linux-kernel - Re: [PATCH v3 1/3] kasan: mark !__SANITIZE_ADDRESS__ stubs __always

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Message-ID: <DF0JIYFQGFCP.9RDI8V58PFNH@google.com>
Date: Wed, 17 Dec 2025 13:53:33 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>, Brendan Jackman <jackmanb@...gle.com>
Cc: Andrey Ryabinin <ryabinin.a.a@...il.com>, Alexander Potapenko <glider@...gle.com>, 
	Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Vincenzo Frascino <vincenzo.frascino@....com>, Marco Elver <elver@...gle.com>, 
	Ard Biesheuvel <ardb@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, <x86@...nel.org>, 
	"H. Peter Anvin" <hpa@...or.com>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, <kasan-dev@...glegroups.com>, 
	<linux-kernel@...r.kernel.org>, <llvm@...ts.linux.dev>
Subject: Re: [PATCH v3 1/3] kasan: mark !__SANITIZE_ADDRESS__ stubs __always_inline

On Tue Dec 16, 2025 at 1:01 PM UTC, Peter Zijlstra wrote:
> On Tue, Dec 16, 2025 at 10:16:34AM +0000, Brendan Jackman wrote:
>> The x86 instrumented bitops in
>> include/asm-generic/bitops/instrumented-non-atomic.h are
>> KASAN-instrumented via explicit calls to instrument_* functions from
>> include/linux/instrumented.h.
>> 
>> This bitops are used from noinstr code in __sev_es_nmi_complete(). This
>> code avoids noinstr violations by disabling __SANITIZE_ADDRESS__ etc for
>> the compilation unit.
>
> Yeah, so don't do that? That's why we use raw_atomic_*() in things like
> smp_text_poke_int3_handler().

Right, this was what Ard suggested in [0]:

> For the short term, we could avoid this by using arch___set_bit()
> directly in the SEV code that triggers this issue today. But for the
> longer term, we should get write of those explicit calls to
> instrumentation intrinsics, as this is fundamentally incompatible with
> per-function overrides.

But, I think the longer term solution is actually now coming from what
Marco described in [1].

So in the meantime what's the cleanest fix? Going straight to the arch_*
calls from SEV seems pretty yucky in its own right. Adding special
un-instrumented wrappers in bitops.h seems overblown for a temporary
workaround. Meanwhile, disabling __SANITIZE_ADDRESS__ is something the
SEV code already relies on as a workaround, so if we can just make that
workaround work for this case too, it seems like a reasonable way
forward?

Anyway, I don't feel too strongly about this, I'm only pushing back
for the sake of hysteresis since I already flipflopped a couple of times
on this fix. If Ard/Marco agree with just using the arch_ functions
directly I'd be fine with that.

And in the meantime, I guess patch 3/3 is OK? As it happens, that will
already make the current error go away without needing the first 2
patches. So maybe we should just merge that and be done with it? There's
probably a good chance no other issues will show up between now and
whenever Marco's nice compiler support arrives.

[0] https://lore.kernel.org/all/CAMj1kXHiA91hH80tHFCO9QjkkfzEGZ2GJgpHnuKrusKhOULMXA@mail.gmail.com/
[1] https://lore.kernel.org/all/CANpmjNNc9vRJbD2e5DPPR8SWNSYa=MqTzniARp4UWKBUEdhh_Q@mail.gmail.com/