linux-kernel - Re: [PATCH v2 1/8] selftests: vDSO: fix -Wunitialized in powerpc VDSO

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Message-ID: <d72bb0f7-8dc3-42ca-84d9-c74b4db02445@csgroup.eu>
Date: Tue, 12 Aug 2025 08:05:51 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
 Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Vincenzo Frascino <vincenzo.frascino@....com>, Shuah Khan
 <shuah@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 "Jason A. Donenfeld" <Jason@...c4.com>
Cc: linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 Shuah Khan <skhan@...uxfoundation.org>, llvm@...ts.linux.dev,
 kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v2 1/8] selftests: vDSO: fix -Wunitialized in powerpc
 VDSO_CALL() wrapper



Le 12/08/2025 à 07:39, Thomas Weißschuh a écrit :
> The _rval register variable is meant to be an output operand of the asm
> statement but is instead used as input operand.
> clang 20.1 notices this and triggers -Wuninitialized warnings:
> 
> tools/testing/selftests/timers/auxclock.c:154:10: error: variable '_rval' is uninitialized when used here [-Werror,-Wuninitialized]
>    154 |                 return VDSO_CALL(self->vdso_clock_gettime64, 2, clockid, ts);
>        |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> tools/testing/selftests/timers/../vDSO/vdso_call.h:59:10: note: expanded from macro 'VDSO_CALL'
>     59 |                 : "r" (_rval)                                           \
>        |                        ^~~~~
> tools/testing/selftests/timers/auxclock.c:154:10: note: variable '_rval' is declared here
> tools/testing/selftests/timers/../vDSO/vdso_call.h:47:2: note: expanded from macro 'VDSO_CALL'
>     47 |         register long _rval asm ("r3");                                 \
>        |         ^
> 
> It seems the list of input and output operands have been switched around.
> However as the argument registers are not always initialized they can not
> be marked as pure inputs as that would trigger -Wuninitialized warnings.
> Adding _rval as another input and output operand does also not work as it
> would collide with the existing _r3 variable.
> 
> Instead reuse _r3 for both the argument and the return value.
> 
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Foe-kbuild-all%2F202506180223.BOOk5jDK-lkp%40intel.com%2F&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C611dd96dfdad4dcd55d708ddd9629305%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638905739611064850%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=QcOje8P4T0%2BsqgkSaP%2BkT3fS875kqCZhxChDmvWVY%2Fg%3D&reserved=0
> Fixes: 6eda706a535c ("selftests: vDSO: fix the way vDSO functions are called for powerpc")
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>

Reviewed-by: Christophe Leroy <christophe.leroy@...roup.eu>

> ---
>   tools/testing/selftests/vDSO/vdso_call.h | 7 +++----
>   1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/testing/selftests/vDSO/vdso_call.h b/tools/testing/selftests/vDSO/vdso_call.h
> index bb237d771051bd4103367fc60b54b505b7586965..e7205584cbdca5e10c13c1e9425d2023b02cda7f 100644
> --- a/tools/testing/selftests/vDSO/vdso_call.h
> +++ b/tools/testing/selftests/vDSO/vdso_call.h
> @@ -44,7 +44,6 @@
>   	register long _r6 asm ("r6");					\
>   	register long _r7 asm ("r7");					\
>   	register long _r8 asm ("r8");					\
> -	register long _rval asm ("r3");					\
>   									\
>   	LOADARGS_##nr(fn, args);					\
>   									\
> @@ -54,13 +53,13 @@
>   		"	bns+	1f\n"					\
>   		"	neg	3, 3\n"					\
>   		"1:"							\
> -		: "+r" (_r0), "=r" (_r3), "+r" (_r4), "+r" (_r5),	\
> +		: "+r" (_r0), "+r" (_r3), "+r" (_r4), "+r" (_r5),	\
>   		  "+r" (_r6), "+r" (_r7), "+r" (_r8)			\
> -		: "r" (_rval)						\
> +		:							\
>   		: "r9", "r10", "r11", "r12", "cr0", "cr1", "cr5",	\
>   		  "cr6", "cr7", "xer", "lr", "ctr", "memory"		\
>   	);								\
> -	_rval;								\
> +	_r3;								\
>   })
>   
>   #else
>