[PATCH] tools/nolibc: MIPS: fix clobbers of 'lo' and 'hi' registers on different ISAs

[PATCH] tools/nolibc: MIPS: fix clobbers of 'lo' and 'hi' registers on different ISAs

[Thomas Weißschuh]

Earlier MIPS64 ISAs still provide the 'lo' and 'hi' special registers.
These are clobbered by system calls and need to be marked as such to
avoid miscompilations.
Also 32-bit ISAs from r6 on do not define the 'lo' and 'hi' registers.

Handle all different combinations of ABI and ISAs.

Fixes: a6a2a8a42972 ("tools/nolibc: MIPS: add support for N64 and N32 ABIs")
Fixes: 66b6f755ad45 ("rcutorture: Import a copy of nolibc")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 tools/include/nolibc/arch-mips.h | 33 +++++++++++++++++++++++++++------
 1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/tools/include/nolibc/arch-mips.h b/tools/include/nolibc/arch-mips.h
index a72506ceec6b..210bd907b51f 100644
--- a/tools/include/nolibc/arch-mips.h
+++ b/tools/include/nolibc/arch-mips.h
@@ -41,23 +41,44 @@
 
 #if defined(_ABIO32)
 
-#define _NOLIBC_SYSCALL_CLOBBERLIST \
-	"memory", "cc", "at", "v1", "hi", "lo", \
-	"t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"
 #define _NOLIBC_SYSCALL_STACK_RESERVE "addiu $sp, $sp, -32\n"
 #define _NOLIBC_SYSCALL_STACK_UNRESERVE "addiu $sp, $sp, 32\n"
 
 #else /* _ABIN32 || _ABI64 */
 
+#define _NOLIBC_SYSCALL_STACK_RESERVE
+#define _NOLIBC_SYSCALL_STACK_UNRESERVE
+
+#endif /* _ABIO32 */
+
+
+#if defined(_ABIO32) && __mips_isa_rev >= 6
+
+#define _NOLIBC_SYSCALL_CLOBBERLIST \
+	"memory", "cc", "at", "v1", \
+	"t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"
+
+#elif defined(_ABIO32) && __mips_isa_rev < 6
+
+#define _NOLIBC_SYSCALL_CLOBBERLIST \
+	"memory", "cc", "at", "v1", "hi", "lo", \
+	"t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"
+
+#elif __mips_isa_rev >= 6 /* _ABIN32 || _ABI64 */
+
 /* binutils, GCC and clang disagree about register aliases, use numbers instead. */
 #define _NOLIBC_SYSCALL_CLOBBERLIST \
 	"memory", "cc", "at", "v1", \
 	"10", "11", "12", "13", "14", "15", "24", "25"
 
-#define _NOLIBC_SYSCALL_STACK_RESERVE
-#define _NOLIBC_SYSCALL_STACK_UNRESERVE
+#else /* __mips_is_rev < 6 && (_ABIN32 || _ABI64) */
+
+#define _NOLIBC_SYSCALL_CLOBBERLIST \
+	"memory", "cc", "at", "v1", "hi", "lo", \
+	"10", "11", "12", "13", "14", "15", "24", "25"
+
+#endif /* __mips_isa_rev and ABI */
 
-#endif /* _ABIO32 */
 
 #define my_syscall0(num)                                                      \
 ({                                                                            \

---
base-commit: 2961f841b025fb234860bac26dfb7fa7cb0fb122
change-id: 20260218-nolibc-mips-clobber-499e9fbdb5bf

Best regards,
-- 
Thomas Weißschuh <thomas.weissschuh@linutronix.de>

Re: [PATCH] tools/nolibc: MIPS: fix clobbers of 'lo' and 'hi' registers on different ISAs

[Willy Tarreau]

On Wed, Feb 18, 2026 at 12:45:48PM +0100, Thomas Weißschuh wrote:
> Earlier MIPS64 ISAs still provide the 'lo' and 'hi' special registers.
> These are clobbered by system calls and need to be marked as such to
> avoid miscompilations.
> Also 32-bit ISAs from r6 on do not define the 'lo' and 'hi' registers.
> 
> Handle all different combinations of ABI and ISAs.
> 
> Fixes: a6a2a8a42972 ("tools/nolibc: MIPS: add support for N64 and N32 ABIs")
> Fixes: 66b6f755ad45 ("rcutorture: Import a copy of nolibc")
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>

I trust that you've contemplated way more doc than me on that topic, so
obviously:

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

Thanks,
Willy

Re: [PATCH] tools/nolibc: MIPS: fix clobbers of 'lo' and 'hi' registers on different ISAs

[Maciej W. Rozycki]

On Wed, 18 Feb 2026, Thomas Weißschuh wrote:

> Earlier MIPS64 ISAs still provide the 'lo' and 'hi' special registers.
> These are clobbered by system calls and need to be marked as such to
> avoid miscompilations.

 Correct.

> Also 32-bit ISAs from r6 on do not define the 'lo' and 'hi' registers.

 There's no MIPS ISA beyond R6 (nanoMIPS is a completely different ISA).  
Also MIPS64r6 is the same in this respect as MIPS32r6 is.

