Masahiro Yamada [off-list ref] writes:

2018-09-25 10:16 GMT+09:00 Michael Ellerman [off-list ref]:

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Christophe LEROY [off-list ref] writes:

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Le 24/09/2018 =C3=A0 14:10, Michael Ellerman a =C3=A9crit :

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Christophe Leroy [off-list ref] writes:

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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.

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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 b=

uilt ?

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Or another way of add -mstack-protector-guard-offset=3Doffsetof(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
+    $(eval KBUILD_CFLAGS +=3D -mstack-protector-guard-offset=3D$(shel=

l awk '{if ($$2 =3D=3D "TSK_STACK_CANARY") print $$3;}' include/generated/a=
sm-offsets.h))

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+

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