linux-kernel - Re: [PATCH] iommu-common: Do not use 64 bit constant 0xffffffffffffffffl for computing align

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Message-ID: <55AC55AD.6070608@roeck-us.net>
Date:	Sun, 19 Jul 2015 18:58:05 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
CC:	Sowmini Varadhan <sowmini.varadhan@...cle.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	benh@...nel.crashing.org, davem@...emloft.net
Subject: Re: [PATCH] iommu-common: Do not use 64 bit  constant 0xffffffffffffffffl
 for computing align_mask

On 07/19/2015 01:25 PM, Rasmus Villemoes wrote:
> On Sun, Jul 19 2015, Guenter Roeck <linux@...ck-us.net> wrote:
>
>> On Sun, Jul 19, 2015 at 02:20:14PM +0200, Sowmini Varadhan wrote:
>>>
>>> Using a 64 bit constant generates "warning: integer constant is too
>>> large for 'long' type" on 32 bit platforms. Instead use ~0l to get
>>> the desired effect.
>>>
>>> Detected by Andrew Morton who has confirmed that this patch
>>> fixes the warning on i386/gcc-4.4.3, i386/gcc-4.4.0 and arm/gcc-4.4.4.
>>>
>>> Signed-off-by: Sowmini Varadhan <sowmini.varadhan@...cle.com>
>>> ---
>>>   lib/iommu-common.c |    2 +-
>>>   1 files changed, 1 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/lib/iommu-common.c b/lib/iommu-common.c
>>> index df30632..fd1297d 100644
>>> --- a/lib/iommu-common.c
>>> +++ b/lib/iommu-common.c
>>> @@ -119,7 +119,7 @@ unsigned long iommu_tbl_range_alloc(struct device *dev,
>>>   	unsigned long align_mask = 0;
>>>
>>>   	if (align_order > 0)
>>> -		align_mask = 0xffffffffffffffffl >> (64 - align_order);
>>> +		align_mask = ~0l >> (64 - align_order);
>>>
>> Wonder if this just hides the real problem. Unless align_order
>> is very large, the resulting mask on 32 bit systems may be 0.
>> Is this really the idea ?
>
> Probably not, but that's not what would happen on x86: the shift
> only depends on the lower 5 or 6 bits - I don't know if other platforms
> also has that behaviour. So for align_order == 2 and x86_32 we'd
> effectively end up with a shift of 62 & 31 == 30 (though technically
> undefined behaviour), and the desired mask of 0x3.
>

#include <stdio.h>

main(int argc, char **argv)
{
         unsigned long am1, am2, am3;
         int align_order = 2;

         am1 = 0xffffffffffffffffl >> (64 - align_order);
         am2 = ~0l >> (64 - align_order);
         am3 = ~0ul >> (64 - align_order);

         printf("0x%lx 0x%lx 0x%lx\n", am1, am2, am3);
}

yields an output of

0x3 0xffffffffffffffff 0x3

So either case ~0l appears to be wrong; it should be ~0ul.
I don't know if ~0ull makes a difference for some architectures.

Guenter

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