linux-kernel - Re: [PATCH] gcov,x86: Mark GCOV broken for x86

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Date:   Mon, 14 Jun 2021 11:05:35 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Johannes Berg <johannes.berg@...el.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] gcov,x86: Mark GCOV broken for x86

On Mon, Jun 14, 2021 at 9:20 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Jun 14, 2021 at 09:05:04AM -0700, Nick Desaulniers wrote:
> > On Mon, Jun 14, 2021 at 3:17 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > >
> > > As recently discovered, there is no function attribute to disable the
> > > -fprofile-generate instrumentation. As such, GCOV is fundamentally
> > > incompatible with architectures that rely on 'noinstr' for correctness.
> >
> > Is there context for comment, or is this patch meant as a joke?
>
> Only if you think recursion in exception entry code is funny.
>
> noinstr *MUST* disable any and all compiler generated instrumentation,
> currently it that isn't the case for -fprofile-gnerate, nor
> -fprofile-arc.
>
> Look for all the fun we had with KCOV back then. Luckily KCOV
> instrumentation was trivial to patch out using objtool, so that's what
> x86 is currently doing.
>
> Luckily both compilers grew a __no_sanitize_coverage recently and we no
> longer have to rely on objtool fixing up the compiler output for much
> longer.
>
>   https://lkml.kernel.org/r/20210527194448.3470080-1-elver@google.com
>
> Now all we need is one more such attribute to kill -fprofile-* stuff.

__attribute__((no_instrument_function)) is already wired to not emit
calls to mcount()/fentry().  I think extending it to also apply to
coverage (-fprofile-arcs) and instrumentation based profiling
(-fprofile-generate) is reasonable.

-- 
Thanks,
~Nick Desaulniers