linux-kernel - Re: [PATCH] bitops: Fix shift overflow in GENMASK macros

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Message-ID: <545890F8.60909@st.com>
Date:	Tue, 4 Nov 2014 09:40:24 +0100
From:	Maxime Coquelin <maxime.coquelin@...com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	<gong.chen@...ux.intel.com>, Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, <tytso@....edu>,
	<linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
	<kernel@...inux.com>, <eric.paire@...com>
Subject: Re: [PATCH] bitops: Fix shift overflow in GENMASK macros

Hi Peter,

On 11/04/2014 09:29 AM, Peter Zijlstra wrote:
> On Mon, Nov 03, 2014 at 06:39:58PM +0100, Maxime COQUELIN wrote:
>> On some 32 bits architectures, including x86, GENMASK(31, 0) returns 0
>> instead of the expected ~0UL.
>>
>> This is the same on some 64 bits architectures with GENMASK_ULL(63, 0).
>>
>> This is due to an overflow in the shift operand, 1 << 32 for GENMASK,
>> 1 << 64 for GENMASK_ULL.
>>
>> Fixes: 10ef6b0dffe404bcc54e94cb2ca1a5b18445a66b
>> Cc: <stable@...r.kernel.org> #v3.13+
>> Reported-by: Eric Paire <eric.paire@...com>
>> Signed-off-by: Maxime Coquelin <maxime.coquelin@...com>
>> ---
>>   include/linux/bitops.h | 8 ++++++--
>>   1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
>> index be5fd38..81f9725 100644
>> --- a/include/linux/bitops.h
>> +++ b/include/linux/bitops.h
>> @@ -18,8 +18,12 @@
>>    * position @h. For example
>>    * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000.
>>    */
>> -#define GENMASK(h, l)		(((U32_C(1) << ((h) - (l) + 1)) - 1) << (l))
>> -#define GENMASK_ULL(h, l)	(((U64_C(1) << ((h) - (l) + 1)) - 1) << (l))
>> +#define GENMASK(h, l) \
>> +	((~0UL >> ((BITS_PER_LONG - 1) - (h))) & ~((1UL << (l)) - 1))
>> +
>> +#define GENMASK_ULL(h, l) \
>> +	((~0ULL >> ((BITS_PER_LONG_LONG - 1) - (h))) & ~((1ULL << (l)) - 1))
>> +
> I was not expecting the mask there, but instead something like:
>
> 	((~0UL >> (BITS_PER_LONG - (h-l+1))) << l)
>
> which shifts the bits to the desired length and then back to the desired
> place. Would that not be more readable?
>
Yes, this is indeed more readable.
I will send a v2 with your implementation.

Thanks,
Maxime


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