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

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Message-ID: <87bm8m5qbs.fsf@concordia.ellerman.id.au>
Date:   Tue, 25 Sep 2018 11:16:39 +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 ?

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 :)

cheers