[PATCH] powerpc/Kconfig: limit the PPC_85xx DATA_SHIFT range to STRICT_KERNEL_RWX

[PATCH] powerpc/Kconfig: limit the PPC_85xx DATA_SHIFT range to STRICT_KERNEL_RWX

[Julian Braha]

On PPC_85xx, enabling DEBUG_PAGEALLOC without STRICT_KERNEL_RWX activates
the DATA_SHIFT range of 20..24, but since the PAGE_SHIFT default is
outside the range (12), kconfig adjusts it to 20, causing 1mb alignment
instead of 4kb. However, the larger alignment is only needed for the
separate kernel permissions provided by STRICT_KERNEL_RWX.

Since PPC_85xx uses page poisoning for DEBUG_PAGEALLOC and does not
support KFENCE, let's restrict this range to STRICT_KERNEL_RWX.

This out-of-range default was detected by kconfirm, a static analysis tool
for Kconfig.

Signed-off-by: Julian Braha <julianbraha@gmail.com>
---
 arch/powerpc/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index c3b6cd655606..5e2a4799438d 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -896,7 +896,7 @@ config DATA_SHIFT
 	default 24 if STRICT_KERNEL_RWX && PPC64
 	range 17 28 if (STRICT_KERNEL_RWX || DEBUG_PAGEALLOC || KFENCE) && PPC_BOOK3S_32
 	range 14 23 if (STRICT_KERNEL_RWX || DEBUG_PAGEALLOC || KFENCE) && PPC_8xx
-	range 20 24 if (STRICT_KERNEL_RWX || DEBUG_PAGEALLOC || KFENCE) && PPC_85xx
+	range 20 24 if STRICT_KERNEL_RWX && PPC_85xx
 	default 22 if STRICT_KERNEL_RWX && PPC_BOOK3S_32
 	default 18 if (DEBUG_PAGEALLOC || KFENCE) && PPC_BOOK3S_32
 	default 23 if (STRICT_KERNEL_RWX || DEBUG_PAGEALLOC || KFENCE) && PPC_8xx && \
-- 
2.55.0

Re: [PATCH] powerpc/Kconfig: limit the PPC_85xx DATA_SHIFT range to STRICT_KERNEL_RWX

[Christophe Leroy (CS GROUP)]

Le 11/10/2026 à 13:31, Julian Braha a écrit :
> On PPC_85xx, enabling DEBUG_PAGEALLOC without STRICT_KERNEL_RWX activates
> the DATA_SHIFT range of 20..24, but since the PAGE_SHIFT default is
> outside the range (12), kconfig adjusts it to 20, causing 1mb alignment
> instead of 4kb. However, the larger alignment is only needed for the
> separate kernel permissions provided by STRICT_KERNEL_RWX.
> 
> Since PPC_85xx uses page poisoning for DEBUG_PAGEALLOC and does not
> support KFENCE, let's restrict this range to STRICT_KERNEL_RWX.
> 
> This out-of-range default was detected by kconfirm, a static analysis tool
> for Kconfig.

Looks like with this change I'm now able to set DATA_SHIFT to any value 
when STRICT_KERNEL_RWX is not set and DEBUG_PAGEALLOC is set.

The range used to protect against that.

 > > Signed-off-by: Julian Braha <julianbraha@gmail.com>
> ---
>   arch/powerpc/Kconfig | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index c3b6cd655606..5e2a4799438d 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -896,7 +896,7 @@ config DATA_SHIFT
>   	default 24 if STRICT_KERNEL_RWX && PPC64
>   	range 17 28 if (STRICT_KERNEL_RWX || DEBUG_PAGEALLOC || KFENCE) && PPC_BOOK3S_32
>   	range 14 23 if (STRICT_KERNEL_RWX || DEBUG_PAGEALLOC || KFENCE) && PPC_8xx
> -	range 20 24 if (STRICT_KERNEL_RWX || DEBUG_PAGEALLOC || KFENCE) && PPC_85xx
> +	range 20 24 if STRICT_KERNEL_RWX && PPC_85xx
>   	default 22 if STRICT_KERNEL_RWX && PPC_BOOK3S_32
>   	default 18 if (DEBUG_PAGEALLOC || KFENCE) && PPC_BOOK3S_32
>   	default 23 if (STRICT_KERNEL_RWX || DEBUG_PAGEALLOC || KFENCE) && PPC_8xx && \