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Date:	Wed, 02 Jul 2014 17:37:36 -0700
From:	Randy Dunlap <rdunlap@...radead.org>
To:	Andi Kleen <ak@...ux.intel.com>
CC:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-next@...r.kernel.org, sfr@...b.auug.org.au, mhocko@...e.cz
Subject: Re: mmotm 2014-07-02-15-07 uploaded (stack protector)

On 07/02/14 17:22, Andi Kleen wrote:
> On Wed, Jul 02, 2014 at 04:21:06PM -0700, Randy Dunlap wrote:
>> On 07/02/14 15:08, akpm@...ux-foundation.org wrote:
>>> The mm-of-the-moment snapshot 2014-07-02-15-07 has been uploaded to
>>>
>>>    http://www.ozlabs.org/~akpm/mmotm/
>>>
>>> mmotm-readme.txt says
>>>
>>> README for mm-of-the-moment:
>>>
>>> http://www.ozlabs.org/~akpm/mmotm/
>>>
>>> This is a snapshot of my -mm patch queue.  Uploaded at random hopefully
>>> more than once a week.
>>
>> Hi Andi,
>>
>> Here is an extract from 10 randconfig builds:
>>
>>
>> $ grep STACKPROTECTOR_STRONG config-r*
>> config-r8735:CONFIG_CC_STACKPROTECTOR_STRONG=y
>> config-r8736:CONFIG_CC_STACKPROTECTOR_STRONG=y
>> config-r8737:# CONFIG_CC_STACKPROTECTOR_STRONG is not set
>> config-r8738:# CONFIG_CC_STACKPROTECTOR_STRONG is not set
>> config-r8739:CONFIG_CC_STACKPROTECTOR_STRONG=y
>> config-r8740:CONFIG_CC_STACKPROTECTOR_STRONG=y
>> config-r8741:CONFIG_CC_STACKPROTECTOR_STRONG=y
>> config-r8742:# CONFIG_CC_STACKPROTECTOR_STRONG is not set
>> config-r8743:# CONFIG_CC_STACKPROTECTOR_STRONG is not set
>> config-r8744:# CONFIG_CC_STACKPROTECTOR_STRONG is not set
>>
>>
>> but the captured output for each one says:
>>
>> grep STACKPROTECTOR_STRONG  build*.out
>> build-r8735.out:/local/lnx/MM/mmotm-2014-0702-1507/Makefile:662: Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong not supported by compiler
>> build-r8736.out:/local/lnx/MM/mmotm-2014-0702-1507/Makefile:662: Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong not supported by compiler
>> build-r8736.out:/local/lnx/MM/mmotm-2014-0702-1507/Makefile:662: Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong not supported by compiler
>> build-r8737.out:/local/lnx/MM/mmotm-2014-0702-1507/Makefile:662: Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong not supported by compiler
>> build-r8739.out:/local/lnx/MM/mmotm-2014-0702-1507/Makefile:662: Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong not supported by compiler
>> build-r8740.out:/local/lnx/MM/mmotm-2014-0702-1507/Makefile:662: Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong not supported by compiler
>> build-r8740.out:/local/lnx/MM/mmotm-2014-0702-1507/Makefile:662: Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong not supported by compiler
>> build-r8741.out:/local/lnx/MM/mmotm-2014-0702-1507/Makefile:662: Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong not supported by compiler
>> build-r8741.out:/local/lnx/MM/mmotm-2014-0702-1507/Makefile:662: Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong not supported by compiler
>> build-r8742.out:/local/lnx/MM/mmotm-2014-0702-1507/Makefile:662: Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong not supported by compiler
>>
>>
>> then the build goes on to produce many errors like this one:
>>
>>   CC      init/do_mounts.o
>> gcc: error: unrecognized command line option '-fstack-protector-strong'
>>
>>
>> Can you explain this, please?  or even better yet, fix it.
> 
> That's the code 
> 
> ifdef CONFIG_CC_STACKPROTECTOR_STRONG
>   stackp-flag := -fstack-protector-strong
>   ifeq ($(call cc-option, $(stackp-flag)),)
>     $(warning Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: \
>               -fstack-protector-strong not supported by compiler)
>   endif
> else
>   # Force off for distro compilers that enable stack protector by default.
>   stackp-flag := $(call cc-option, -fno-stack-protector)
> endif
> endif
> KBUILD_CFLAGS += $(stackp-flag)
> 
> So -fstack-protector-strong itself is never protected by a cc-option call.

ouch.

> You could add it to the stack-flag line, then it would only warn.
> 
>   stackp-flag := $(call cc-option, -fstack-protector-strong)
> 
> It's hard to tell what the original author wanted, perhaps they wanted
> it to error out. It looks intentional. Clearly they didn't think of randconfig
> though.
> 
> Or get a compiler that supports stack protection?

Yeah, I have plans to upgrade, just been too busy.

thanks for your help.
-- 
~Randy
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