[PATCH v2 0/2] Fix and add warning of misuse of type##_replace_bits()

[PATCH v2 0/2] Fix and add warning of misuse of type##_replace_bits()

[Ben Horgan]

By inspection there is one mistake in the use of u64_replace_bits(). Fix
this and while I'm here add a __must_check annotation to help avoid the
same mistake happening again.

Changes since v1:

Patch 1: Add Rb tag from Zenghui Yu - thanks!
Patch 2: Extend to add __must_check on _get_bits() and _encode_bits()

Ben Horgan (2):
  KVM: arm64: Fix enforcement of upper bound on MDCR_EL2.HPMN
  bitfield: Ensure the return values of helper functions are checked

 arch/arm64/kvm/sys_regs.c | 2 +-
 include/linux/bitfield.h  | 8 ++++----
 2 files changed, 5 insertions(+), 5 deletions(-)

-- 
2.43.0

[PATCH v2 1/2] KVM: arm64: Fix enforcement of upper bound on MDCR_EL2.HPMN

[Ben Horgan]

Previously, u64_replace_bits() was used to no effect as the return value
was ignored. Convert to u64p_replace_bits() so the value is updated in
place.

Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Fixes: efff9dd2fee7 ("KVM: arm64: Handle out-of-bound write to MDCR_EL2.HPMN")
Cc: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
---
 arch/arm64/kvm/sys_regs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 76c2f0da821f..c20bd6f21e60 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -2624,7 +2624,7 @@ static bool access_mdcr(struct kvm_vcpu *vcpu,
 	 */
 	if (hpmn > vcpu->kvm->arch.nr_pmu_counters) {
 		hpmn = vcpu->kvm->arch.nr_pmu_counters;
-		u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
+		u64p_replace_bits(&val, hpmn, MDCR_EL2_HPMN);
 	}
 
 	__vcpu_assign_sys_reg(vcpu, MDCR_EL2, val);
-- 
2.43.0

[PATCH v2 2/2] bitfield: Ensure the return values of helper functions are checked

[Ben Horgan]

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.

Although, type_##_get_bits() and type_##_encode_bits() are harder to misuse
they are still only useful if the return value is checked. For
consistency, also add __must_check to these.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
 include/linux/bitfield.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 6d9a53db54b6..5355f8f806a9 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -189,14 +189,14 @@ static __always_inline u64 field_mask(u64 field)
 }
 #define field_max(field)	((typeof(field))field_mask(field))
 #define ____MAKE_OP(type,base,to,from)					\
-static __always_inline __##type type##_encode_bits(base v, base field)	\
+static __always_inline __##type __must_check type##_encode_bits(base v, base field)	\
 {									\
 	if (__builtin_constant_p(v) && (v & ~field_mask(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);	\
 }									\
@@ -205,7 +205,7 @@ static __always_inline void type##p_replace_bits(__##type *p,		\
 {									\
 	*p = (*p & ~to(field)) | type##_encode_bits(val, field);	\
 }									\
-static __always_inline base type##_get_bits(__##type v, base field)	\
+static __always_inline base __must_check type##_get_bits(__##type v, base field)	\
 {									\
 	return (from(v) & field)/field_multiplier(field);		\
 }
-- 
2.43.0

Re: (subset) [PATCH v2 1/2] KVM: arm64: Fix enforcement of upper bound on MDCR_EL2.HPMN

[Marc Zyngier]

On Wed, 09 Jul 2025 10:38:07 +0100, Ben Horgan wrote:
> Previously, u64_replace_bits() was used to no effect as the return value
> was ignored. Convert to u64p_replace_bits() so the value is updated in
> place.
> 
> 

Applied to fixes, thanks!

I have dropped the Cc: stable, as ths bug only exists in 6.16, and we
are not backporting anything related to NV to previous kernel versions.

[1/2] KVM: arm64: Fix enforcement of upper bound on MDCR_EL2.HPMN
      commit: 2265c08ec393ef1f5ef5019add0ab1e3a7ee0b79

Cheers,

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

Re: [PATCH v2 2/2] bitfield: Ensure the return values of helper functions are checked

[Yury Norov]

On Wed, Jul 09, 2025 at 10:38:08AM +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.
> 
> Although, type_##_get_bits() and type_##_encode_bits() are harder to misuse
> they are still only useful if the return value is checked. For
> consistency, also add __must_check to these.
> 
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>

Applied.

Thanks,
Yury