stackprotector: update make rules

Impact: no default -fno-stack-protector if stackp is enabled, cleanup

Stackprotector make rules had the following problems.

* cc support test and warning are scattered across makefile and
  kernel/panic.c.

* -fno-stack-protector was always added regardless of configuration.

Update such that cc support test and warning are contained in makefile
and -fno-stack-protector is added iff stackp is turned off.  While at
it, prepare for 32bit support.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Makefile

@@ -532,8 +532,9 @@ KBUILD_CFLAGS += $(call cc-option,-Wframe-larger-than=${CONFIG_FRAME_WARN})
 endif
 
 # Force gcc to behave correct even for buggy distributions
-# Arch Makefiles may override this setting
+ifndef CONFIG_CC_STACKPROTECTOR
 KBUILD_CFLAGS += $(call cc-option, -fno-stack-protector)
+endif
 
 ifdef CONFIG_FRAME_POINTER
 KBUILD_CFLAGS	+= -fno-omit-frame-pointer -fno-optimize-sibling-calls

arch/x86/Makefile

@@ -70,14 +70,17 @@ else
         # this works around some issues with generating unwind tables in older gccs
         # newer gccs do it by default
         KBUILD_CFLAGS += -maccumulate-outgoing-args
+endif
 
-        stackp := $(CONFIG_SHELL) $(srctree)/scripts/gcc-x86_64-has-stack-protector.sh
-        stackp-$(CONFIG_CC_STACKPROTECTOR) := $(shell $(stackp) \
-                "$(CC)" "-fstack-protector -DGCC_HAS_SP" )
-        stackp-$(CONFIG_CC_STACKPROTECTOR_ALL) += $(shell $(stackp) \
-                "$(CC)" -fstack-protector-all )
-
+ifdef CONFIG_CC_STACKPROTECTOR
+	cc_has_sp := $(srctree)/scripts/gcc-x86_$(BITS)-has-stack-protector.sh
+        ifeq ($(shell $(CONFIG_SHELL) $(cc_has_sp) $(CC)),y)
+                stackp-y := -fstack-protector
+                stackp-$(CONFIG_CC_STACKPROTECTOR_ALL) += -fstack-protector-all
                 KBUILD_CFLAGS += $(stackp-y)
+        else
+                $(warning stack protector enabled but no compiler support)
+        endif
 endif
 
 # Stackpointer is addressed different for 32 bit and 64 bit x86

kernel/panic.c

@@ -359,10 +359,6 @@ EXPORT_SYMBOL(warn_slowpath);
 
 #ifdef CONFIG_CC_STACKPROTECTOR
 
-#ifndef GCC_HAS_SP
-#warning You have selected the CONFIG_CC_STACKPROTECTOR option, but the gcc used does not support this.
-#endif
-
 /*
  * Called when gcc's -fstack-protector feature is used, and
  * gcc detects corruption of the on-stack canary value

scripts/gcc-x86_64-has-stack-protector.sh

@@ -2,5 +2,7 @@
 
 echo "int foo(void) { char X[200]; return 3; }" | $1 -S -xc -c -O0 -mcmodel=kernel -fstack-protector - -o - 2> /dev/null | grep -q "%gs"
 if [ "$?" -eq "0" ] ; then
-	echo $2
+	echo y
+else
+	echo n
 fi