Re: [PATCH v2] mm: fix KASAN build error due to p*d_populate_kernel()

[Dave Hansen]

On 8/21/25 18:11, Harry Yoo wrote:
> On Thu, Aug 21, 2025 at 10:36:12AM -0700, Dave Hansen wrote:
>> On 8/21/25 04:57, Harry Yoo wrote:
>>> However, {pgd,p4d}_populate_kernel() is defined as a function regardless
>>> of the number of page table levels, so the compiler may not optimize
>>> them away. In this case, the following linker error occurs:
> 
> Hi, thanks for taking a look, Dave!
> 
> First of all, this is a fix-up patch of a mm-hotfixes patch series that
> fixes a bug (I should have explained that in the changelog) [1].
> 
> [1] https://lore.kernel.org/linux-mm/20250818020206.4517-1-harry.yoo@xxxxxxxxxx
> 
> I think we can continue discussing it and perhaps do that as part of
> a follow-up series, because the current patch series need to be backported
> to -stable and your suggestion to improve existing code doesn't require
> -stable backports.
> 
> Does that sound fine?
> 
>> This part of the changelog confused me. I think it's focusing on the
>> wrong thing.
>>
>> The code that's triggering this is literally:
>>
>>>                         pgd_populate(&init_mm, pgd,
>>>                                         lm_alias(kasan_early_shadow_p4d));
>>
>> It sure _looks_ like it's unconditionally referencing the
>> 'kasan_early_shadow_p4d' symbol. I think it's wrong to hide that with
>> macro magic and just assume that the macros won't reference it.
>>
>> If a symbol isn't being defined, it shouldn't be referenced in C code.:q
> 
> A fair point, and that's what KASAN code has been doing for years.
> 
>> The right way to do it is to have an #ifdef in a header that avoids
>> compiling in the reference to the symbol.
> 
> You mean defining some wrapper functions for p*d_populate_kernel() in
> KASAN with different implementations based on ifdeffery?

That would work.

So would something like:

#if CONFIG_PGTABLE_LEVELS >= 4
extern p4d_t kasan_early_shadow_p4d[MAX_PTRS_PER_P4D];
#else
#define kasan_early_shadow_p4d NULL
#endif

> Just to clarify, what should be the exact ifdeffery to cover these cases?
> #if CONFIG_PGTABLE_LEVELS == 4 and 5, or
> #ifdef __PAGETABLE_P4D_FOLDED and __PAGETABLE_PUD_FOLDED ?
> 
> I have no strong opinion on this, let's hear what KASAN folks think.

I think CONFIG_PGTABLE_LEVELS works, but in the end I'm not picky about
the specific #ifdefs that work.