[RFC PATCH 7/7] Test stackprotector options in Kconfig to kill CC_STACKPROTECTOR_AUTO

[Masahiro Yamada <yamada.masahiro@socionext.com>]

Add CC_HAS_STACKPROTECTOR(_STRONG) and proper dependency.

I re-arranged the choice values, _STRONG, _REGULAR, _NONE in this order
because the default of choice is the first visible symbol.

TODO:
Broken stackprotector is not tested.
scripts/gcc-$(SRCARCH)_$(BITS)-has-stack-protector.sh should be
evaluated in Kconfig.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Test stackprotector options in Kconfig to kill CC_STACKPROTECTOR_AUTO

Add CC_HAS_STACKPROTECTOR(_STRONG) and proper dependency.

I re-arranged the choice values, _STRONG, _REGULAR, _NONE in this order
because the default of choice is the first visible symbol.

TODO:
Broken stackprotector is not tested.
scripts/gcc-$(SRCARCH)_$(BITS)-has-stack-protector.sh should be
evaluated in Kconfig.

---

 Makefile     | 58 +++++++++++-----------------------------------------------
 arch/Kconfig | 54 +++++++++++++++++++++++++++++++-----------------------
 2 files changed, 42 insertions(+), 70 deletions(-)

diff --git a/Makefile b/Makefile
index 9afd617..8123ccf 100644
--- a/Makefile
+++ b/Makefile
@@ -679,56 +679,20 @@ ifneq ($(CONFIG_FRAME_WARN),0)
 KBUILD_CFLAGS += $(call cc-option,-Wframe-larger-than=${CONFIG_FRAME_WARN})
 endif
 
-# This selects the stack protector compiler flag. Testing it is delayed
-# until after .config has been reprocessed, in the prepare-compiler-check
-# target.
-ifdef CONFIG_CC_STACKPROTECTOR_AUTO
-  stackp-flag := $(call cc-option,-fstack-protector-strong,$(call cc-option,-fstack-protector))
-  stackp-name := AUTO
-else
-ifdef CONFIG_CC_STACKPROTECTOR_REGULAR
-  stackp-flag := -fstack-protector
-  stackp-name := REGULAR
-else
-ifdef CONFIG_CC_STACKPROTECTOR_STRONG
-  stackp-flag := -fstack-protector-strong
-  stackp-name := STRONG
-else
-  # If either there is no stack protector for this architecture or
-  # CONFIG_CC_STACKPROTECTOR_NONE is selected, we're done, and $(stackp-name)
-  # is empty, skipping all remaining stack protector tests.
-  #
-  # Force off for distro compilers that enable stack protector by default.
-  KBUILD_CFLAGS += $(call cc-option, -fno-stack-protector)
-endif
-endif
-endif
-# Find arch-specific stack protector compiler sanity-checking script.
-ifdef stackp-name
-ifneq ($(stackp-flag),)
-  stackp-path := $(srctree)/scripts/gcc-$(SRCARCH)_$(BITS)-has-stack-protector.sh
-  stackp-check := $(wildcard $(stackp-path))
-  # If the wildcard test matches a test script, run it to check functionality.
-  ifdef stackp-check
-    ifneq ($(shell $(CONFIG_SHELL) $(stackp-check) $(CC) $(KBUILD_CPPFLAGS) $(biarch)),y)
-      stackp-broken := y
-    endif
-  endif
-  ifndef stackp-broken
-    # If the stack protector is functional, enable code that depends on it.
-    KBUILD_CPPFLAGS += -DCONFIG_CC_STACKPROTECTOR
-    # Either we've already detected the flag (for AUTO) or we'll fail the
-    # build in the prepare-compiler-check rule (for specific flag).
-    KBUILD_CFLAGS += $(stackp-flag)
-  else
-    # We have to make sure stack protector is unconditionally disabled if
-    # the compiler is broken (in case we're going to continue the build in
-    # AUTO mode).
-    KBUILD_CFLAGS += $(call cc-option, -fno-stack-protector)
-  endif
+ifeq ($(CONFIG_CC_STACKPROTECTOR_STRONG),y)
+KBUILD_CFLAGS += -fstack-protector-strong
 endif
+ifeq ($(CONFIG_CC_STACKPROTECTOR_REGULAR),y)
+KBUILD_CFLAGS += -fstack-protector
 endif
 
