Re: [PATCH 2/2] bitfield: Ensure the return value of type##_replace_bits() is checked

[Yury Norov <yury.norov@gmail.com>]

On Tue, Jul 08, 2025 at 10:45:50AM +0100, Marc Zyngier wrote:
> On Tue, 08 Jul 2025 10:42:06 +0100,
> Ben Horgan <ben.horgan@arm.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@arm.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?

Yes please.
 
> > > 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.

OK. Then I'll take patch #2 v2 by myself.

Thanks,
Yury