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

[Julian Braha <julianbraha@gmail.com>]

On 10/12/26 11:28, Christophe Leroy (CS GROUP) wrote:
> 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. 

Sorry, yeah kconfig can be tricky...

> +    range 20 24 if STRICT_KERNEL_RWX && PPC_85xx
I think if we keep this ^ and then add another range:

    range PAGE_SHIFT 24 if !STRICT_KERNEL_RWX && PPC_85xx

this would cover your STRICT_KERNEL_RWX=n and DEBUG_PAGEALLOC=y
configuration while resolving the 12 -> 20

- Julian Braha