gcc-plugins: Force full rebuild when plugins change

There was no dependency between the plugins changing and the rest of the
kernel being built. This could cause strange behaviors as instrumentation
could vary between targets depending on when they were built.

Generate a new header file, gcc-plugins.h, any time the GCC plugins
change. Include the header file in compiler-version.h when its associated
feature name, GCC_PLUGINS, is defined. This will be picked up by fixdep
and force rebuilds where needed.

Add a generic "touch" kbuild command, which will be used again in
a following patch. Add a "normalize_path" string helper to make the
"TOUCH" output less ugly.

Link: https://lore.kernel.org/r/[e-mail]
Tested-by: Nicolas Schier <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>

include/linux/compiler-version.h

···
  * and add dependency on include/config/CC_VERSION_TEXT, which is touched
  * by Kconfig when the version string from the compiler changes.
  */
+
+/* Additional tree-wide dependencies start here. */
+
+/*
+ * If any of the GCC plugins change, we need to rebuild everything that
+ * was built with them, as they may have changed their behavior and those
+ * behaviors may need to be synchronized across all translation units.
+ */
+#ifdef GCC_PLUGINS
+#include <generated/gcc-plugins.h>
+#endif

scripts/Makefile.gcc-plugins

···
 
 # All the plugin CFLAGS are collected here in case a build target needs to
 # filter them out of the KBUILD_CFLAGS.
-GCC_PLUGINS_CFLAGS := $(strip $(addprefix -fplugin=$(objtree)/scripts/gcc-plugins/, $(gcc-plugin-y)) $(gcc-plugin-cflags-y))
+GCC_PLUGINS_CFLAGS := $(strip $(addprefix -fplugin=$(objtree)/scripts/gcc-plugins/, $(gcc-plugin-y)) $(gcc-plugin-cflags-y)) -DGCC_PLUGINS
 export GCC_PLUGINS_CFLAGS
 
 # Add the flags to the build!

scripts/Makefile.lib

···
 	$(addprefix $(obj)/, $(call suffix-search, $(patsubst $(obj)/%,%,$m), $2, $3))))
 endef
 
+# Remove ".." and "." from a path, without using "realpath"
+# Usage:
+#   $(call normalize_path,path/to/../file)
+define normalize_path
+$(strip $(eval elements :=) \
+$(foreach elem,$(subst /, ,$1), \
+	$(if $(filter-out .,$(elem)), \
+	     $(if $(filter ..,$(elem)), \
+		  $(eval elements := $(wordlist 2,$(words $(elements)),x $(elements))), \
+		  $(eval elements := $(elements) $(elem))))) \
+$(subst $(space),/,$(elements)))
+endef
+
 # Build commands
 # ===========================================================================
 # These are shared by some Makefile.* files.
···
 
 $(obj)/%: $(src)/%_shipped
 	$(call cmd,copy)
+
+# Touch a file
+# ===========================================================================
+quiet_cmd_touch = TOUCH   $(call normalize_path,$@)
+      cmd_touch = touch $@
 
 # Commands useful for building a boot image
 # ===========================================================================

scripts/gcc-plugins/Makefile

···
 
 $(plugin-objs): $(obj)/%.o: $(src)/%.c FORCE
 	$(call if_changed_dep,plugin_cxx_o_c)
+
+$(obj)/../../include/generated/gcc-plugins.h: $(plugin-single) $(plugin-multi) FORCE
+	$(call if_changed,touch)
+always-y += ../../include/generated/gcc-plugins.h