linux-kernel - Re: [PATCH] kasan: use IS_ALIGNED in memory_is_poisoned

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Date:	Mon, 14 Sep 2015 09:17:43 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Andrey Konovalov <adech.fo@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Michal Marek <mmarek@...e.cz>,
	"long.wanglong" <long.wanglong@...wei.com>,
	Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>
Subject: Re: [PATCH] kasan: use IS_ALIGNED in memory_is_poisoned_8()

On 2015/9/12 6:47, Andrew Morton wrote:

> On Fri, 11 Sep 2015 10:02:29 +0800 Xishi Qiu <qiuxishi@...wei.com> 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.

> 
>> --- a/mm/kasan/kasan.c
>> +++ b/mm/kasan/kasan.c
>> @@ -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?
> 

OK, I'll send V2.

> 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"

> 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

> 
> 
> .
> 



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