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kasan: use IS_ALIGNED in memory_is_poisoned_8()

PATCH Superseded Xishi Qiu 3 participants 4 messages Sep 11, 2015 – Sep 14, 2015

Subsystems (2)
Memory management KASAN

v1 of 3 Superseded Latest is v3 →

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Xishi Qiu

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#0 lore raw

PATCH 1/1 kasan: use IS_ALIGNED in memory_is_poisoned_8()

Use IS_ALIGNED() to determine whether the shadow span two bytes. It generates less code and more readable.

Signed-off-by Xishi Qiu <[email protected]>
---
 mm/kasan/kasan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
mm/kasan/kasan.c +1 -1
index 7b28e9c..c6ddff1 100644
@@ -120,7 +120,7 @@ static __always_inline bool memory_is_poisoned_8(unsigned long addr)
 		if (memory_is_poisoned_1(addr + 7))
 			return true;
 
-		if (likely(((addr + 7) & KASAN_SHADOW_MASK) >= 7))
+		if (likely(IS_ALIGNED(addr, 8)))
 			return false;
 
 		return unlikely(*(u8 *)shadow_addr);
Signature · 1 line
2.0.0
maintainer
#1 lore raw
Quoted 3 lines · On Fri, 11 Sep 2015 10:02:29 +0800 Xishi Qiu <[email protected]> wrote:

Use IS_ALIGNED() to determine whether the shadow span two bytes. It generates less code and more readable.

Please cc Andrey Ryabinin on kasan patches.

mm/kasan/kasan.c static __always_inline bool memory_is_poisoned_8(unsigned long addr) · line 124

Quoted 9 lines
mm/kasan/kasan.c +1 -1
@@ -120,7 +120,7 @@ static __always_inline bool memory_is_poisoned_8(unsigned long addr)
 		if (memory_is_poisoned_1(addr + 7))
 			return true;
 
-		if (likely(((addr + 7) & KASAN_SHADOW_MASK) >= 7))
+		if (likely(IS_ALIGNED(addr, 8)))
 			return false;

Wouldn't IS_ALIGNED(addr, KASAN_SHADOW_SCALE_SIZE) be more appropriate?

But I'm not really sure what the original code is trying to do.

	if ((addr + 7) & 7) >= 7)

can only evaluate true if ((addr + 7) & 7) equals 7, so the ">=" could be "==".

I think. The code looks a bit weird. A code comment would help.

And how come memory_is_poisoned_16() does IS_ALIGNED(addr, 8)? Should it be 16?

author
#2 lore raw
Quoted 7 lines · Andrew Morton
Quoted 3 lines · On Fri, 11 Sep 2015 10:02:29 +0800 Xishi Qiu <[email protected]> wrote:

Use IS_ALIGNED() to determine whether the shadow span two bytes. It generates less code and more readable.

Please cc Andrey Ryabinin on kasan patches.

Sorry, my mistake.

Quoted 7 lines
Quoted 7 lines
mm/kasan/kasan.c +0 -0
@@ -120,7 +120,7 @@ static __always_inline bool memory_is_poisoned_8(unsigned long addr)
 		if (memory_is_poisoned_1(addr + 7))
 			return true;
Quoted 6 lines

mm/kasan/kasan.c static __always_inline bool memory_is_poisoned_8(unsigned long addr) · line 124

Quoted 3 lines
mm/kasan/kasan.c +1 -1
-		if (likely(((addr + 7) & KASAN_SHADOW_MASK) >= 7))
+		if (likely(IS_ALIGNED(addr, 8)))
 			return false;

Wouldn't IS_ALIGNED(addr, KASAN_SHADOW_SCALE_SIZE) be more appropriate?

OK, I'll send V2.

Quoted 7 lines

But I'm not really sure what the original code is trying to do.

	if ((addr + 7) & 7) >= 7)

can only evaluate true if ((addr + 7) & 7) equals 7, so the ">=" could be "==".

I think it should be "==", the value will not "> 7"

Quoted 4 lines

I think. The code looks a bit weird. A code comment would help.

And how come memory_is_poisoned_16() does IS_ALIGNED(addr, 8)? Should it be 16?

No, it is to determine whether the shadow span two bytes(8 bytes, not 16).

Thanks,
Xishi Qiu

Quoted 4 lines

.

maintainer
#3 lore raw
In reply to Andrew Morton

2015-09-12 1:47 GMT+03:00 Andrew Morton <[email protected]>:

Quoted 9 lines · On Fri, 11 Sep 2015 10:02:29 +0800 Xishi Qiu <[email protected]> wrote:

mm/kasan/kasan.c static __always_inline bool memory_is_poisoned_8(unsigned long addr) · line 124

Quoted 3 lines
mm/kasan/kasan.c +1 -1
-             if (likely(((addr + 7) & KASAN_SHADOW_MASK) >= 7))
+             if (likely(IS_ALIGNED(addr, 8)))
                      return false;

Wouldn't IS_ALIGNED(addr, KASAN_SHADOW_SCALE_SIZE) be more appropriate?

But I'm not really sure what the original code is trying to do.

Original code is trying to estimate whether we should check 2 shadow bytes or just 1 should be enough.

Quoted 5 lines
        if ((addr + 7) & 7) >= 7)

can only evaluate true if ((addr + 7) & 7) equals 7, so the ">=" could be "==".

Yes, it could be "==".
">=" is just for consistency with similar code in memory_is_poisoned_2/4.

If I'm not mistaken generic formula for such check looks like this:

        ((addr + size - 1) & KASAN_SHADOW_MASK) >= ((size - 1) &

KASAN_SHADOW_MASK)

But when size >= KASAN_SHADOW_SCALE_SIZE we could just check for alignment.

Quoted 5 lines

I think. The code looks a bit weird. A code comment would help.

And how come memory_is_poisoned_16() does IS_ALIGNED(addr, 8)? Should it be 16?

No, If 16 bytes are 8-byte aligned, then shadow is 2-bytes.

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