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From:   Marco Elver <elver@google.com>
Date:   Tue, 2 Jun 2020 21:25:47 +0200
Message-ID: <CANpmjNP3kAZt3kXuABVqJLAJAW0u9-=kzr-QKDLmO6V_S7qXvQ@mail.gmail.com>
Subject: Re: [PATCH -tip 1/2] Kconfig: Bump required compiler version of KASAN
 and UBSAN
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Will Deacon <will@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, 2 Jun 2020 at 21:19, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Jun 02, 2020 at 11:57:15AM -0700, Nick Desaulniers wrote:
> > On Tue, Jun 2, 2020 at 11:44 AM 'Marco Elver' via Clang Built Linux
> > <clang-built-linux@googlegroups.com> wrote:
> > >
> > > Adds config variable CC_HAS_WORKING_NOSANITIZE, which will be true if we
> > > have a compiler that does not fail builds due to no_sanitize functions.
> > > This does not yet mean they work as intended, but for automated
> > > build-tests, this is the minimum requirement.
> > >
> > > For example, we require that __always_inline functions used from
> > > no_sanitize functions do not generate instrumentation. On GCC <= 7 this
> > > fails to build entirely, therefore we make the minimum version GCC 8.
> > >
> > > For KCSAN this is a non-functional change, however, we should add it in
> > > case this variable changes in future.
> > >
> > > Link: https://lkml.kernel.org/r/20200602175859.GC2604@hirez.programming.kicks-ass.net
> > > Suggested-by: Peter Zijlstra <peterz@infradead.org>
> > > Signed-off-by: Marco Elver <elver@google.com>
> >
> > Is this a problem only for x86?  If so, that's quite a jump in minimal
> > compiler versions for a feature that I don't think is currently
> > problematic for other architectures?  (Based on
> > https://lore.kernel.org/lkml/20200529171104.GD706518@hirez.programming.kicks-ass.net/
> > )
>
> Currently x86 only, but I know other arch maintainers are planning to
> have a hard look at their code based on our findings.

I've already spotted a bunch of 'noinstr' outside arch/x86 e.g. in
kernel/{locking,rcu}, and a bunch of these functions use atomic_*, all
of which are __always_inline. The noinstr uses outside arch/x86 would
break builds on all architecture with GCC <= 7 when using sanitizers.
At least that's what led me to conclude we need this for all
architectures.

Thanks,
-- Marco