linux-kernel - Re: [PATCH AUTOSEL 6.7 07/26] arm64: jump_label: use constraints "Si" instead of "i"

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Message-ID: <ZeCpOPDi18OBEclz@FVFF77S0Q05N>
Date: Thu, 29 Feb 2024 15:56:40 +0000
From: Mark Rutland <mark.rutland@....com>
To: Sasha Levin <sashal@...nel.org>
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Fangrui Song <maskray@...gle.com>, Ard Biesheuvel <ardb@...nel.org>,
	Will Deacon <will@...nel.org>, peterz@...radead.org,
	jpoimboe@...nel.org, jbaron@...mai.com, catalin.marinas@....com,
	nathan@...nel.org, linux-arm-kernel@...ts.infradead.org,
	llvm@...ts.linux.dev
Subject: Re: [PATCH AUTOSEL 6.7 07/26] arm64: jump_label: use constraints
 "Si" instead of "i"

On Thu, Feb 29, 2024 at 10:48:26AM -0500, Sasha Levin wrote:
> From: Fangrui Song <maskray@...gle.com>
> 
> [ Upstream commit f9daab0ad01cf9d165dbbbf106ca4e61d06e7fe8 ]

That upstream patch caused issues and was subsequently reverted in commit:

  a6b3eb304a82c29665a0ab947cfe276f6d29f523
  ("Revert "arm64: jump_label: use constraints "Si" instead of "i""")

Please drop this; it'll break stable in the same way, and it wasn't necessary
to backport in the first place.

As an aside, that revert has been upstream since v6.8-rc4, ~2.5 weeks ago; it's
unfortunate for the broken patch to be AUTOSEL'd now! It'd be good if something
could automatically check for reverts in mainline...

Mark.

> 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@...nel.org>
> Signed-off-by: Fangrui Song <maskray@...gle.com>
> Link: https://github.com/llvm/llvm-project/pull/80255 [1]
> Acked-by: Mark Rutland <mark.rutland@....com>
> Link: https://lore.kernel.org/r/20240206074552.541154-1-maskray@google.com
> Signed-off-by: Will Deacon <will@...nel.org>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  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 48ddc0f45d228..b7716b215f91a 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
>