Subject: Re: [RFC PATCH 2/2] arm64: Expose EPAN support via HWCAPS2_EPAN

From: Catalin Marinas <[email-masked]>
Date: Tue, 17 Nov 2020 16:59:37 +0000

On Fri, Nov 13, 2020 at 03:20:23PM +0000, Vladimir Murzin wrote:
> So user have a clue whether exec-only permissions will work.

I do think we should tell user the PROT_EXEC actually gives execute-only
permission.

> --- a/arch/arm64/include/uapi/asm/hwcap.h
> +++ b/arch/arm64/include/uapi/asm/hwcap.h
> @@ -75,5 +75,6 @@
>  #define HWCAP2_RNG		(1 << 16)
>  #define HWCAP2_BTI		(1 << 17)
>  #define HWCAP2_MTE		(1 << 18)
> +#define HWCAP2_EPAN		(1 << 19)

However, I wonder whether EPAN is meaningful to the user. PAN is a
kernel protection that doesn't say much from a user perspective. Maybe
something like HWCAP2_EXECONLY?

That said, we do have a precedent on 32-bit where we exposed HWCAP_LPAE
to the user meaning that 64-bit atomics are available.

-- 
Catalin

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