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From: Michael Ellerman <mpe@ellerman.id.au>
To: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Christophe LEROY <christophe.leroy@c-s.fr>,
 linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
 Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
 Michal Marek <michal.lkml@markovi.net>,
 Segher Boessenkool <segher@kernel.crashing.org>,
 LKML <linux-kernel@vger.kernel.org>
Subject: Re: How to define some additional KBUILD_CFLAGS after building
 include/generated/asm-offsets.h ?
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Date: Thu, 27 Sep 2018 16:31:48 +1000
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Masahiro Yamada <yamada.masahiro@socionext.com> writes:
> 2018-09-25 10:16 GMT+09:00 Michael Ellerman <mpe@ellerman.id.au>:
>> Christophe LEROY <christophe.leroy@c-s.fr> writes:
>>
>>> Le 24/09/2018 =C3=A0 14:10, Michael Ellerman a =C3=A9crit :
>>>> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>>>>
>>>>> I'm trying to implement TLS based stack protector in the Linux Kernel.
>>>>> For that I need to give to GCC the offset at which it will find the
>>>>> canary (register r2 is pointing to the current task struct).
>>>>>
>>>>> I have been able to do it with the below patch, but it only works when
>>>>> include/generated/asm-offsets.h already exists from the start of the =
build.
>>>>>
>>>>> Is there a way to evaluate CANARY_OFFSET and add the stack-protector
>>>>> flags to KBUILD_FLAGS only after include/generated/asm-offsets.h is b=
uilt ?
>>>>>
>>>>> Or another way of add -mstack-protector-guard-offset=3Doffsetof(struct
>>>>> task_struct, stack_canary) ?
>>>>
>>>> This seems to work, at least I see the value in CFLAGS:
>>>>
>>>> diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
>>>> index 07d9dce..39ee113 100644
>>>> --- a/arch/powerpc/Makefile
>>>> +++ b/arch/powerpc/Makefile
>>>> @@ -404,6 +394,11 @@ archclean:
>>>>
>>>>   archprepare: checkbin
>>>>
>>>> +prepare: stack_protector_prepare
>>>> +
>>>> +stack_protector_prepare: prepare0
>>>> +    $(eval KBUILD_CFLAGS +=3D -mstack-protector-guard-offset=3D$(shel=
l awk '{if ($$2 =3D=3D "TSK_STACK_CANARY") print $$3;}' include/generated/a=
sm-offsets.h))
>>>> +
>>>
>>> Great, it works !
>>> Thanks, I have sent v3 of the patches.
>>
>> Cool.
>>
>> It would be good to here from someone who knows Kbuild better than me if
>> this is acceptable or just a gross hack :)
>
> I am fine with this solution.

Thanks.

cheers