Andrey Ryabinin [off-list ref] writes:

2015-08-26 11:26 GMT+03:00 Aneesh Kumar K.V [off-list ref]:

quoted

We add enable/disable callbacks in this patch which architecture
can implemement. We will use this in the later patches for architecture
like ppc64, that cannot have early zero page kasan shadow region for the
entire virtual address space. Such architectures also cannot use
inline kasan support.

Signed-off-by: Aneesh Kumar K.V <redacted>
---
 mm/kasan/kasan.c |  9 +++++++++
 mm/kasan/kasan.h | 15 +++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
index 7b28e9cdf1c7..e4d33afd0eaf 100644
--- a/mm/kasan/kasan.c
+++ b/mm/kasan/kasan.c

@@ -43,6 +43,9 @@ static void kasan_poison_shadow(const void *address, size_t size, u8 value)
 {
        void *shadow_start, *shadow_end;

+       if (!kasan_enabled())
+               return;
+

By the time this function called we already should have shadow,
so it should be safe to remove this check.

I remember hitting that call before enabling kasan completely. Will
check that again. 

quoted

        shadow_start = kasan_mem_to_shadow(address);
        shadow_end = kasan_mem_to_shadow(address + size);

@@ -51,6 +54,9 @@ static void kasan_poison_shadow(const void *address, size_t size, u8 value)

 void kasan_unpoison_shadow(const void *address, size_t size)
 {
+       if (!kasan_enabled())
+               return;
+

Ditto.

quoted

        kasan_poison_shadow(address, size, 0);

        if (size & KASAN_SHADOW_MASK) {

@@ -238,6 +244,9 @@ static __always_inline void check_memory_region(unsigned long addr,
 {
        struct kasan_access_info info;

+       if (!kasan_enabled())
+               return;
+
        if (unlikely(size == 0))
                return;

diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index a6b46cc94907..deb547d5a916 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h

@@ -68,6 +68,21 @@ static inline bool kasan_report_enabled(void)
        return !current->kasan_depth;
 }

+#ifndef kasan_enabled
+/*
+ * Some archs may want to disable kasan callbacks.
+ */
+static inline bool kasan_enabled(void)
+{
+       return true;
+}
+#define kasan_enabled kasan_enabled

Why we need this define?

That is to make sure that we don't endup with different definition of
kasan_enabled due to header include errors. Once we select a
particular definition of the overloaded function, it is always good to mark
the function as defined, so that checks like #ifndef kasan_enabled will
always fail. 

quoted

+#else
+#ifdef CONFIG_KASAN_INLINE
+#error "Kasan inline support cannot work with KASAN arch hooks"
+#endif
+#endif
+
 void kasan_report(unsigned long addr, size_t size,
                bool is_write, unsigned long ip);

--
2.5.0

-aneesh