linux-kernel - Re: [PATCH v3 30/60] arm64: idreg-override: Create a pseudo feature for rodata=off

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Message-ID: <c4d4f543-9dae-9514-3411-7061192af530@arm.com>
Date:   Mon, 17 Apr 2023 15:28:47 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org
Cc:     linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v3 30/60] arm64: idreg-override: Create a pseudo feature
 for rodata=off

On 07/03/2023 14:04, Ard Biesheuvel wrote:
> Add rodata=off to the set of kernel command line options that is parsed
> early using the CPU feature override detection code, so we can easily
> refer to it when creating the kernel mapping.
> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  arch/arm64/include/asm/cpufeature.h   | 1 +
>  arch/arm64/kernel/pi/idreg-override.c | 2 ++
>  2 files changed, 3 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index bc10098901808c00..edc7733aa49846b2 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -16,6 +16,7 @@
>  #define cpu_feature(x)		KERNEL_HWCAP_ ## x
>  
>  #define ARM64_SW_FEATURE_OVERRIDE_NOKASLR	0
> +#define ARM64_SW_FEATURE_OVERRIDE_RODATA_OFF	4

I assume these are bit numbers? Why not just use the next available bit (bit 1)
for this new flag?

>  
>  #ifndef __ASSEMBLY__
>  
> diff --git a/arch/arm64/kernel/pi/idreg-override.c b/arch/arm64/kernel/pi/idreg-override.c
> index 4e76db6eb72c2087..6c547cccaf6a9e9c 100644
> --- a/arch/arm64/kernel/pi/idreg-override.c
> +++ b/arch/arm64/kernel/pi/idreg-override.c
> @@ -151,6 +151,7 @@ static const struct ftr_set_desc sw_features __prel64_initconst = {
>  	.override	= &arm64_sw_feature_override,
>  	.fields		= {
>  		FIELD("nokaslr", ARM64_SW_FEATURE_OVERRIDE_NOKASLR, NULL),
> +		FIELD("rodataoff", ARM64_SW_FEATURE_OVERRIDE_RODATA_OFF, NULL),
>  		{}
>  	},
>  };
> @@ -183,6 +184,7 @@ static const struct {
>  	  "id_aa64isar2.gpa3=0 id_aa64isar2.apa3=0"	   },
>  	{ "arm64.nomte",		"id_aa64pfr1.mte=0" },
>  	{ "nokaslr",			"arm64_sw.nokaslr=1" },
> +	{ "rodata=off",			"arm64_sw.rodataoff=1" },
>  };
>  
>  static int __init parse_hexdigit(const char *p, u64 *v)