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

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Date:	Wed, 22 Oct 2014 13:13:08 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Andrey Ryabinin <a.ryabinin@...sung.com>,
	n-horiguchi@...jp.nec.com, aarcange@...hat.com, mgorman@...e.de,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm, hugetlb: correct bit shift in hstate_sizelog

On Wed, 22 Oct 2014 14:50:22 -0400 Sasha Levin <sasha.levin@...cle.com> wrote:

> On 10/22/2014 02:44 PM, Andrew Morton wrote:
> > 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)
> 
> True, but "(1 << page_size_log)" produces an integer rather than long because "1"
> is an int and not long.

My point is that there's no point in using 1ULL because
size_to_hstate() will truncate it anyway.

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