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

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Message-ID: <Z0ChYA7bsmnZRI2d@sashalap>
Date: Fri, 22 Nov 2024 10:21:04 -0500
From: Sasha Levin <sashal@...nel.org>
To: Nathan Chancellor <nathan@...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

On Wed, Nov 20, 2024 at 08:13:38AM -0700, Nathan Chancellor wrote:
>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.

Ack, will do.

-- 
Thanks,
Sasha