Re: [PATCH v2] arm64: jump_label: use constraints "Si" instead of "i"

[Mark Rutland]

On Mon, Feb 05, 2024 at 11:45:52PM -0800, Fangrui Song wrote:
> The generic constraint "i" seems to be copied from x86 or arm (and with
> a redundant generic operand modifier "c"). It works with -fno-PIE but
> not with -fPIE/-fPIC in GCC's aarch64 port.
> 
> The machine constraint "S", which denotes a symbol or label reference
> with a constant offset, supports PIC and has been available in GCC since
> 2012 and in Clang since 7.0. However, Clang before 19 does not support
> "S" on a symbol with a constant offset [1] (e.g.
> `static_key_false(&nf_hooks_needed[pf][hook])` in
> include/linux/netfilter.h), so we use "i" as a fallback.
> 
> Suggested-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>
> Link: https://github.com/llvm/llvm-project/pull/80255 [1]

This looks reasonable to me, and works with the toolchains I've tried, so:

Acked-by: Mark Rutland <mark.rutland@xxxxxxx>

Mark.

> ---
> Changes from
> arm64: jump_label: use constraint "S" instead of "i" (https://lore.kernel.org/all/20240131065322.1126831-1-maskray@xxxxxxxxxx/)
> 
> * Use "Si" as Ard suggested to support Clang<19
> * Make branch a separate operand
> 
> Changes from v1:
> 
> * Use asmSymbolicName for readability
> ---
>  arch/arm64/include/asm/jump_label.h | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/jump_label.h b/arch/arm64/include/asm/jump_label.h
> index 48ddc0f45d22..b7716b215f91 100644
> --- a/arch/arm64/include/asm/jump_label.h
> +++ b/arch/arm64/include/asm/jump_label.h
> @@ -15,6 +15,10 @@
>  
>  #define JUMP_LABEL_NOP_SIZE		AARCH64_INSN_SIZE
>  
> +/*
> + * Prefer the constraint "S" to support PIC with GCC. Clang before 19 does not
> + * support "S" on a symbol with a constant offset, so we use "i" as a fallback.
> + */
>  static __always_inline bool arch_static_branch(struct static_key * const key,
>  					       const bool branch)
>  {
> @@ -23,9 +27,9 @@ 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		(%[key] - .) + %[bit0]  \n\t"
>  		 "	.popsection				\n\t"
> -		 :  :  "i"(&((char *)key)[branch]) :  : l_yes);
> +		 :  :  [key]"Si"(key), [bit0]"i"(branch) :  : l_yes);
>  
>  	return false;
>  l_yes:
> @@ -40,9 +44,9 @@ 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		(%[key] - .) + %[bit0]  \n\t"
>  		 "	.popsection				\n\t"
> -		 :  :  "i"(&((char *)key)[branch]) :  : l_yes);
> +		 :  :  [key]"Si"(key), [bit0]"i"(branch) :  : l_yes);
>  
>  	return false;
>  l_yes:
> -- 
> 2.43.0.594.gd9cf4e227d-goog
> 
>