Re: [PATCH] mm, hugetlb: correct bit shift in hstate_sizelog

[Andrey Ryabinin]

2014-10-23 0:13 GMT+04:00 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>:
> On Wed, 22 Oct 2014 14:50:22 -0400 Sasha Levin <sasha.levin@xxxxxxxxxx> wrote:
>
>> On 10/22/2014 02:44 PM, Andrew Morton wrote:
>> > On Wed, 22 Oct 2014 09:42:46 +0400 Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx> 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@xxxxxxxxxx>
>> >>> > > ---
>> >>> > >  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.
>

There is a point to use 1ULL
On 32-bit with size >= 32
(1UL << size) - undefined, so size_to_hstate() will truncate it to
undefined as well. E.g. It definitely won't be zero on x86.
While (1ULL << size) - is defined and size_to_hstate() will truncate it to zero.


-- 
Best regards,
Andrey Ryabinin
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