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

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Message-Id: <20150911154730.3a2151a0b111fed01acdaaa1@linux-foundation.org>
Date:	Fri, 11 Sep 2015 15:47:30 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Xishi Qiu <qiuxishi@...wei.com>
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 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.

> --- 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?

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?

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