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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aKfDrKBaMc24cNgC@hyeyoo>
Date: Fri, 22 Aug 2025 10:11:08 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Liam.Howlett@...cle.com, akpm@...ux-foundation.org, andreyknvl@...il.com,
        aneesh.kumar@...ux.ibm.com, anshuman.khandual@....com,
        apopple@...dia.com, ardb@...nel.org, arnd@...db.de, bp@...en8.de,
        cl@...two.org, dave.hansen@...ux.intel.com, david@...hat.com,
        dennis@...nel.org, dev.jain@....com, dvyukov@...gle.com,
        glider@...gle.com, gwan-gyeong.mun@...el.com, hpa@...cr.com,
        jane.chu@...cle.com, jgross@...e.de, jhubbard@...dia.com,
        joao.m.martins@...cle.com, joro@...tes.org, kas@...nel.org,
        kevin.brodsky@....com, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        lorenzo.stoakes@...cle.com, luto@...nel.org, maobibo@...ngson.cn,
        mhocko@...e.com, mingo@...hat.com, osalvador@...e.de,
        peterx@...hat.com, peterz@...radead.org, rppt@...nel.org,
        ryabinin.a.a@...il.com, ryan.roberts@....com, stable@...r.kernel.org,
        surenb@...gle.com, tglx@...utronix.de, thuth@...hat.com, tj@...nel.org,
        urezki@...il.com, vbabka@...e.cz, vincenzo.frascino@....com,
        x86@...nel.org, zhengqi.arch@...edance.com
Subject: Re: [PATCH v2] mm: fix KASAN build error due to p*d_populate_kernel()

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?

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.

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

At least that's what KASAN has relied on p*d_populate() to do...

-- 
Cheers,
Harry / Hyeonggon