Re: [PATCH] LoongArch: vDSO: correctly use asm parameters in syscall wrappers

[Nathan Chancellor <nathan@kernel.org>]

On Tue, Jun 03, 2025 at 01:48:54PM +0200, Thomas Weißschuh wrote:
> The syscall wrappers use the "a0" register for two different register
> variables, both the first argument and the return value. The "ret"
> variable is used as both input and output while the argument register is
> only used as input. Clang treats the conflicting input parameters as
> undefined behaviour and optimizes away the argument assignment.
> 
> The code seems to work by chance for the most part today but that may
> change in the future. Specifically clock_gettime_fallback() fails with
> clockids from 16 to 23, as implemented by the upcoming auxiliary clocks.
> 
> Switch the "ret" register variable to a pure output, similar to the other
> architectures' vDSO code. This works in both clang and GCC.
> 
> Link: https://lore.kernel.org/lkml/20250602102825-42aa84f0-23f1-4d10-89fc-e8bbaffd291a@linutronix.de/
> Link: https://lore.kernel.org/lkml/20250519082042.742926976@linutronix.de/
> Fixes: c6b99bed6b8f ("LoongArch: Add VDSO and VSYSCALL support")
> Fixes: 18efd0b10e0f ("LoongArch: vDSO: Wire up getrandom() vDSO implementation")
> Cc: stable@vger.kernel.org
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>

This is definitely an odd interaction because of the register variables
using the same value.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

> ---
>  arch/loongarch/include/asm/vdso/getrandom.h    | 2 +-
>  arch/loongarch/include/asm/vdso/gettimeofday.h | 6 +++---
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/loongarch/include/asm/vdso/getrandom.h b/arch/loongarch/include/asm/vdso/getrandom.h
> index 48c43f55b039b42168698614d0479b7a872d20f3..a81724b69f291ee49dd1f46b12d6893fc18442b8 100644
> --- a/arch/loongarch/include/asm/vdso/getrandom.h
> +++ b/arch/loongarch/include/asm/vdso/getrandom.h
> @@ -20,7 +20,7 @@ static __always_inline ssize_t getrandom_syscall(void *_buffer, size_t _len, uns
>  
>  	asm volatile(
>  	"      syscall 0\n"
> -	: "+r" (ret)
> +	: "=r" (ret)
>  	: "r" (nr), "r" (buffer), "r" (len), "r" (flags)
>  	: "$t0", "$t1", "$t2", "$t3", "$t4", "$t5", "$t6", "$t7", "$t8",
>  	  "memory");
> diff --git a/arch/loongarch/include/asm/vdso/gettimeofday.h b/arch/loongarch/include/asm/vdso/gettimeofday.h
> index 88cfcf13311630ed5f1a734d23a2bc3f65d79a88..f15503e3336ca1bdc9675ec6e17bbb77abc35ef4 100644
> --- a/arch/loongarch/include/asm/vdso/gettimeofday.h
> +++ b/arch/loongarch/include/asm/vdso/gettimeofday.h
> @@ -25,7 +25,7 @@ static __always_inline long gettimeofday_fallback(
>  
>  	asm volatile(
>  	"       syscall 0\n"
> -	: "+r" (ret)
> +	: "=r" (ret)
>  	: "r" (nr), "r" (tv), "r" (tz)
>  	: "$t0", "$t1", "$t2", "$t3", "$t4", "$t5", "$t6", "$t7",
>  	  "$t8", "memory");
> @@ -44,7 +44,7 @@ static __always_inline long clock_gettime_fallback(
>  
>  	asm volatile(
>  	"       syscall 0\n"
> -	: "+r" (ret)
> +	: "=r" (ret)
>  	: "r" (nr), "r" (clkid), "r" (ts)
>  	: "$t0", "$t1", "$t2", "$t3", "$t4", "$t5", "$t6", "$t7",
>  	  "$t8", "memory");
> @@ -63,7 +63,7 @@ static __always_inline int clock_getres_fallback(
>  
>  	asm volatile(
>  	"       syscall 0\n"
> -	: "+r" (ret)
> +	: "=r" (ret)
>  	: "r" (nr), "r" (clkid), "r" (ts)
>  	: "$t0", "$t1", "$t2", "$t3", "$t4", "$t5", "$t6", "$t7",
>  	  "$t8", "memory");
> 
> ---
> base-commit: 546b1c9e93c2bb8cf5ed24e0be1c86bb089b3253
> change-id: 20250603-loongarch-vdso-syscall-f585a99bea03
> 
> Best regards,
> -- 
> Thomas Weißschuh <thomas.weissschuh@linutronix.de>
>