linux-kernel - Re: [PATCH v2] mm: fix KASAN build error due to p*d_populate

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Message-ID: <3976ef5d-a959-408a-b538-7feba1f0ab7a@intel.com>
Date: Thu, 21 Aug 2025 10:36:12 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Harry Yoo <harry.yoo@...cle.com>
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Subject: Re: [PATCH v2] mm: fix KASAN build error due to p*d_populate_kernel()

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:

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

The right way to do it is to have an #ifdef in a header that avoids
compiling in the reference to the symbol.

But just changing the 'static inline' to a #define seems like a fragile
hack to me.