linux-kernel - Re: [PATCH 4/4] x86/percpu: Use C for percpu read/write accessors

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Date:   Mon, 9 Oct 2023 14:00:45 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Nadav Amit <namit@...are.com>, Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 4/4] x86/percpu: Use C for percpu read/write accessors

On Mon, Oct 9, 2023 at 1:51 PM Ingo Molnar <mingo@...nel.org> wrote:
>
>
> * Ingo Molnar <mingo@...nel.org> wrote:
>
> >
> > * Uros Bizjak <ubizjak@...il.com> wrote:
> >
> > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > > index ecb256954351..1edf4a5b93ca 100644
> > > --- a/arch/x86/Kconfig
> > > +++ b/arch/x86/Kconfig
> > > @@ -2393,7 +2393,7 @@ config CC_HAS_NAMED_AS
> > >
> > >  config USE_X86_SEG_SUPPORT
> > >     def_bool y
> > > -   depends on CC_HAS_NAMED_AS && SMP
> > > +   depends on CC_HAS_NAMED_AS && SMP && !KASAN
> > > +   depends on CC_HAS_NAMED_AS && SMP && !KASAN
> >
> > So I'd rather express this as a Kconfig quirk line, and explain each quirk.
> >
> > Something like:
> >
> >       depends on CC_HAS_NAMED_AS
> >       depends on SMP
> >       #
> >       # -fsanitize=kernel-address (KASAN) is at the moment incompatible
> >       # with named address spaces - see GCC bug #12345.
> >       #
> >       depends on !KASAN
> >
> > ... or so.
>
> BTW., while this OK for testing, this is too heavy handed for release
> purposes, so please only disable the KASAN instrumentation for the affected
> percpu accessors.
>
> See the various __no_sanitize* attributes available.

These attributes are for function declarations. The percpu casts can
not be implemented with separate static inline functions. Also,
__no_sanitize_address is mutually exclusive with __always_inline.

Uros.

> I'd even suggest introducing a new attribute variant, specific to x86,
> prefixed with __no_sanitize_x86_seg or so, which would allow the eventual
> KASAN-instrumentation of the percpu accessors once the underlying GCC bug
> is fixed.
>
> Thanks,
>
>         Ingo