linux-kernel - Re: [PATCH] arm64: jump_label: use constraint "S" instead of "i"

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Date: Wed, 31 Jan 2024 08:16:04 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Fangrui Song <maskray@...gle.com>
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	linux-arm-kernel@...ts.infradead.org, Jisheng Zhang <jszhang@...nel.org>, 
	llvm@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: jump_label: use constraint "S" instead of "i"

Hello Fangrui,

On Wed, 31 Jan 2024 at 07:53, Fangrui Song <maskray@...gle.com> wrote:
>
> The constraint "i" seems to be copied from x86 (and with a redundant
> modifier "c"). It works with -fno-PIE but not with -fPIE/-fPIC in GCC's
> aarch64 port.
>
> The constraint "S", which denotes a symbol reference (e.g. function,
> global variable) or label reference, is more appropriate, and has been
> available in GCC since 2012 and in Clang since 7.0.
>
> Signed-off-by: Fangrui Song <maskray@...gle.com>
> Link: https://maskray.me/blog/2024-01-30-raw-symbol-names-in-inline-assembly
> ---
>  arch/arm64/include/asm/jump_label.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/include/asm/jump_label.h b/arch/arm64/include/asm/jump_label.h
> index 48ddc0f45d22..31862b3bb33d 100644
> --- a/arch/arm64/include/asm/jump_label.h
> +++ b/arch/arm64/include/asm/jump_label.h
> @@ -23,9 +23,9 @@ static __always_inline bool arch_static_branch(struct static_key * const key,
>                  "      .pushsection    __jump_table, \"aw\"    \n\t"
>                  "      .align          3                       \n\t"
>                  "      .long           1b - ., %l[l_yes] - .   \n\t"
> -                "      .quad           %c0 - .                 \n\t"
> +                "      .quad           %0 - .                  \n\t"
>                  "      .popsection                             \n\t"
> -                :  :  "i"(&((char *)key)[branch]) :  : l_yes);
> +                :  :  "S"(&((char *)key)[branch]) :  : l_yes);

'key' is not used as a raw symbol name. We should make this

"    .quad   %0 + %1 - ."

and

::  "S"(key), "i"(branch) :: l_yes);

if we want to really clean this up.



>
>         return false;
>  l_yes:
> @@ -40,9 +40,9 @@ static __always_inline bool arch_static_branch_jump(struct static_key * const ke
>                  "      .pushsection    __jump_table, \"aw\"    \n\t"
>                  "      .align          3                       \n\t"
>                  "      .long           1b - ., %l[l_yes] - .   \n\t"
> -                "      .quad           %c0 - .                 \n\t"
> +                "      .quad           %0 - .                  \n\t"
>                  "      .popsection                             \n\t"
> -                :  :  "i"(&((char *)key)[branch]) :  : l_yes);
> +                :  :  "S"(&((char *)key)[branch]) :  : l_yes);
>
>         return false;
>  l_yes:
> --
> 2.43.0.429.g432eaa2c6b-goog
>