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

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Date:	Mon, 14 Sep 2015 16:19:24 +0300
From:	Andrey Ryabinin <ryabinin.a.a@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Xishi Qiu <qiuxishi@...wei.com>,
	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>
Subject: Re: [PATCH] kasan: use IS_ALIGNED in memory_is_poisoned_8()

2015-09-12 1:47 GMT+03:00 Andrew Morton <akpm@...ux-foundation.org>:
> On Fri, 11 Sep 2015 10:02:29 +0800 Xishi Qiu <qiuxishi@...wei.com> wrote:
>> -             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.

>         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.

> 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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