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From:   Marco Elver <elver@google.com>
Date:   Mon, 14 Jun 2021 12:31:27 +0200
Message-ID: <CANpmjNN2Jv=2AqH0ZbwrVwSG9XMzkyHGdhM6tYhoK5DokYbnig@mail.gmail.com>
Subject: Re: [PATCH] gcov,x86: Mark GCOV broken for x86
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "the arch/x86 maintainers" <x86@kernel.org>,
        Peter Oberparleiter <oberpar@linux.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>, johannes.berg@intel.com,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Mark Rutland <mark.rutland@arm.com>
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On Mon, 14 Jun 2021 at 12:17, Peter Zijlstra <peterz@infradead.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.

GCOV today uses only -fprofile-arcs -ftest-coverage. But the problem
is the same: https://godbolt.org/z/fr9cs4sar

> Until such time as that compilers have added a function attribute to
> disable this instrumentation, mark GCOV as broken.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/Kconfig    | 2 +-
>  kernel/gcov/Kconfig | 4 ++++
>  2 files changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 86dae426798b..c0f8c9d4c31a 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -75,7 +75,7 @@ config X86
>         select ARCH_HAS_FAST_MULTIPLIER
>         select ARCH_HAS_FILTER_PGPROT
>         select ARCH_HAS_FORTIFY_SOURCE
> -       select ARCH_HAS_GCOV_PROFILE_ALL
> +       select ARCH_HAS_GCOV_BROKEN
>         select ARCH_HAS_KCOV                    if X86_64 && STACK_VALIDATION
>         select ARCH_HAS_MEM_ENCRYPT
>         select ARCH_HAS_MEMBARRIER_SYNC_CORE
> diff --git a/kernel/gcov/Kconfig b/kernel/gcov/Kconfig
> index 58f87a3092f3..74b028a66ebe 100644
> --- a/kernel/gcov/Kconfig
> +++ b/kernel/gcov/Kconfig
> @@ -1,10 +1,14 @@
>  # SPDX-License-Identifier: GPL-2.0-only
>  menu "GCOV-based kernel profiling"
>
> +config ARCH_HAS_GCOV_BROKEN
> +       def_bool n
> +
>  config GCOV_KERNEL
>         bool "Enable gcov-based kernel profiling"
>         depends on DEBUG_FS
>         depends on !CC_IS_CLANG || CLANG_VERSION >= 110000
> +       depends on !ARCH_HAS_GCOV_BROKEN
>         select CONSTRUCTORS
>         default n
>         help