fortify: use if_changed_dep to record header dependency in *.cmd files

After building with CONFIG_FORTIFY_SOURCE=y, many .*.d files are left
in lib/test_fortify/ because the compiler outputs header dependencies
into *.d without fixdep being invoked.

When compiling C files, if_changed_dep should be used so that the
auto-generated header dependencies are recorded in .*.cmd files.

Currently, if_changed is incorrectly used, and only two headers are
hard-coded in lib/Makefile.

In the previous patch version, the kbuild test robot detected new errors
on GCC 7.

GCC 7 or older does not produce test.d with the following test code:

 $ echo 'void b(void) __attribute__((__error__(""))); void a(void) { b(); }' |
   gcc -Wp,-MMD,test.d -c -o /dev/null -x c -

Perhaps, this was a bug that existed in older GCC versions.

Skip the tests for GCC<=7 for now, as this will be eventually solved
when we bump the minimal supported GCC version.

Link: https://lore.kernel.org/oe-kbuild-all/CAK7LNARmJcyyzL-jVJfBPi3W684LTDmuhMf1koF0TXoCpKTmcw@mail.gmail.com/T/#m13771bf78ae21adff22efc4d310c973fb4bcaf67
Signed-off-by: Masahiro Yamada <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

Author: masahir0y

lib/test_fortify/Makefile

@@ -6,11 +6,8 @@ quiet_cmd_test_fortify = TEST    $@
       cmd_test_fortify = $(CONFIG_SHELL) $(src)/test_fortify.sh \
 			$< $@ "$(NM)" $(CC) $(c_flags) -DKBUILD_EXTRA_WARN1
 
-$(obj)/%.log: $(src)/%.c $(src)/test_fortify.sh \
-	      $(src)/test_fortify.h \
-	      $(srctree)/include/linux/fortify-string.h \
-	      FORCE
-	$(call if_changed,test_fortify)
+$(obj)/%.log: $(src)/%.c $(src)/test_fortify.sh FORCE
+	$(call if_changed_dep,test_fortify)
 
 logs = $(patsubst $(src)/%.c, %.log, $(wildcard $(src)/*-*.c))
 targets += $(logs)
@@ -21,7 +18,10 @@ quiet_cmd_gen_fortify_log = CAT     $@
 $(obj)/test_fortify.log: $(addprefix $(obj)/, $(logs)) FORCE
 	$(call if_changed,gen_fortify_log)
 
-always-y += test_fortify.log
+# GCC<=7 does not always produce *.d files.
+# Run the tests only for GCC>=8 or Clang.
+always-$(call gcc-min-version, 80000) += test_fortify.log
+always-$(CONFIG_CC_IS_CLANG)          += test_fortify.log
 
 # Some architectures define __NO_FORTIFY if __SANITIZE_ADDRESS__ is undefined.
 # Pass CFLAGS_KASAN to avoid warnings.