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

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Date:   Tue, 25 Sep 2018 22:39:36 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe LEROY <christophe.leroy@....fr>,
        linuxppc-dev@...ts.ozlabs.org, linux-kbuild@...r.kernel.org,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Segher Boessenkool <segher@...nel.crashing.org>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: How to define some additional KBUILD_CFLAGS after building include/generated/asm-offsets.h ?

Michael Ellerman <mpe@...erman.id.au> writes:
> 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
                      ^
                      hear

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