Re: [PATCH AUTOSEL 6.6 06/21] arm64: jump_label: use constraints "Si" instead of "i"

Mark Rutland
2024-02-29T15:58:56Z

On Thu, Feb 29, 2024 at 10:49:26AM -0500, Sasha Levin wrote:
> From: Fangrui Song <maskray@google.com>
>
> [ Upstream commit f9daab0ad01cf9d165dbbbf106ca4e61d06e7fe8 ]

Please drop this; same story as wit the v6.7 backport:

https://lore.kernel.org/lkml/ZeCpOPDi18OBEclz@FVFF77S0Q05N/

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