[PATCH] kbuild: introduce include/generated/global-rebuild.h

[Masahiro Yamada <masahiroy@kernel.org>]

Sometimes we need to trigger a global rebuild of the kernel tree -
for instance, when any of the GCC plugins changes. [1]

This commit provides a simple mechanism to force a global rebuild
using a single header file.

The top-level Makefile creates include/generated/global-rebuild.h if
it does not exist, and this file is included by all kernel sources.

You can touch it in a build rule when a global rebuild is required.

The following is an example change to the GCC plugin build rule.

  quiet_cmd_plugin_cxx_so_c = HOSTCXX $@
 -      cmd_plugin_cxx_so_c = $(HOSTCXX) $(plugin_cxxflags) $(plugin_ldflags) -o $@ $<
 +      cmd_plugin_cxx_so_c = $(HOSTCXX) $(plugin_cxxflags) $(plugin_ldflags) -o $@ $<; touch include/generated/global-rebuild.h

Link: https://lore.kernel.org/linux-kbuild/CAK7LNATDbxc+3HQ6zoSk9t-Lkf4MSNmEUN6S5EqoVWnBQw_K6g@mail.gmail.com/T/#me069145443a17f0b464c13814424dbba0d970863 [1]
Cc: Kees Cook <kees@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

 Makefile         | 9 ++++++++-
 arch/um/Makefile | 3 ++-
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 5aa9ee52a765..64811eebd549 100644
--- a/Makefile
+++ b/Makefile
@@ -558,7 +558,8 @@ USERINCLUDE    := \
 		-I$(srctree)/include/uapi \
 		-I$(objtree)/include/generated/uapi \
                 -include $(srctree)/include/linux/compiler-version.h \
-                -include $(srctree)/include/linux/kconfig.h
+                -include $(srctree)/include/linux/kconfig.h \
+                -include $(objtree)/include/generated/global-rebuild.h
 
 # Use LINUXINCLUDE when you must reference the include/ directory.
 # Needed to be compatible with the O= option
@@ -1257,6 +1258,11 @@ endif
 include/config/kernel.release: FORCE
 	$(call filechk,kernel.release)
 
+filechk_empty = printf ''
+
+include/generated/global-rebuild.h: FORCE
+	$(call filechk,empty)
+
 # Additional helpers built in scripts/
 # Carefully list dependencies so we do not try to build scripts twice
 # in parallel
@@ -1273,6 +1279,7 @@ scripts: scripts_basic scripts_dtc
 PHONY += prepare archprepare
 
 archprepare: outputmakefile archheaders archscripts scripts include/config/kernel.release \
+	include/generated/global-rebuild.h \
 	asm-generic $(version_h) include/generated/utsrelease.h \
 	include/generated/compile.h include/generated/autoconf.h \
 	include/generated/rustc_cfg remove-stale-files
diff --git a/arch/um/Makefile b/arch/um/Makefile
index 1d36a613aad8..f564a26c1364 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -73,7 +73,8 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
 		-D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \
 		-idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__ \
 		-include $(srctree)/include/linux/compiler-version.h \
-		-include $(srctree)/include/linux/kconfig.h
+		-include $(srctree)/include/linux/kconfig.h \
+		-include $(objtree)/include/generated/global-rebuild.h
 
 #This will adjust *FLAGS accordingly to the platform.
 include $(srctree)/$(ARCH_DIR)/Makefile-os-Linux
-- 
2.43.0