[PATCH] tools/nolibc: mark arg1 operand in __nolibc_syscall0() as write-only

[PATCH] tools/nolibc: mark arg1 operand in __nolibc_syscall0() as write-only

[Thomas Weißschuh]

__nolibc_syscall0() does not set the arg1 variable before passing it to
the asm block. This uninitialized variable read is undefined behavior.
Clang can miscompile this.

Mark the asm operand as write-only to fix this.

Fixes: 8e1930296f92 ("tools/nolibc: Add support for SPARC")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
---
 tools/include/nolibc/arch-sparc.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/include/nolibc/arch-sparc.h b/tools/include/nolibc/arch-sparc.h
index ddae9bc10dfe..23fab40accfa 100644
--- a/tools/include/nolibc/arch-sparc.h
+++ b/tools/include/nolibc/arch-sparc.h
@@ -45,7 +45,7 @@
 									      \
 	__asm__ volatile (                                                    \
 		_NOLIBC_SYSCALL                                               \
-		: "+r"(_arg1)                                                 \
+		: "=r"(_arg1)                                                 \
 		: "r"(_num)                                                   \
 		: "memory", "cc"                                              \
 	);                                                                    \

---
base-commit: d92642aeb2b9b0c670194d5768c3a6b227002d0d
change-id: 20260703-nolibc-sparc-asm-3544ec0f1edc

Best regards,
--  
Thomas Weißschuh <linux@weissschuh.net>

Re: [PATCH] tools/nolibc: mark arg1 operand in __nolibc_syscall0() as write-only

[Willy Tarreau]

Hi Thomas!

On Fri, Jul 03, 2026 at 07:30:16PM +0200, Thomas Weißschuh wrote:
> __nolibc_syscall0() does not set the arg1 variable before passing it to
> the asm block. This uninitialized variable read is undefined behavior.
> Clang can miscompile this.
 
Interesting. I don't know how we ended up like this, most likely just
due to reusing the same code blocks for the different variants. I'm
noting that m68k is affected as well then. I'm CCing Daniel for a
double-check.

> Mark the asm operand as write-only to fix this.
> 
> Fixes: 8e1930296f92 ("tools/nolibc: Add support for SPARC")
> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> ---
>  tools/include/nolibc/arch-sparc.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/include/nolibc/arch-sparc.h b/tools/include/nolibc/arch-sparc.h
> index ddae9bc10dfe..23fab40accfa 100644
> --- a/tools/include/nolibc/arch-sparc.h
> +++ b/tools/include/nolibc/arch-sparc.h
> @@ -45,7 +45,7 @@
>  									      \
>  	__asm__ volatile (                                                    \
>  		_NOLIBC_SYSCALL                                               \
> -		: "+r"(_arg1)                                                 \
> +		: "=r"(_arg1)                                                 \
>  		: "r"(_num)                                                   \
>  		: "memory", "cc"                                              \
>  	);                                                                    \

Agreed, this totally makes sense.

Acked-by: Willy Tarreau <w@1wt.eu>

Thanks!
Willy

Re: [PATCH] tools/nolibc: mark arg1 operand in __nolibc_syscall0() as write-only

[Willy Tarreau]

On Fri, Jul 03, 2026 at 07:41:58PM +0200, Willy Tarreau wrote:
> Hi Thomas!
> 
> On Fri, Jul 03, 2026 at 07:30:16PM +0200, Thomas Weißschuh wrote:
> > __nolibc_syscall0() does not set the arg1 variable before passing it to
> > the asm block. This uninitialized variable read is undefined behavior.
> > Clang can miscompile this.
>  
> Interesting. I don't know how we ended up like this, most likely just
> due to reusing the same code blocks for the different variants. I'm
> noting that m68k is affected as well then. I'm CCing Daniel for a
> double-check.

Hmmm no I'm wrong, sorry Daniel for the noise, on m68k _num is initialized
before being passed to the asm statement so "+r" is fine there.

Willy