[PATCH] ARM: stackprotector: prefer compiler for TLS based per-task protector

[Ard Biesheuvel]

Currently, we implement the per-task stack protector for ARM using a GCC
plugin, due to lack of native compiler support. However, work is
underway to get this implemented in the compiler, which means we will be
able to deprecate the GCC plugin at some point.

In the meantime, we will need to support both, where the native compiler
implementation is obviously preferred. So let's wire this up in Kconfig
and the Makefile.

Cc: Kees Cook <keescook at chromium.org>
Cc: Nick Desaulniers <ndesaulniers at google.com>
Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm/Kconfig  | 8 ++++++--
 arch/arm/Makefile | 9 +++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 4f61c9789e7f..130449332521 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1604,10 +1604,14 @@ config XEN
 	help
 	  Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
 
+config CC_HAVE_STACKPROTECTOR_TLS
+	def_bool $(cc-option,-mtp=cp15 -mstack-protector-guard=tls -mstack-protector-guard-offset=0)
+
 config STACKPROTECTOR_PER_TASK
 	bool "Use a unique stack canary value for each task"
-	depends on GCC_PLUGINS && STACKPROTECTOR && THREAD_INFO_IN_TASK && !XIP_DEFLATED_DATA
-	select GCC_PLUGIN_ARM_SSP_PER_TASK
+	depends on STACKPROTECTOR && THREAD_INFO_IN_TASK && !XIP_DEFLATED_DATA
+	depends on GCC_PLUGINS || CC_HAVE_STACKPROTECTOR_TLS
+	select GCC_PLUGIN_ARM_SSP_PER_TASK if !CC_HAVE_STACKPROTECTOR_TLS
 	default y
 	help
 	  Due to the fact that GCC uses an ordinary symbol reference from
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 1c540157e283..bfa861d3ccbb 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -275,6 +275,14 @@ endif
 
 ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
 prepare: stack_protector_prepare
+ifeq ($(CONFIG_CC_HAVE_STACKPROTECTOR_TLS),y)
+stack_protector_prepare: prepare0
+	$(eval KBUILD_CFLAGS += \
+		-mstack-protector-guard=tls \
+		-mstack-protector-guard-offset=$(shell	\
+			awk '{if ($$2 == "TSK_STACK_CANARY") print $$3;}'\
+				include/generated/asm-offsets.h))
+else
 stack_protector_prepare: prepare0
 	$(eval SSP_PLUGIN_CFLAGS := \
 		-fplugin-arg-arm_ssp_per_task_plugin-offset=$(shell	\
@@ -283,6 +291,7 @@ stack_protector_prepare: prepare0
 	$(eval KBUILD_CFLAGS += $(SSP_PLUGIN_CFLAGS))
 	$(eval GCC_PLUGINS_CFLAGS += $(SSP_PLUGIN_CFLAGS))
 endif
+endif
 
 all:	$(notdir $(KBUILD_IMAGE))
 
-- 
2.30.2