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From: Michael Ellerman <mpe@ellerman.id.au>
To: Christophe LEROY <christophe.leroy@c-s.fr>, linuxppc-dev@lists.ozlabs.org,
 linux-kbuild@vger.kernel.org, Masahiro Yamada <yamada.masahiro@socionext.com>,
 Michal Marek <michal.lkml@markovi.net>,
 Segher Boessenkool <segher@kernel.crashing.org>
Cc: 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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 <87mus75c5m.fsf@concordia.ellerman.id.au>
 <56227965-464a-d972-1c25-d6a4d190e1a1@c-s.fr>
Date: Tue, 25 Sep 2018 11:16:39 +1000
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Christophe LEROY <christophe.leroy@c-s.fr> writes:

> Le 24/09/2018 =C3=A0 14:10, Michael Ellerman a =C3=A9crit=C2=A0:
>> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>>=20
>>> 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 bu=
ild.
>>>
>>> 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 bui=
lt ?
>>>
>>> Or another way of add -mstack-protector-guard-offset=3Doffsetof(struct
>>> task_struct, stack_canary) ?
>>=20
>> This seems to work, at least I see the value in CFLAGS:
>>=20
>> 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:
>>=20=20=20
>>   archprepare: checkbin
>>=20=20=20
>> +prepare: stack_protector_prepare
>> +
>> +stack_protector_prepare: prepare0
>> +	$(eval KBUILD_CFLAGS +=3D -mstack-protector-guard-offset=3D$(shell awk=
 '{if ($$2 =3D=3D "TSK_STACK_CANARY") print $$3;}' include/generated/asm-of=
fsets.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 :)

cheers