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Date:   Wed, 19 May 2021 10:32:40 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Will Deacon <will@kernel.org>,
        "Jose E. Marchesi" <jemarch@gnu.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        linux-toolchains@vger.kernel.org
Subject: Re: Linux Plumbers Conf 2021
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On Tue, May 18, 2021 at 10:03:06PM +0200, Peter Zijlstra wrote:
> On Tue, May 18, 2021 at 09:57:26AM -0700, Nick Desaulniers wrote:
> > On Tue, May 18, 2021 at 6:23 AM Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > >  - validates noinstr annotation; *HOWEVER* we rely on objtool to NOP
> > >    all __sanitizer_cov_* calls in .noinstr/.entry text sections because
> > >    __no_sanitize_cov is 'broken' in all known compilers.
> > 
> > ^ Do you have more information on this?
> 
> Random link in the middle of a relevant thread:
> 
>   https://lkml.kernel.org/r/CANpmjNPNa2f=kAF6c199oYVJ0iSyirQRGxeOBLxa9PmakSXRbA@mail.gmail.com
> 
> Notable quote:
> 
> "It's everywhere. We cannot mark noinstr functions to not be
> instrumented by KCOV/-fsanitize-coverage, because no compiler provides
> an attribute to do so. GCC doesn't have
> __attribute__((no_sanitize_coverage)) and Clang doesn't have
> __attribute__((no_sanitize("coverage")), and therefore we can't have
> __no_sanitize_coverage."

Yuck; I wasn't aware of that.

This means that arm64's entry code is unsound (along with some other
parts) if kcov is enabled.

That *really* needs an attribute, and going forward we shouldn't accept
new compiler instrumentation that we can't opt-out of like this.

Thanks,
Mark.