linux-kernel - Re: [PATCH 00/23] kconfig: move compiler capability tests to Kconfig

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Message-ID: <CAK8P3a1DrhxWXaEs6E85W5HEhvFr23E-GDsu=0X-sG0pX+PLag@mail.gmail.com>
Date:   Wed, 21 Feb 2018 11:52:50 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Rich Felker <dalias@...c.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        X86 ML <x86@...nel.org>, Paul Mackerras <paulus@...ba.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        sparclinux <sparclinux@...r.kernel.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Jonathan Corbet <corbet@....net>,
        Richard Weinberger <richard@....at>,
        Linux-sh list <linux-sh@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Emese Revfy <re.emese@...il.com>,
        Kees Cook <keescook@...omium.org>,
        uml-devel <user-mode-linux-devel@...ts.sourceforge.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>,
        Jeff Dike <jdike@...toit.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        user-mode-linux-user@...ts.sourceforge.net,
        Thomas Gleixner <tglx@...utronix.de>,
        Michal Marek <michal.lkml@...kovi.net>,
        Ulf Magnusson <ulfalizer@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 00/23] kconfig: move compiler capability tests to Kconfig

On Wed, Feb 21, 2018 at 11:20 AM, Masahiro Yamada
<yamada.masahiro@...ionext.com> wrote:
> 2018-02-21 18:56 GMT+09:00 Arnd Bergmann <arnd@...db.de>:
>> On Wed, Feb 21, 2018 at 8:38 AM, Masahiro Yamada
>> <yamada.masahiro@...ionext.com> wrote:
>>> 2018-02-20 0:18 GMT+09:00 Ulf Magnusson <ulfalizer@...il.com>:
>
> Let me clarify my concern.
>
> When we test the compiler flag, is there a case
> where a particular flag depends on -m{32,64} ?
>
> For example, is there a compiler that supports -fstack-protector
> for 64bit mode, but unsupports it for 32bit mode?
>
>   $(cc-option -m32)                     ->  y
>   $(cc-option -m64)                     ->  y
>   $(cc-option -fstack-protector)        ->  y
>   $(cc-option -m32 -fstack-protector)   ->  n
>   $(cc-option -m64 -fstack-protector)   ->  y
>
> I guess this is unlikely to happen,
> but I am not whether it is zero possibility.
>
> If this could happen,
> $(cc-option ) must be evaluated together with
> correct bi-arch option (either -m32 or -m64).
>
>
> Currently, -m32/-m64 is specified in Makefile,
> but we are moving compiler tests to Kconfig
> and, CONFIG_64BIT can be dynamically toggled in Kconfig.

I don't think it can happen for this particular combination (stack protector
and word size), but I'm sure we'll eventually run into options that
need to be tested in combination. For the current CFLAGS_KERNEL
setting, we definitely have the case of needing the variables to be
evaluated in a specific order.

      Arnd