Re: [PATCH v2 1/2] powerpc: fix KUAP warning in VMX usercopy path

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Hi Again,

Le 03/03/2026 à 15:57, Christophe Leroy (CS GROUP) a écrit :
> Hi,
> 
> Le 03/03/2026 à 10:19, Sayali Patil a écrit :
>>
>> On 02/03/26 16:42, Christophe Leroy (CS GROUP) wrote:
>>>
>> Hi Christophe,
>> Thanks for the review.
>> With the suggested change, we are hitting a compilation error.
>>
>> The issue is related to how KUAP enforces the access direction.
>> allow_user_access() contains:
>>
>> BUILD_BUG_ON(!__builtin_constant_p(dir));
>>
>> which requires that the access direction is a compile-time constant.
>> If we pass a runtime value (for example, an unsigned long), the
>> __builtin_constant_p() check fails and triggers the following build 
>> error.
>>
>> Error:
>> In function 'allow_user_access', inlined from '__copy_tofrom_user_vmx' 
>> at arch/powerpc/lib/vmx-helper.c:19:3:
>> BUILD_BUG_ON failed: !__builtin_constant_p(dir) 706
>>
>>
>> The previous implementation worked because allow_user_access() was 
>> invoked with enum
>> constants (READ, WRITE, READ_WRITE), which satisfied the 
>> __builtin_constant_p() requirement.
>> So in this case, the function must be called with a compile-time 
>> constant to satisfy KUAP.
>>
>> Please let me know if you would prefer a different approach.
>>
> 
> Ah, right, I missed that. The problem should only be in vmx-helper.c
> 

Thinking about it once more, I realised that powerpc does not define 
INLINE_COPY_FROM_USER nor INLINE_COPY_TO_USER.

This means that raw_copy_from_user() and raw_copy_to_user() will in 
really not be called much. Therefore __copy_tofrom_user_vmx() could 
remain in uaccess.h as static __always_inline allthough it requires 
exporting enter_vmx_usercopy() and exit_vmx_usercopy().

Christophe