kbuild: rebuild GCC plugins when the compiler is upgraded

Linus reported a build error due to the GCC plugin incompatibility
when the compiler is upgraded. [1]

GCC plugins are tied to a particular GCC version. So, they must be
rebuilt when the compiler is upgraded.

This seems to be a long-standing flaw since the initial support of
GCC plugins.

Extend commit 8b59cd8 ("kbuild: ensure full rebuild when the
compiler is updated"), so that GCC plugins are covered by the
compiler upgrade detection.

[1]: https://lore.kernel.org/lkml/CAHk-=wieoN5ttOy7SnsGwZv+Fni3R6m-Ut=oxih6bbZ28G+4dw@mail.gmail.com/

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>

Makefile

@@ -479,6 +479,7 @@ USERINCLUDE    := \
 		-I$(objtree)/arch/$(SRCARCH)/include/generated/uapi \
 		-I$(srctree)/include/uapi \
 		-I$(objtree)/include/generated/uapi \
+                -include $(srctree)/include/linux/compiler-version.h \
                 -include $(srctree)/include/linux/kconfig.h
 
 # Use LINUXINCLUDE when you must reference the include/ directory.

include/linux/compiler-version.h

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifdef  __LINUX_COMPILER_VERSION_H
+#error "Please do not include <linux/compiler-version.h>. This is done by the build system."
+#endif
+#define __LINUX_COMPILER_VERSION_H
+
+/*
+ * This header exists to force full rebuild when the compiler is upgraded.
+ *
+ * When fixdep scans this, it will find this string "CONFIG_CC_VERSION_TEXT"
+ * and add dependency on include/config/cc/version/text.h, which is touched
+ * by Kconfig when the version string from the compiler changes.
+ */

include/linux/kconfig.h

@@ -2,8 +2,6 @@
 #ifndef __LINUX_KCONFIG_H
 #define __LINUX_KCONFIG_H
 
-/* CONFIG_CC_VERSION_TEXT (Do not delete this comment. See help in Kconfig) */
-
 #include <generated/autoconf.h>
 
 #ifdef CONFIG_CPU_BIG_ENDIAN

init/Kconfig

@@ -20,10 +20,10 @@ config CC_VERSION_TEXT
 	    When the compiler is updated, Kconfig will be invoked.
 
 	  - Ensure full rebuild when the compiler is updated
-	    include/linux/kconfig.h contains this option in the comment line so
-	    fixdep adds include/config/cc/version/text.h into the auto-generated
-	    dependency. When the compiler is updated, syncconfig will touch it
-	    and then every file will be rebuilt.
+	    include/linux/compiler-version.h contains this option in the comment
+	    line so fixdep adds include/config/cc/version/text.h into the
+	    auto-generated dependency. When the compiler is updated, syncconfig
+	    will touch it and then every file will be rebuilt.
 
 config CC_IS_GCC
 	def_bool $(success,test "$(cc-name)" = GCC)

scripts/gcc-plugins/Makefile

@@ -22,6 +22,7 @@ always-y += $(GCC_PLUGIN)
 GCC_PLUGINS_DIR = $(shell $(CC) -print-file-name=plugin)
 
 plugin_cxxflags	= -Wp,-MMD,$(depfile) $(KBUILD_HOSTCXXFLAGS) -fPIC \
+		  -include $(srctree)/include/linux/compiler-version.h \
 		   -I $(GCC_PLUGINS_DIR)/include -I $(obj) -std=gnu++11 \
 		   -fno-rtti -fno-exceptions -fasynchronous-unwind-tables \
 		   -ggdb -Wno-narrowing -Wno-unused-variable \