kasan: fix clang compilation warning due to stack protector

KASAN uses a single cc-option invocation to disable both conserve-stack
and stack-protector flags.  The former flag is not present in Clang,
which causes cc-option to fail, and results in stack-protector being
enabled.

Fix by using separate cc-option calls for each flag.  Also collect all
flags in a variable to avoid calling cc-option multiple times for
different files.
Reported-by: Qian Cai <cai@lca.pw>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Marco Elver <elver@google.com>
Link: http://lkml.kernel.org/r/c2f0c8e4048852ae014f4a391d96ca42d27e3255.1590779332.git.andreyknvl@google.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/kasan/Makefile

@@ -15,14 +15,19 @@ CFLAGS_REMOVE_tags_report.o = $(CC_FLAGS_FTRACE)
 
 # Function splitter causes unnecessary splits in __asan_load1/__asan_store1
 # see: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63533
-CFLAGS_common.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector) -DDISABLE_BRANCH_PROFILING
-CFLAGS_generic.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector) -DDISABLE_BRANCH_PROFILING
-CFLAGS_generic_report.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector) -DDISABLE_BRANCH_PROFILING
-CFLAGS_init.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector) -DDISABLE_BRANCH_PROFILING
-CFLAGS_quarantine.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector) -DDISABLE_BRANCH_PROFILING
-CFLAGS_report.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector) -DDISABLE_BRANCH_PROFILING
-CFLAGS_tags.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector) -DDISABLE_BRANCH_PROFILING
-CFLAGS_tags_report.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector) -DDISABLE_BRANCH_PROFILING
+CC_FLAGS_KASAN_RUNTIME := $(call cc-option, -fno-conserve-stack)
+CC_FLAGS_KASAN_RUNTIME += $(call cc-option, -fno-stack-protector)
+# Disable branch tracing to avoid recursion.
+CC_FLAGS_KASAN_RUNTIME += -DDISABLE_BRANCH_PROFILING
+
+CFLAGS_common.o := $(CC_FLAGS_KASAN_RUNTIME)
+CFLAGS_generic.o := $(CC_FLAGS_KASAN_RUNTIME)
+CFLAGS_generic_report.o := $(CC_FLAGS_KASAN_RUNTIME)
+CFLAGS_init.o := $(CC_FLAGS_KASAN_RUNTIME)
+CFLAGS_quarantine.o := $(CC_FLAGS_KASAN_RUNTIME)
+CFLAGS_report.o := $(CC_FLAGS_KASAN_RUNTIME)
+CFLAGS_tags.o := $(CC_FLAGS_KASAN_RUNTIME)
+CFLAGS_tags_report.o := $(CC_FLAGS_KASAN_RUNTIME)
 
 obj-$(CONFIG_KASAN) := common.o init.o report.o
 obj-$(CONFIG_KASAN_GENERIC) += generic.o generic_report.o quarantine.o