For some unaligned memory accesses we have to check additional
byte of the shadow memory. Currently we load that byte speculatively
to have only single load + branch on the optimistic fast path.
However, this approach have some downsides:
- It's unaligned access, so this prevents porting KASAN on architectures
which doesn't support unaligned accesses.
- We have to map additional shadow page to prevent crash if
speculative load happens near the end of the mapped memory.
This would significantly complicate upcoming memory hotplug support.
I wasn't able to notice any performance degradation with this patch.
So these speculative loads is just a pain with no gain, let's remove
them.
Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
---
mm/kasan/kasan.c | 98 +++++++++-----------------------------------------------
1 file changed, 16 insertions(+), 82 deletions(-)
diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
index 85ee45b07615..e6fe07a98677 100644
--- a/mm/kasan/kasan.c
+++ b/mm/kasan/kasan.c
@@ -134,94 +134,30 @@ static __always_inline bool memory_is_poisoned_1(unsigned long addr)
return false;
}
-static __always_inline bool memory_is_poisoned_2(unsigned long addr)
+static __always_inline bool memory_is_poisoned_2_4_8(unsigned long addr,
+ unsigned long size)
{
- u16 *shadow_addr = (u16 *)kasan_mem_to_shadow((void *)addr);
-
- if (unlikely(*shadow_addr)) {
- if (memory_is_poisoned_1(addr + 1))
- return true;
-
- /*
- * If single shadow byte covers 2-byte access, we don't
- * need to do anything more. Otherwise, test the first
- * shadow byte.
- */
- if (likely(((addr + 1) & KASAN_SHADOW_MASK) != 0))
- return false;
-
- return unlikely(*(u8 *)shadow_addr);
- }
+ u8 *shadow_addr = (u8 *)kasan_mem_to_shadow((void *)addr);
- return false;
-}
-
-static __always_inline bool memory_is_poisoned_4(unsigned long addr)
-{
- u16 *shadow_addr = (u16 *)kasan_mem_to_shadow((void *)addr);
-
- if (unlikely(*shadow_addr)) {
- if (memory_is_poisoned_1(addr + 3))
- return true;
-
- /*
- * If single shadow byte covers 4-byte access, we don't
- * need to do anything more. Otherwise, test the first
- * shadow byte.
- */
- if (likely(((addr + 3) & KASAN_SHADOW_MASK) >= 3))
- return false;
-
- return unlikely(*(u8 *)shadow_addr);
- }
-
- return false;
-}
-
-static __always_inline bool memory_is_poisoned_8(unsigned long addr)
-{
- u16 *shadow_addr = (u16 *)kasan_mem_to_shadow((void *)addr);
-
- if (unlikely(*shadow_addr)) {
- if (memory_is_poisoned_1(addr + 7))
- return true;
-
- /*
- * If single shadow byte covers 8-byte access, we don't
- * need to do anything more. Otherwise, test the first
- * shadow byte.
- */
- if (likely(IS_ALIGNED(addr, KASAN_SHADOW_SCALE_SIZE)))
- return false;
-
- return unlikely(*(u8 *)shadow_addr);
- }
+ /*
+ * Access crosses 8(shadow size)-byte boundary. Such access maps
+ * into 2 shadow bytes, so we need to check them both.
+ */
+ if (unlikely(((addr + size - 1) & KASAN_SHADOW_MASK) < size - 1))
+ return *shadow_addr || memory_is_poisoned_1(addr + size - 1);
- return false;
+ return memory_is_poisoned_1(addr + size - 1);
}
static __always_inline bool memory_is_poisoned_16(unsigned long addr)
{
- u32 *shadow_addr = (u32 *)kasan_mem_to_shadow((void *)addr);
-
- if (unlikely(*shadow_addr)) {
- u16 shadow_first_bytes = *(u16 *)shadow_addr;
-
- if (unlikely(shadow_first_bytes))
- return true;
-
- /*
- * If two shadow bytes covers 16-byte access, we don't
- * need to do anything more. Otherwise, test the last
- * shadow byte.
- */
- if (likely(IS_ALIGNED(addr, KASAN_SHADOW_SCALE_SIZE)))
- return false;
+ u16 *shadow_addr = (u16 *)kasan_mem_to_shadow((void *)addr);
- return memory_is_poisoned_1(addr + 15);
- }
+ /* Unaligned 16-bytes access maps into 3 shadow bytes. */
+ if (unlikely(!IS_ALIGNED(addr, KASAN_SHADOW_SCALE_SIZE)))
+ return *shadow_addr || memory_is_poisoned_1(addr + 15);
- return false;
+ return *shadow_addr;
}
static __always_inline unsigned long bytes_is_nonzero(const u8 *start,
@@ -292,11 +228,9 @@ static __always_inline bool memory_is_poisoned(unsigned long addr, size_t size)
case 1:
return memory_is_poisoned_1(addr);
case 2:
- return memory_is_poisoned_2(addr);
case 4:
- return memory_is_poisoned_4(addr);
case 8:
- return memory_is_poisoned_8(addr);
+ return memory_is_poisoned_2_4_8(addr, size);
case 16:
return memory_is_poisoned_16(addr);
default:
--
2.13.0
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