linux-kernel - Re: [PATCH 07/43] compiler_attributes.h: add __disable_sanitizer

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Date:   Wed, 15 Dec 2021 14:33:34 +0100
From:   Marco Elver <elver@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Christoph Hellwig <hch@....de>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/43] compiler_attributes.h: add __disable_sanitizer_instrumentation

A

On Wed, 15 Dec 2021 at 14:24, Mark Rutland <mark.rutland@....com> wrote:
>
> On Tue, Dec 14, 2021 at 05:20:14PM +0100, Alexander Potapenko wrote:
> > The new attribute maps to
> > __attribute__((disable_sanitizer_instrumentation)), which will be
> > supported by Clang >= 14.0. Future support in GCC is also possible.
> >
> > This attribute disables compiler instrumentation for kernel sanitizer
> > tools, making it easier to implement noinstr. It is different from the
> > existing __no_sanitize* attributes, which may still allow certain types
> > of instrumentation to prevent false positives.
>
> When you say the __no_sanitize* attributes allow some instrumentation, does
> that apply to any of the existing KASAN/KCSAN/KCOV support, or just for KMSAN?
>
> The documentation just says the same as the commit message:
>
> | This is not the same as __attribute__((no_sanitize(...))), which depending on
> | the tool may still insert instrumentation to prevent false positive reports.
>
> ... which implies the other instrumentation might not be suprressed.
>
> I ask because architectures which select ARCH_WANTS_NO_INSTR *need* to be able
> to suppress all instrumentation. It's fine if that means they need a new
> version of clang for KMSAN, but if there's latent instrumentation we have more
> bugs to fix first...

Thus far, none of the existing K*SANs added other instrumentation.
Apart from KMSAN here, this will change with KCSAN's barrier
instrumentation, which is why this patch is also part of KCSAN's
upcoming changes -- recall I said I fixed barrier instrumentation for
arm64 as well, this is how :-)

See https://lore.kernel.org/all/20211130114433.2580590-26-elver@google.com/
how I resolved it for KCSAN on architectures that don't have objtool.

I expect this patch will be dropped from the KMSAN series once it
reaches mainline through the KCSAN changes.

Also note, this applies only for bug-detection tools, that may want to
avoid false positives. So by definition, it is irrelevant for KCOV
(which had its own attribute woes a while back though). Yeah, it's
been a long road to get the compilers to play along ... :-/

Thanks,
-- Marco