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

[Andrey Ryabinin <ryabinin.a.a@gmail.com>]

On 8/22/25 7:08 PM, Dave Hansen wrote:
> 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@oracle.com
>>
>> 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
> 

This won't work. It will fix the linker error, but will introduce runtime bug instead:

lm_alias(kasan_early_shadow_p4d) -> __va(__phys_addr_symbol(NULL))

On arm64:

phys_addr_t __phys_addr_symbol(unsigned long x)
	VIRTUAL_BUG_ON(x < (unsigned long) KERNEL_START ||
		       x > (unsigned long) KERNEL_END);

And NULL is < KERNEL_START.

Since __phys_addr_symbol() isn't pure or const,  compiler has no right to eliminate such
call even though the return value is unused.