linux-kernel - Re: How to define some additional KBUILD_CFLAGS after building include/generated/asm-offsets.h ?

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Message-ID: <CAK7LNAQA2JyK9aOp9QQF32bwsz2MitaZ5HrqSVjdEvRdCQpO3g@mail.gmail.com>
Date:   Thu, 27 Sep 2018 14:24:48 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Christophe LEROY <christophe.leroy@....fr>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: How to define some additional KBUILD_CFLAGS after building
 include/generated/asm-offsets.h ?

Hi.

2018-09-25 10:16 GMT+09:00 Michael Ellerman <mpe@...erman.id.au>:
> Christophe LEROY <christophe.leroy@....fr> writes:
>
>> Le 24/09/2018 à 14:10, Michael Ellerman a écrit :
>>> Christophe Leroy <christophe.leroy@....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 built ?
>>>>
>>>> Or another way of add -mstack-protector-guard-offset=offsetof(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
>>> +    $(eval KBUILD_CFLAGS += -mstack-protector-guard-offset=$(shell awk '{if ($$2 == "TSK_STACK_CANARY") print $$3;}' include/generated/asm-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.



-- 
Best Regards
Masahiro Yamada