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

From: Michael Ellerman
Date: Mon Sep 24 2018 - 08:10:38 EST

Christophe Leroy <christophe.leroy@xxxxxx> 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))
# Use the file '.tmp_gas_check' for binutils tests, as gas won't output
# to stdout and these checks are run even on install targets.
TOUT := .tmp_gas_check