kbuild: Add extra gcc checks

Add a 'W=1' Makefile switch which adds additional checking per build
object.

The idea behind this option is targeted at developers who, in the
process of writing their code, want to do the occasional

make W=1 [target.o]

and let gcc do more extensive code checking for them. Then, they
could eyeball the output for valid gcc warnings about various
bugs/discrepancies which are not reported during the normal build
process.

For more background information and a use case, read through this
thread: http://marc.info/?l=kernel-janitors&m=129802065918147&w=2

Cc: Michal Marek <mmarek@suse.cz>
Cc: linux-kbuild@vger.kernel.org
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/Makefile.build

@@ -49,6 +49,40 @@ ifeq ($(KBUILD_NOPEDANTIC),)
                 $(error CFLAGS was changed in "$(kbuild-file)". Fix it to use EXTRA_CFLAGS)
         endif
 endif
+
+#
+# make W=1 settings
+#
+# $(call cc-option... ) handles gcc -W.. options which
+# are not supported by all versions of the compiler
+ifdef KBUILD_ENABLE_EXTRA_GCC_CHECKS
+KBUILD_EXTRA_WARNINGS := -Wextra
+KBUILD_EXTRA_WARNINGS += -Wunused -Wno-unused-parameter
+KBUILD_EXTRA_WARNINGS += -Waggregate-return
+KBUILD_EXTRA_WARNINGS += -Wbad-function-cast
+KBUILD_EXTRA_WARNINGS += -Wcast-qual
+KBUILD_EXTRA_WARNINGS += -Wcast-align
+KBUILD_EXTRA_WARNINGS += -Wconversion
+KBUILD_EXTRA_WARNINGS += -Wdisabled-optimization
+KBUILD_EXTRA_WARNINGS += -Wlogical-op
+KBUILD_EXTRA_WARNINGS += -Wmissing-declarations
+KBUILD_EXTRA_WARNINGS += -Wmissing-format-attribute
+KBUILD_EXTRA_WARNINGS += $(call cc-option, -Wmissing-include-dirs,)
+KBUILD_EXTRA_WARNINGS += -Wmissing-prototypes
+KBUILD_EXTRA_WARNINGS += -Wnested-externs
+KBUILD_EXTRA_WARNINGS += -Wold-style-definition
+KBUILD_EXTRA_WARNINGS += $(call cc-option, -Woverlength-strings,)
+KBUILD_EXTRA_WARNINGS += -Wpacked
+KBUILD_EXTRA_WARNINGS += -Wpacked-bitfield-compat
+KBUILD_EXTRA_WARNINGS += -Wpadded
+KBUILD_EXTRA_WARNINGS += -Wpointer-arith
+KBUILD_EXTRA_WARNINGS += -Wredundant-decls
+KBUILD_EXTRA_WARNINGS += -Wshadow
+KBUILD_EXTRA_WARNINGS += -Wswitch-default
+KBUILD_EXTRA_WARNINGS += $(call cc-option, -Wvla,)
+KBUILD_CFLAGS += $(KBUILD_EXTRA_WARNINGS)
+endif
+
 include scripts/Makefile.lib
 
 ifdef host-progs
@@ -403,7 +437,6 @@ ifneq ($(cmd_files),)
   include $(cmd_files)
 endif
 
-
 # Declare the contents of the .PHONY variable as phony.  We keep that
 # information in a variable se we can use it in if_changed and friends.