kbuild: compile constant module information only once

Various information about modules is compiled into the info sections.
For that a dedicated .mod.c file is generated by modpost for each module
and then linked into the module.
However most of the information in the .mod.c is the same for all
modules, internal and external.
Split the shared information into a dedicated source file that is
compiled once and then linked into all modules.

This avoids frequent rebuilds for all .mod.c files when using
CONFIG_LOCALVERSION_AUTO because the local version ends up in .mod.c
through UTS_RELEASE and VERMAGIC_STRING.
The modules are still relinked in this case.

The code is also easier to maintain as it's now in a proper source file
instead of an inline string literal.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Author: t-8ch

scripts/Makefile.modfinal

@@ -30,6 +30,9 @@ quiet_cmd_cc_o_c = CC [M]  $@
 %.mod.o: %.mod.c FORCE
 	$(call if_changed_dep,cc_o_c)
 
+$(extmod_prefix).module-common.o: $(srctree)/scripts/module-common.c FORCE
+	$(call if_changed_dep,cc_o_c)
+
 quiet_cmd_ld_ko_o = LD [M]  $@
       cmd_ld_ko_o +=							\
 	$(LD) -r $(KBUILD_LDFLAGS)					\
@@ -54,13 +57,13 @@ if_changed_except = $(if $(call newer_prereqs_except,$(2))$(cmd-check),      \
 	printf '%s\n' 'savedcmd_$@ := $(make-cmd)' > $(dot-target).cmd, @:)
 
 # Re-generate module BTFs if either module's .ko or vmlinux changed
-%.ko: %.o %.mod.o scripts/module.lds $(and $(CONFIG_DEBUG_INFO_BTF_MODULES),$(KBUILD_BUILTIN),vmlinux) FORCE
+%.ko: %.o %.mod.o $(extmod_prefix).module-common.o scripts/module.lds $(and $(CONFIG_DEBUG_INFO_BTF_MODULES),$(KBUILD_BUILTIN),vmlinux) FORCE
 	+$(call if_changed_except,ld_ko_o,vmlinux)
 ifdef CONFIG_DEBUG_INFO_BTF_MODULES
 	+$(if $(newer-prereqs),$(call cmd,btf_ko))
 endif
 
-targets += $(modules:%.o=%.ko) $(modules:%.o=%.mod.o)
+targets += $(modules:%.o=%.ko) $(modules:%.o=%.mod.o) $(extmod_prefix).module-common.o
 
 # Add FORCE to the prerequisites of a target to force it to be always rebuilt.
 # ---------------------------------------------------------------------------

scripts/mod/modpost.c

@@ -1755,26 +1755,9 @@ static void check_modname_len(struct module *mod)
 static void add_header(struct buffer *b, struct module *mod)
 {
 	buf_printf(b, "#include <linux/module.h>\n");
-	/*
-	 * Include build-salt.h after module.h in order to
-	 * inherit the definitions.
-	 */
-	buf_printf(b, "#define INCLUDE_VERMAGIC\n");
-	buf_printf(b, "#include <linux/build-salt.h>\n");
-	buf_printf(b, "#include <linux/elfnote-lto.h>\n");
 	buf_printf(b, "#include <linux/export-internal.h>\n");
-	buf_printf(b, "#include <linux/vermagic.h>\n");
 	buf_printf(b, "#include <linux/compiler.h>\n");
 	buf_printf(b, "\n");
-	buf_printf(b, "#ifdef CONFIG_UNWINDER_ORC\n");
-	buf_printf(b, "#include <asm/orc_header.h>\n");
-	buf_printf(b, "ORC_HEADER;\n");
-	buf_printf(b, "#endif\n");
-	buf_printf(b, "\n");
-	buf_printf(b, "BUILD_SALT;\n");
-	buf_printf(b, "BUILD_LTO_INFO;\n");
-	buf_printf(b, "\n");
-	buf_printf(b, "MODULE_INFO(vermagic, VERMAGIC_STRING);\n");
 	buf_printf(b, "MODULE_INFO(name, KBUILD_MODNAME);\n");
 	buf_printf(b, "\n");
 	buf_printf(b, "__visible struct module __this_module\n");
@@ -1792,12 +1775,6 @@ static void add_header(struct buffer *b, struct module *mod)
 	if (!external_module)
 		buf_printf(b, "\nMODULE_INFO(intree, \"Y\");\n");
 
-	buf_printf(b,
-		   "\n"
-		   "#ifdef CONFIG_MITIGATION_RETPOLINE\n"
-		   "MODULE_INFO(retpoline, \"Y\");\n"
-		   "#endif\n");
-
 	if (strstarts(mod->name, "drivers/staging"))
 		buf_printf(b, "\nMODULE_INFO(staging, \"Y\");\n");
 

scripts/module-common.c

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/module.h>
+/*
+ * Include build-salt.h after module.h in order to
+ * inherit the definitions.
+ */
+#define INCLUDE_VERMAGIC
+#include <linux/build-salt.h>
+#include <linux/elfnote-lto.h>
+#include <linux/vermagic.h>
+
+#ifdef CONFIG_UNWINDER_ORC
+#include <asm/orc_header.h>
+ORC_HEADER;
+#endif
+
+BUILD_SALT;
+BUILD_LTO_INFO;
+
+MODULE_INFO(vermagic, VERMAGIC_STRING);
+
+#ifdef CONFIG_MITIGATION_RETPOLINE
+MODULE_INFO(retpoline, "Y");
+#endif