Re: [PATCH v2 11/21] stack-protector: test compiler capability in Kconfig and drop AUTO mode

From: Masahiro Yamada <yamada.masahiro@socionext.com>
To: Kees Cook <keescook@chromium.org>
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>
Subject: Re: [PATCH v2 11/21] stack-protector: test compiler capability in Kconfig and drop AUTO mode
Date: Tue, 10 Apr 2018 12:15:30 +0900	[thread overview]
Message-ID: <CAK7LNATULgbS6mxYJUumnkVZU_1LhQwS0jL=v_=HAjtRiF12yA@mail.gmail.com> (raw)
In-Reply-To: <CAGXu5jJCpDtVcFckLzeTg=Du0TWL=KGUH1_k2LSbv8PTONQ2Mw@mail.gmail.com>

2018-04-10 0:04 GMT+09:00 Kees Cook <keescook@chromium.org>:
> 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).
>

It is still hard to understand this.

When we "its compiler supports the -fstack-protector option",
we have two meanings

[1] the stack protector feature is implemented in GCC source code.

[2] -fstack-protector is recognized as a valid option in the GCC being used.
    This can be tested by $(cc-option -fstack-protector)

I guess you were talking about [1], where as I [2].
Is this correct?

Does [2] happen only after [1] happens?
Or, are they independent?

If there is a case where GCC recognizes -fstack-protector,
but not implemented?

For x86, there are cases where the option is recognized but not working.
That's why we have
scripts/gcc-x86_{32,64}-has-stack-protector.sh

Generally, if GCC accepts -fstack-protector as a valid option,
we expect "it is working".

I wonder why we need additional information about the compiler
even after $(cc-option -fstack-protector) succeeds.

This is just a matter of comment.

Can you clarify your problem?

> 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).
>

All of these are talking about the kernel side implementation.
So, it is included in the following comment I am still keeping.

  - it has implemented a stack canary (e.g. __stack_chk_guard)

-- 
Best Regards
Masahiro Yamada