[PATCH] arm64: cpufeature: Warn if we attempt to read a zero width field

[PATCH] arm64: cpufeature: Warn if we attempt to read a zero width field

[Mark Brown]

Add a WARN_ON_ONCE() when extracting a field if no width is specified. This
should never happen outside of development since it will be triggered with
or without the feature so long as the relevant ID register is present.

Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/include/asm/cpufeature.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 2fe54369ac3f..4c8badfa9e8b 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -577,6 +577,7 @@ static inline u64 arm64_ftr_reg_user_value(const struct arm64_ftr_reg *reg)
 static inline int __attribute_const__
 cpuid_feature_extract_field_width(u64 features, int field, int width, bool sign)
 {
+	WARN_ON_ONCE(!width);
 	return (sign) ?
 		cpuid_feature_extract_signed_field_width(features, field, width) :
 		cpuid_feature_extract_unsigned_field_width(features, field, width);
-- 
2.30.2


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Re: [PATCH] arm64: cpufeature: Warn if we attempt to read a zero width field

[Will Deacon]

On Wed, Mar 02, 2022 at 01:47:02PM +0000, Mark Brown wrote:
> Add a WARN_ON_ONCE() when extracting a field if no width is specified. This
> should never happen outside of development since it will be triggered with
> or without the feature so long as the relevant ID register is present.
> 
> Suggested-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>  arch/arm64/include/asm/cpufeature.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 2fe54369ac3f..4c8badfa9e8b 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -577,6 +577,7 @@ static inline u64 arm64_ftr_reg_user_value(const struct arm64_ftr_reg *reg)
>  static inline int __attribute_const__
>  cpuid_feature_extract_field_width(u64 features, int field, int width, bool sign)
>  {
> +	WARN_ON_ONCE(!width);

I think this makes sense, but lets also override width to be 4 in this case,
as that is much more likely to "work" than 0. i.e.

	if (WARN_ON_ONCE(!width))
		width = 4;

Will

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