> diff --git a/tools/include/nolibc/arch-mips.h b/tools/include/nolibc/arch-mips.h
> index a72506ceec6b..210bd907b51f 100644
> --- a/tools/include/nolibc/arch-mips.h
> +++ b/tools/include/nolibc/arch-mips.h
> @@ -41,23 +41,44 @@
>  
>  #if defined(_ABIO32)
>  
> -#define _NOLIBC_SYSCALL_CLOBBERLIST \
> -	"memory", "cc", "at", "v1", "hi", "lo", \
> -	"t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"
>  #define _NOLIBC_SYSCALL_STACK_RESERVE "addiu $sp, $sp, -32\n"
>  #define _NOLIBC_SYSCALL_STACK_UNRESERVE "addiu $sp, $sp, 32\n"
>  
>  #else /* _ABIN32 || _ABI64 */
>  
> +#define _NOLIBC_SYSCALL_STACK_RESERVE
> +#define _NOLIBC_SYSCALL_STACK_UNRESERVE
> +
> +#endif /* _ABIO32 */
> +
> +
> +#if defined(_ABIO32) && __mips_isa_rev >= 6
> +
> +#define _NOLIBC_SYSCALL_CLOBBERLIST \
> +	"memory", "cc", "at", "v1", \
> +	"t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"
> +
> +#elif defined(_ABIO32) && __mips_isa_rev < 6
> +
> +#define _NOLIBC_SYSCALL_CLOBBERLIST \
> +	"memory", "cc", "at", "v1", "hi", "lo", \
> +	"t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"
> +
> +#elif __mips_isa_rev >= 6 /* _ABIN32 || _ABI64 */
> +
>  /* binutils, GCC and clang disagree about register aliases, use numbers instead. */
>  #define _NOLIBC_SYSCALL_CLOBBERLIST \
>  	"memory", "cc", "at", "v1", \
>  	"10", "11", "12", "13", "14", "15", "24", "25"
>  
> -#define _NOLIBC_SYSCALL_STACK_RESERVE
> -#define _NOLIBC_SYSCALL_STACK_UNRESERVE
> +#else /* __mips_is_rev < 6 && (_ABIN32 || _ABI64) */

 s/__mips_is_rev/__mips_isa_rev/

> +
> +#define _NOLIBC_SYSCALL_CLOBBERLIST \
> +	"memory", "cc", "at", "v1", "hi", "lo", \
> +	"10", "11", "12", "13", "14", "15", "24", "25"
> +
> +#endif /* __mips_isa_rev and ABI */
>  
> -#endif /* _ABIO32 */

 I think this would be more readable/maintainable if you did something 
such as:

#if __mips_isa_rev < 6
#define _NOLIBC_SYSCALL_CLOBBER_HI "hi"
#define _NOLIBC_SYSCALL_CLOBBER_LO "lo"
#else
#define _NOLIBC_SYSCALL_CLOBBER_HI "$0"
#define _NOLIBC_SYSCALL_CLOBBER_LO "$0"
#endif

and then used the macros rather than duplicating the clobber lists.  Or 
you could reuse the approach taken with VDSO_SYSCALL_CLOBBERS, perhaps a 
bit messy, also when it comes to the name of the macro.

Re: [PATCH] tools/nolibc: MIPS: fix clobbers of 'lo' and 'hi' registers on different ISAs

[Thomas Weißschuh]

On Sat, Mar 14, 2026 at 06:01:20PM +0000, Maciej W. Rozycki wrote:
> On Wed, 18 Feb 2026, Thomas Weißschuh wrote:
> 
> > Earlier MIPS64 ISAs still provide the 'lo' and 'hi' special registers.
> > These are clobbered by system calls and need to be marked as such to
> > avoid miscompilations.
> 
>  Correct.
> 
> > Also 32-bit ISAs from r6 on do not define the 'lo' and 'hi' registers.
> 
>  There's no MIPS ISA beyond R6 (nanoMIPS is a completely different ISA).  
> Also MIPS64r6 is the same in this respect as MIPS32r6 is.

I'll reword this.

> > diff --git a/tools/include/nolibc/arch-mips.h b/tools/include/nolibc/arch-mips.h
> > index a72506ceec6b..210bd907b51f 100644
> > --- a/tools/include/nolibc/arch-mips.h
> > +++ b/tools/include/nolibc/arch-mips.h
> > @@ -41,23 +41,44 @@

(...)

> #if __mips_isa_rev < 6
> #define _NOLIBC_SYSCALL_CLOBBER_HI "hi"
> #define _NOLIBC_SYSCALL_CLOBBER_LO "lo"
> #else
> #define _NOLIBC_SYSCALL_CLOBBER_HI "$0"
> #define _NOLIBC_SYSCALL_CLOBBER_LO "$0"
> #endif

Ack, will use this.

> and then used the macros rather than duplicating the clobber lists.  Or 
> you could reuse the approach taken with VDSO_SYSCALL_CLOBBERS, perhaps a 
> bit messy, also when it comes to the name of the macro.

Reusing code from the core kernel in nolibc is problematic,
as nolibc is licensed more liberally.


Thomas