linux-kernel - Re: [PATCH] mm, hugetlb: correct bit shift in hstate

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Message-ID: <CAPAsAGxELx7+A=dujXeo4gqA+DUvQHgKw=gp8iLHvvQwCy2VNw@mail.gmail.com>
Date:	Wed, 22 Oct 2014 23:04:10 +0400
From:	Andrey Ryabinin <ryabinin.a.a@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	aarcange@...hat.com, Mel Gorman <mgorman@...e.de>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] mm, hugetlb: correct bit shift in hstate_sizelog

2014-10-22 22:44 GMT+04:00 Andrew Morton <akpm@...ux-foundation.org>:
> On Wed, 22 Oct 2014 09:42:46 +0400 Andrey Ryabinin <a.ryabinin@...sung.com> wrote:
>
>> On 10/21/2014 10:15 PM, Sasha Levin wrote:
>> > hstate_sizelog() would shift left an int rather than long, triggering
>> > undefined behaviour and passing an incorrect value when the requested
>> > page size was more than 4GB, thus breaking >4GB pages.
>>
>> >
>> > Signed-off-by: Sasha Levin <sasha.levin@...cle.com>
>> > ---
>> >  include/linux/hugetlb.h |    3 ++-
>> >  1 file changed, 2 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
>> > index 65e12a2..57e0dfd 100644
>> > --- a/include/linux/hugetlb.h
>> > +++ b/include/linux/hugetlb.h
>> > @@ -312,7 +312,8 @@ static inline struct hstate *hstate_sizelog(int page_size_log)
>> >  {
>> >     if (!page_size_log)
>> >             return &default_hstate;
>> > -   return size_to_hstate(1 << page_size_log);
>> > +
>> > +   return size_to_hstate(1UL << page_size_log);
>>
>> That still could be undefined on 32-bits. Either use 1ULL or reduce SHM_HUGE_MASK on 32bits.
>>
>
> But
>
> struct hstate *size_to_hstate(unsigned long size)
>

What's wrong? On 32 bits, if page_size_log >= 32, then  (unsingned
long)(1ULL << page_size_log) will be truncated to 0. I guess it's ok.
size_to_hstate will just return NULL in that case.


-- 
Best regards,
Andrey Ryabinin
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