linux-kernel - Re: [PATCH AUTOSEL 6.6 5/6] ARM: 9434/1: cfi: Fix compilation corner case

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Message-ID: <20241120151338.GA3158726@thelio-3990X>
Date: Wed, 20 Nov 2024 08:13:38 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Sasha Levin <sashal@...nel.org>
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Linus Walleij <linus.walleij@...aro.org>,
	kernel test robot <lkp@...el.com>,
	Russell King <rmk+kernel@...linux.org.uk>, linux@...linux.org.uk,
	arnd@...db.de, samitolvanen@...gle.com,
	linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev
Subject: Re: [PATCH AUTOSEL 6.6 5/6] ARM: 9434/1: cfi: Fix compilation corner
 case

Hi Sasha,

On Wed, Nov 20, 2024 at 09:06:35AM -0500, Sasha Levin wrote:
> From: Linus Walleij <linus.walleij@...aro.org>
> 
> [ Upstream commit 4aea16b7cfb76bd3361858ceee6893ef5c9b5570 ]
> 
> When enabling expert mode CONFIG_EXPERT and using that power
> user mode to disable the branch prediction hardening
> !CONFIG_HARDEN_BRANCH_PREDICTOR, the assembly linker
> in CLANG notices that some assembly in proc-v7.S does
> not have corresponding C call sites, i.e. the prototypes
> in proc-v7-bugs.c are enclosed in ifdef
> CONFIG_HARDEN_BRANCH_PREDICTOR so this assembly:
> 
> SYM_TYPED_FUNC_START(cpu_v7_smc_switch_mm)
> SYM_TYPED_FUNC_START(cpu_v7_hvc_switch_mm)
> 
> Results in:
> 
> ld.lld: error: undefined symbol: __kcfi_typeid_cpu_v7_smc_switch_mm
> >>> referenced by proc-v7.S:94 (.../arch/arm/mm/proc-v7.S:94)
> >>> arch/arm/mm/proc-v7.o:(.text+0x108) in archive vmlinux.a
> 
> ld.lld: error: undefined symbol: __kcfi_typeid_cpu_v7_hvc_switch_mm
> >>> referenced by proc-v7.S:105 (.../arch/arm/mm/proc-v7.S:105)
> >>> arch/arm/mm/proc-v7.o:(.text+0x124) in archive vmlinux.a
> 
> Fix this by adding an additional requirement that
> CONFIG_HARDEN_BRANCH_PREDICTOR has to be enabled to compile
> these assembly calls.
> 
> Closes: https://lore.kernel.org/oe-kbuild-all/202411041456.ZsoEiD7T-lkp@intel.com/
> 
> Reported-by: kernel test robot <lkp@...el.com>
> Reviewed-by: Nathan Chancellor <nathan@...nel.org>
> Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
> Signed-off-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  arch/arm/mm/proc-v7.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm/mm/proc-v7.S b/arch/arm/mm/proc-v7.S
> index 193c7aeb67039..bea11f9bfe856 100644
> --- a/arch/arm/mm/proc-v7.S
> +++ b/arch/arm/mm/proc-v7.S
> @@ -93,7 +93,7 @@ ENTRY(cpu_v7_dcache_clean_area)
>  	ret	lr
>  ENDPROC(cpu_v7_dcache_clean_area)
>  
> -#ifdef CONFIG_ARM_PSCI
> +#if defined(CONFIG_ARM_PSCI) && defined(CONFIG_HARDEN_BRANCH_PREDICTOR)
>  	.arch_extension sec
>  ENTRY(cpu_v7_smc_switch_mm)

This patch is unnecessary in branches prior to 6.10 (when ARM started
supporting kCFI) because SYM_TYPED_FUNC_START() is not used here. I
would just drop it for 6.6 and earlier.

>  	stmfd	sp!, {r0 - r3}
> -- 
> 2.43.0
>