Re: [PATCH] arm64/cpufeature: Validate feature bits spacing in arm64_ftr_regs[]

[Suzuki K Poulose]

On 06/16/2020 03:25 AM, Anshuman Khandual wrote:
> arm64_feature_bits for a register in arm64_ftr_regs[] are in a descending
> order as per their shift values. Validate that these features bits are
> defined correctly and do not overlap with each other. This check protects
> against any inadvertent erroneous changes to the register definitions.
>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> Cc: Mark Brown <broonie@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
> Applies on 5.8-rc1.
>
>   arch/arm64/kernel/cpufeature.c | 45 +++++++++++++++++++++++++++++++---
>   1 file changed, 42 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 4ae41670c2e6..2270eda9a7fb 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -697,11 +697,50 @@ static s64 arm64_ftr_safe_value(const struct arm64_ftr_bits *ftrp, s64 new,
>   
>   static void __init sort_ftr_regs(void)
>   {
> -	int i;
> +	const struct arm64_ftr_reg *ftr_reg;
> +	const struct arm64_ftr_bits *ftr_bits;
> +	unsigned int i, j, width, shift, prev_shift;
> +
> +	for (i = 0; i < ARRAY_SIZE(arm64_ftr_regs); i++) {
> +		/*
> +		 * Features here must be sorted in descending order with respect
> +		 * to their shift values and should not overlap with each other.
> +		 */
> +		ftr_reg = arm64_ftr_regs[i].reg;
> +		for (ftr_bits = ftr_reg->ftr_bits, j = 0;
> +				ftr_bits->width != 0; ftr_bits++, j++) {
> +			if (WARN_ON(ftr_bits->shift  + ftr_bits->width > 64))
> +				pr_err("%s has invalid feature at shift %d\n",
> +					ftr_reg->name, ftr_bits->shift);

nit:

			WARN((ftr_bits->shift + ftr_bits->width) > 64,
				"%s......);?

> +
> +			/*
> +			 * Skip the first feature. There is nothing to
> +			 * compare against for now.
> +			 */
> +			if (j == 0)
> +				continue;
> +
> +			prev_shift = ftr_reg->ftr_bits[j - 1].shift;
> +			width = ftr_reg->ftr_bits[j].width;
> +			shift = ftr_reg->ftr_bits[j].shift;
> +			if (WARN_ON(prev_shift < shift + width))
> +				pr_err("%s has feature overlap at shift %d\n",
> +					ftr_reg->name, ftr_bits->shift);

same as above ?

> +		}
>   
> -	/* Check that the array is sorted so that we can do the binary search */
> -	for (i = 1; i < ARRAY_SIZE(arm64_ftr_regs); i++)
> +		/*
> +		 * Skip the first register. There is nothing to
> +		 * compare against for now.
> +		 */
> +		if (i == 0)
> +			continue;

You are starting at 1 already, so you may skip this check.

> +		/*
> +		 * Registers here must be sorted in ascending order with respect
> +		 * to sys_id for subsequent binary search in get_arm64_ftr_reg()
> +		 * to work correctly.
> +		 */
>   		BUG_ON(arm64_ftr_regs[i].sys_id < arm64_ftr_regs[i - 1].sys_id);
> +	}
>   }
>   
>   /*

Otherwise looks good to me.

Suzuki