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

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Message-ID: <87ftxvv4bv.fsf@concordia.ellerman.id.au>
Date:   Thu, 27 Sep 2018 16:31:48 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
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 ?

Masahiro Yamada <yamada.masahiro@...ionext.com> writes:
> 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.

cheers