linux-kernel - Re: [PATCH 2/2] bitfield: Ensure the return value of type##_replace

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Message-ID: <03a76e9a-86ac-4791-9f0a-494b28c07fcc@arm.com>
Date: Tue, 8 Jul 2025 10:42:06 +0100
From: Ben Horgan <ben.horgan@....com>
To: Yury Norov <yury.norov@...il.com>
Cc: catalin.marinas@....com, will@...nel.org, maz@...nel.org,
 oliver.upton@...ux.dev, joey.gouly@....com, suzuki.poulose@....com,
 yuzenghui@...wei.com, linux-arm-kernel@...ts.infradead.org,
 kvmarm@...ts.linux.dev, linux@...musvillemoes.dk,
 linux-kernel@...r.kernel.org, james.morse@....com
Subject: Re: [PATCH 2/2] bitfield: Ensure the return value of
 type##_replace_bits() is checked

Hi Yury,

On 7/7/25 17:31, Yury Norov wrote:
> Hi Ben,
> 
> On Thu, Jul 03, 2025 at 02:57:29PM +0100, Ben Horgan wrote:
>> As type##_replace_bits() has no side effects it is only useful if its
>> return value is checked. Add __must_check to enforce this usage. To have
>> the bits replaced in-place typep##_replace_bits() can be used instead.
>>
>> Signed-off-by: Ben Horgan <ben.horgan@....com>
>> ---
>>   include/linux/bitfield.h | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
>> index 6d9a53db54b6..39333b80d22b 100644
>> --- a/include/linux/bitfield.h
>> +++ b/include/linux/bitfield.h
>> @@ -195,8 +195,8 @@ static __always_inline __##type type##_encode_bits(base v, base field)	\
>>   		__field_overflow();					\
>>   	return to((v & field_mask(field)) * field_multiplier(field));	\
>>   }									\
>> -static __always_inline __##type type##_replace_bits(__##type old,	\
>> -					base val, base field)		\
>> +static __always_inline __##type __must_check type##_replace_bits(__##type old,	\
>> +							base val, base field)	\
>>   {									\
>>   	return (old & ~to(field)) | type##_encode_bits(val, field);	\
>>   }									\
> 
> So, would it make sense to mark _encode_bits() and _get_bits() as
> __must_check as well? At least from the point of unification, it
> would.
Could do. It seems less important as there are no obvious foot-guns that 
these would guards against. Would you like me to add this in a v2?
> 
> How would we move this - with my bitmap-for next or with arm branch?

I'm not familiar with the branch machinery so can't comment on this.
> 
> Thanks,
> Yury
> 

Thanks,

Ben