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 for <linuxppc-dev@lists.ozlabs.org>; Tue, 25 Sep 2018 22:39:38 +1000 (AEST)
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 ?
In-Reply-To: <87bm8m5qbs.fsf@concordia.ellerman.id.au>
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 <56227965-464a-d972-1c25-d6a4d190e1a1@c-s.fr>
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Date: Tue, 25 Sep 2018 22:39:36 +1000
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Michael Ellerman <mpe@ellerman.id.au> writes:
> 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:
>>>> 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 b=
uild.
>>>>
>>>> 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 bu=
ilt ?
>>>>
>>>> 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 aw=
k '{if ($$2 =3D=3D "TSK_STACK_CANARY") print $$3;}' include/generated/asm-o=
ffsets.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
                      ^
                      hear

Still learning English.

cheers