+# is this necessary?
+#ifeq ($(CONFIG_CC_STACKPROTECTOR_NONE),y)
+#KBUILD_CFLAGS += -fno-stack-protector
+#endif
+
+# TODO: run scripts/gcc-$(SRCARCH)_$(BITS)-has-stack-protector.sh from Kconfig
+
 ifeq ($(cc-name),clang)
 KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
 KBUILD_CFLAGS += $(call cc-disable-warning, unused-variable)
diff --git a/arch/Kconfig b/arch/Kconfig
index 76c0b54..50723d8 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -538,10 +538,20 @@ config HAVE_CC_STACKPROTECTOR
 	  - its compiler supports the -fstack-protector option
 	  - it has implemented a stack canary (e.g. __stack_chk_guard)
 
+config CC_HAS_STACKPROTECTOR
+	bool
+	option shell="$CC -Werror -fstack-protector -c -x c /dev/null"
+
+config CC_HAS_STACKPROTECTOR_STRONG
+	bool
+	option shell="$CC -Werror -fstack-protector-strong -c -x c /dev/null"
+
+config CC_STACKPROTECTOR
+	bool
+
 choice
 	prompt "Stack Protector buffer overflow detection"
 	depends on HAVE_CC_STACKPROTECTOR
-	default CC_STACKPROTECTOR_AUTO
 	help
 	  This option turns on the "stack-protector" GCC feature. This
 	  feature puts, at the beginning of functions, a canary value on
@@ -551,26 +561,10 @@ choice
 	  overwrite the canary, which gets detected and the attack is then
 	  neutralized via a kernel panic.
 
-config CC_STACKPROTECTOR_NONE
-	bool "None"
-	help
-	  Disable "stack-protector" GCC feature.
-
-config CC_STACKPROTECTOR_REGULAR
-	bool "Regular"
-	help
-	  Functions will have the stack-protector canary logic added if they
-	  have an 8-byte or larger character array on the stack.
-
-	  This feature requires gcc version 4.2 or above, or a distribution
-	  gcc with the feature backported ("-fstack-protector").
-
-	  On an x86 "defconfig" build, this feature adds canary checks to
-	  about 3% of all kernel functions, which increases kernel code size
-	  by about 0.3%.
-
 config CC_STACKPROTECTOR_STRONG
 	bool "Strong"
+	depends on CC_HAS_STACKPROTECTOR_STRONG
+	select CC_STACKPROTECTOR
 	help
 	  Functions will have the stack-protector canary logic added in any
 	  of the following conditions:
@@ -588,11 +582,25 @@ config CC_STACKPROTECTOR_STRONG
 	  about 20% of all kernel functions, which increases the kernel code
 	  size by about 2%.
 
-config CC_STACKPROTECTOR_AUTO
-	bool "Automatic"
+config CC_STACKPROTECTOR_REGULAR
+	bool "Regular"
+	depends on CC_HAS_STACKPROTECTOR
+	select CC_STACKPROTECTOR
+	help
+	  Functions will have the stack-protector canary logic added if they
+	  have an 8-byte or larger character array on the stack.
+
+	  This feature requires gcc version 4.2 or above, or a distribution
+	  gcc with the feature backported ("-fstack-protector").
+
+	  On an x86 "defconfig" build, this feature adds canary checks to
+	  about 3% of all kernel functions, which increases kernel code size
+	  by about 0.3%.
+
+config CC_STACKPROTECTOR_NONE
+	bool "None"
 	help
-	  If the compiler supports it, the best available stack-protector
-	  option will be chosen.
+	  Disable "stack-protector" GCC feature.
 
 endchoice
 
-- 
2.7.4