linux-kernel - Re: [PATCH v2 16/21] kcov: imply GCC_PLUGINS and GCC_PLUGIN

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Message-ID: <CAGXu5jKXtGgyP8zh0Gg1k-U=A4KYEuyPsxiXxtFeaNJPn+metg@mail.gmail.com>
Date:   Wed, 28 Mar 2018 04:53:39 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     linux-kbuild <linux-kbuild@...r.kernel.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Ulf Magnusson <ulfalizer@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>,
        Nicolas Pitre <nico@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 16/21] kcov: imply GCC_PLUGINS and GCC_PLUGIN_SANCOV
 instead of select'ing them

On Mon, Mar 26, 2018 at 10:29 PM, Masahiro Yamada
<yamada.masahiro@...ionext.com> wrote:
> As Documentation/kbuild/kconfig-language.txt notes, 'select' should be
> used with care - it forces a lower limit of another symbol, ignoring
> the dependency.  In this case, KCOV can select GCC_PLUGINS even if
> 'depends on HAVE_GCC_PLUGINS' is unmet.
>
> 'imply' is modest enough to observe the dependency, and this makes
> sense.  If you enable KCOV, you will probably want to enable
> GCC_PLUGIN_SANCOV, but it should not break the dependency.
>
> I also remove unneeded code, I just happened to notice.
>
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
>   - Drop depends on GCC_VERSION
>
> ---
>
> Changes in v2: None
>
>  lib/Kconfig.debug | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 5be22e4..8243c9d 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -743,8 +743,8 @@ config KCOV
>         bool "Code coverage for fuzzing"
>         depends on ARCH_HAS_KCOV
>         select DEBUG_FS
> -       select GCC_PLUGINS if !COMPILE_TEST
> -       select GCC_PLUGIN_SANCOV if !COMPILE_TEST
> +       imply GCC_PLUGINS
> +       imply GCC_PLUGIN_SANCOV
>         help
>           KCOV exposes kernel code coverage information in a form suitable
>           for coverage-guided fuzzing (randomized testing).

Actually, I think this can be improved, as KCOV should depend on either:

-fsanitize-coverage=trace-pc support (in gcc 6+, see scripts/Makefile.kcov)

or

GCC_PLUGIN_SANCOV (which provides the trace-pc support via the plugin
for gcc before 6)

> @@ -758,7 +758,6 @@ config KCOV
>  config KCOV_ENABLE_COMPARISONS
>         bool "Enable comparison operands collection by KCOV"
>         depends on KCOV
> -       default n
>         help
>           KCOV also exposes operands of every comparison in the instrumented
>           code along with operand sizes and PCs of the comparison instructions.

Similarly, this depends on -fsanitize-coverage=trace-cmp

> @@ -768,7 +767,7 @@ config KCOV_ENABLE_COMPARISONS
>  config KCOV_INSTRUMENT_ALL
>         bool "Instrument all code by default"
>         depends on KCOV
> -       default y if KCOV
> +       default y
>         help
>           If you are doing generic system call fuzzing (like e.g. syzkaller),
>           then you will want to instrument the whole kernel and you should
> --
> 2.7.4
>

-Kees

-- 
Kees Cook
Pixel Security