:31:47Z

__nr_to_section should check for array bound overflow.
We should better get NULL dereference then silently
pass some memory snippet out of bounds to a caller.

Also add a comment about mem_section structure.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
---
include/linux/mmzone.h | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)

Index: linux-2.6.git/include/linux/mmzone.h
===================================================================
--- linux-2.6.git.orig/include/linux/mmzone.h
+++ linux-2.6.git/include/linux/mmzone.h
@@ -935,6 +935,12 @@ static inline unsigned long early_pfn_to

struct page;
struct page_cgroup;
+
+/*
+ * NOTE: sizeof(struct mem_section) _must_ be power of 2
+ * otherwise SECTION_ROOT_MASK will be broken so be
+ * really cautious while modifying this structure
+ */
struct mem_section {
/*
* This is, logically, a pointer to an array of struct
@@ -980,9 +986,14 @@ extern struct mem_section mem_section[NR

static inline struct mem_section *__nr_to_section(unsigned long nr)
{
- if (!mem_section[SECTION_NR_TO_ROOT(nr)])
+ unsigned long idx = SECTION_NR_TO_ROOT(nr);
+
+ if (WARN_ON(idx >= NR_SECTION_ROOTS))
+ return NULL;
+
+ if (!mem_section[idx])
return NULL;
- return &mem_section[SECTION_NR_TO_ROOT(nr)][nr & SECTION_ROOT_MASK];
+ return &mem_section[idx][nr & SECTION_ROOT_MASK];
}
extern int __section_nr(struct mem_section* ms);
extern unsigned long usemap_size(void);