[PATCH] KVM: arm64: fix u64_replace_bits() usage

[PATCH] KVM: arm64: fix u64_replace_bits() usage

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

u64_replace_bits() returns a modified word but does not actually modify
its argument, as pointed out by this new warning:

arch/arm64/kvm/sys_regs.c: In function 'access_mdcr':
arch/arm64/kvm/sys_regs.c:2654:17: error: ignoring return value of 'u64_replace_bits' declared with attribute 'warn_unused_result' [-Werror=unused-result]
 2654 |                 u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);

The intention here must have been to update 'val', so do that instead.

Fixes: efff9dd2fee7 ("KVM: arm64: Handle out-of-bound write to MDCR_EL2.HPMN")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 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 33aa4f5071b8..793fb19bebd6 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -2651,7 +2651,7 @@ 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);
+		val = u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
 	}
 
 	__vcpu_assign_sys_reg(vcpu, MDCR_EL2, val);
-- 
2.39.5

Re: [PATCH] KVM: arm64: fix u64_replace_bits() usage

[Zenghui Yu]

On 2025/7/11 15:27, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> u64_replace_bits() returns a modified word but does not actually modify
> its argument, as pointed out by this new warning:
> 
> arch/arm64/kvm/sys_regs.c: In function 'access_mdcr':
> arch/arm64/kvm/sys_regs.c:2654:17: error: ignoring return value of 'u64_replace_bits' declared with attribute 'warn_unused_result' [-Werror=unused-result]
>  2654 |                 u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
> 
> The intention here must have been to update 'val', so do that instead.
> 
> Fixes: efff9dd2fee7 ("KVM: arm64: Handle out-of-bound write to MDCR_EL2.HPMN")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  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 33aa4f5071b8..793fb19bebd6 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -2651,7 +2651,7 @@ 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);
> +		val = u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
>  	}
>  
>  	__vcpu_assign_sys_reg(vcpu, MDCR_EL2, val);

Already fixed by Ben's patch [*], thanks!

[*]
https://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm.git/commit/?h=fixes&id=2265c08ec393ef1f5ef5019add0ab1e3a7ee0b79

Zenghui

Re: [PATCH] KVM: arm64: fix u64_replace_bits() usage

[Marc Zyngier]

On Fri, 11 Jul 2025 08:27:47 +0100,
Arnd Bergmann <arnd@kernel.org> wrote:
> 
> From: Arnd Bergmann <arnd@arndb.de>
> 
> u64_replace_bits() returns a modified word but does not actually modify
> its argument, as pointed out by this new warning:
> 
> arch/arm64/kvm/sys_regs.c: In function 'access_mdcr':
> arch/arm64/kvm/sys_regs.c:2654:17: error: ignoring return value of 'u64_replace_bits' declared with attribute 'warn_unused_result' [-Werror=unused-result]
>  2654 |                 u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
> 
> The intention here must have been to update 'val', so do that instead.
> 
> Fixes: efff9dd2fee7 ("KVM: arm64: Handle out-of-bound write to MDCR_EL2.HPMN")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  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 33aa4f5071b8..793fb19bebd6 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -2651,7 +2651,7 @@ 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);
> +		val = u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
>  	}
>  
>  	__vcpu_assign_sys_reg(vcpu, MDCR_EL2, val);

This is only in -next, right? Because I have a fix for this already
queued for 6.16, as per [1].

Thanks,

	M.

[1] https://lore.kernel.org/r/20250709093808.920284-2-ben.horgan@arm.com

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

Re: [PATCH] KVM: arm64: fix u64_replace_bits() usage

[Arnd Bergmann]

On Fri, Jul 11, 2025, at 10:36, Marc Zyngier wrote:
> On Fri, 11 Jul 2025 08:27:47 +0100, Arnd Bergmann <arnd@kernel.org> wrote:
>>  	if (hpmn > vcpu->kvm->arch.nr_pmu_counters) {
>>  		hpmn = vcpu->kvm->arch.nr_pmu_counters;
>> -		u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
>> +		val = u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
>>  	}
>>  
>>  	__vcpu_assign_sys_reg(vcpu, MDCR_EL2, val);
>
> This is only in -next, right? Because I have a fix for this already
> queued for 6.16, as per [1].

Yes, as far as I can tell, the warning only showed up in linux-next
after f66f9c3d09c1 ("bitfield: Ensure the return values of helper
functions are checked").

As far as I can tell, Ben added the check in linux/bitfield.h
when he sent you his version of the fix, they just ended up
in linux-next in the wrong order, so I ended up recreating his
original fix slightly differently.

      Arnd

Re: [PATCH] KVM: arm64: fix u64_replace_bits() usage

[Marc Zyngier]

On Fri, 11 Jul 2025 09:44:23 +0100,
"Arnd Bergmann" <arnd@arndb.de> wrote:
> 
> On Fri, Jul 11, 2025, at 10:36, Marc Zyngier wrote:
> > On Fri, 11 Jul 2025 08:27:47 +0100, Arnd Bergmann <arnd@kernel.org> wrote:
> >>  	if (hpmn > vcpu->kvm->arch.nr_pmu_counters) {
> >>  		hpmn = vcpu->kvm->arch.nr_pmu_counters;
> >> -		u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
> >> +		val = u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
> >>  	}
> >>  
> >>  	__vcpu_assign_sys_reg(vcpu, MDCR_EL2, val);
> >
> > This is only in -next, right? Because I have a fix for this already
> > queued for 6.16, as per [1].
> 
> Yes, as far as I can tell, the warning only showed up in linux-next
> after f66f9c3d09c1 ("bitfield: Ensure the return values of helper
> functions are checked").
> 
> As far as I can tell, Ben added the check in linux/bitfield.h
> when he sent you his version of the fix, they just ended up
> in linux-next in the wrong order, so I ended up recreating his
> original fix slightly differently.

I don't think Ben's fix is in -next, as I queued it in the kvmarm
fixes branch, which isn't pulled by -next.

Hopefully Paolo will send this to Linus shortly (pull request
here[1]), and -next will be clean again.

Thanks,

	M.

[1] https://lore.kernel.org/r/20250711084835.2411230-1-maz@kernel.org

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

Re: [PATCH] KVM: arm64: fix u64_replace_bits() usage

[Mark Brown]

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On Fri, Jul 11, 2025 at 09:53:41AM +0100, Marc Zyngier wrote:
> "Arnd Bergmann" <arnd@arndb.de> wrote:

> > Yes, as far as I can tell, the warning only showed up in linux-next
> > after f66f9c3d09c1 ("bitfield: Ensure the return values of helper
> > functions are checked").

> > As far as I can tell, Ben added the check in linux/bitfield.h
> > when he sent you his version of the fix, they just ended up
> > in linux-next in the wrong order, so I ended up recreating his
> > original fix slightly differently.

> I don't think Ben's fix is in -next, as I queued it in the kvmarm
> fixes branch, which isn't pulled by -next.

We should really get that added to -next, that'll ensure it gets covered
in the pending-fixes testing coverage and avoid issues like this with
the main -next tree.

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