linux-kernel - Re: [PATCH 1/3] retpolines: Only enable retpoline when compiler support it

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Message-ID: <1e62b1b7-8c37-b9a1-bcda-41aa99155420@oracle.com>
Date:   Wed, 31 Oct 2018 13:55:14 +0800
From:   Zhenzhong Duan <zhenzhong.duan@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>, konrad.wilk@...cle.com,
        David Woodhouse <dwmw@...zon.co.uk>,
        Thomas Gleixner <tglx@...utronix.de>, srinivas.eeda@...cle.com,
        Borislav Petkov <bp@...e.de>,
        Daniel Borkmann <daniel@...earbox.net>,
        Michal Marek <michal.lkml@...kovi.net>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/3] retpolines: Only enable retpoline when compiler
 support it

On 2018/10/30 18:09, Peter Zijlstra wrote:
> On Tue, Oct 30, 2018 at 06:39:24PM +0900, Masahiro Yamada wrote:
>> Hi,
>>
>>
>>
>> On Tue, Oct 30, 2018 at 3:57 PM Zhenzhong Duan
>> <zhenzhong.duan@...cle.com> wrote:
>>>
>>> Since retpoline capable compilers are widely available, make
>>> CONFIG_RETPOLINE hard depend on it.
>>>
>>> Change KBUILD to use CONFIG_RETPOLINE_SUPPORT to avoid conflict with
>>> CONFIG_RETPOLINE which is used by kernel.
>>>
>>> With all that stuff, the check of RETPOLINE is changed to
>>> CONFIG_RETPOLINE.
>>>
>>> This change is based on suggestion in https://lkml.org/lkml/2018/9/18/1016
>>>
>>> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@...cle.com>
>>> Cc: Thomas Gleixner <tglx@...utronix.de>
>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>> Cc: Borislav Petkov <bp@...e.de>
>>> Cc: Daniel Borkmann <daniel@...earbox.net>
>>> Cc: David Woodhouse <dwmw@...zon.co.uk>
>>> Cc: H. Peter Anvin <hpa@...or.com>
>>> Cc: Ingo Molnar <mingo@...hat.com>
>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
>>> Cc: Andy Lutomirski <luto@...nel.org>
>>> Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>
>>> Cc: Michal Marek <michal.lkml@...kovi.net>
>>> ---
>>
>>
>> Instead of adding another CONFIG option,
>> does it make sense to add compiler support checks
>> to 'depends on' syntax ?
>>
>>
>> config RETPOLINE
>>           bool "Avoid speculative indirect branches in kernel"
>>           depends on $(cc-option,-mindirect-branch=thunk-extern
>> -mindirect-branch-register) || \
>>                      $(cc-option,-mretpoline-external-thunk)
>>           default y
>>           select STACK_VALIDATION if HAVE_STACK_VALIDATION

Looks better, thanks for suggestion.

> 
> That seems to be what we did for stackprotector, which is similar in
> that it used to fail the build. So yes, this seems sane.

Should I add a scripts/gcc-x86_64-has-retpoline.sh like what 
stackprotector does as below or there is a simpler way?

config CC_HAS_SANE_STACKPROTECTOR
         bool
         default 
$(success,$(srctree)/scripts/gcc-x86_64-has-stack-protector.sh $(CC)) if 
64BIT
         default 
$(success,$(srctree)/scripts/gcc-x86_32-has-stack-protector.sh $(CC))
         help
            We have to make sure stack protector is unconditionally 
disabled if
            the compiler produces broken code.

Thanks
Zhenzhong