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

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Message-ID: <86tt3n9fsh.wl-maz@kernel.org>
Date: Tue, 08 Jul 2025 10:45:50 +0100
From: Marc Zyngier <maz@...nel.org>
To: Ben Horgan <ben.horgan@....com>
Cc: Yury Norov <yury.norov@...il.com>,
	catalin.marinas@....com,
	will@...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

On Tue, 08 Jul 2025 10:42:06 +0100,
Ben Horgan <ben.horgan@....com> wrote:
> 
> 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.

The first patch will definitely go in via the KVM/arm64 tree, probably
as a fix for 6.16.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.