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From:   Kees Cook <keescook@chromium.org>
Date:   Mon, 9 Apr 2018 08:04:16 -0700
Message-ID: <CAGXu5jJCpDtVcFckLzeTg=Du0TWL=KGUH1_k2LSbv8PTONQ2Mw@mail.gmail.com>
Subject: Re: [PATCH v2 11/21] stack-protector: test compiler capability in
 Kconfig and drop AUTO mode
To:     Masahiro Yamada <yamada.masahiro@socionext.com>
Cc:     linux-kbuild <linux-kbuild@vger.kernel.org>,
        Sam Ravnborg <sam@ravnborg.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Ulf Magnusson <ulfalizer@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Luis R . Rodriguez" <mcgrof@kernel.org>,
        Nicolas Pitre <nico@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>
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On Mon, Apr 9, 2018 at 1:54 AM, Masahiro Yamada
<yamada.masahiro@socionext.com> wrote:
> 2018-03-28 20:18 GMT+09:00 Kees Cook <keescook@chromium.org>:
>> On Mon, Mar 26, 2018 at 10:29 PM, Masahiro Yamada
>> <yamada.masahiro@socionext.com> wrote:
>>> diff --git a/arch/Kconfig b/arch/Kconfig
>>> index 8e0d665..b42378d 100644
>>> --- a/arch/Kconfig
>>> +++ b/arch/Kconfig
>>> @@ -535,13 +535,13 @@ config HAVE_CC_STACKPROTECTOR
>>>         bool
>>>         help
>>>           An arch should select this symbol if:
>>> -         - its compiler supports the -fstack-protector option
>>
>> Please leave this note: it's still valid. An arch must still have
>> compiler support for this to be sensible.
>>
>
> No.
>
> "its compiler supports the -fstack-protector option"
> is tested by $(cc-option -fstack-protector)
>
> ARCH does not need to know the GCC support level.

That's not correct: if you enable stack protector for a kernel
architecture that doesn't having it enabled, it's unlikely for the
resulting kernel to boot. An architecture must handle the changes that
the compiler introduces when adding -fstack-protector (for example,
having the stack protector canary value defined, having the failure
function defined, handling context switches changing canaries, etc).

-Kees

-- 
Kees Cook
Pixel Security