kbuild: do section mismatch check on full vmlinux

Previously we did do the check on the .o files used to link
vmlinux but that failed to find questionable references across
the .o files.
Create a dedicated vmlinux.o file used only for section mismatch checks
that uses the defualt linker script so section does not get renamed.

The vmlinux.o may later be used as part of the the final link of vmlinux
but for now it is used fo section mismatch only.
For a defconfig build this is instant but for an allyesconfig this
add two minutes to a full build (that anyways takes ~2 hours).
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -612,7 +612,7 @@ quiet_cmd_vmlinux__ ?= LD      $@
       cmd_vmlinux__ ?= $(LD) $(LDFLAGS) $(LDFLAGS_vmlinux) -o $@ \
       -T $(vmlinux-lds) $(vmlinux-init)                          \
       --start-group $(vmlinux-main) --end-group                  \
-      $(filter-out $(vmlinux-lds) $(vmlinux-init) $(vmlinux-main) FORCE ,$^)
+      $(filter-out $(vmlinux-lds) $(vmlinux-init) $(vmlinux-main) vmlinux.o FORCE ,$^)
 
 # Generate new vmlinux version
 quiet_cmd_vmlinux_version = GEN     .version
@@ -736,15 +736,31 @@ debug_kallsyms: .tmp_map$(last_kallsyms)
 
 endif # ifdef CONFIG_KALLSYMS
 
+# Do modpost on a prelinked vmlinux. The finally linked vmlinux has
+# relevant sections renamed as per the linker script.
+quiet_cmd_vmlinux-modpost = LD      $@
+      cmd_vmlinux-modpost = $(LD) $(LDFLAGS) -r -o $@                          \
+	 $(vmlinux-init) --start-group $(vmlinux-main) --end-group             \
+	 $(filter-out $(vmlinux-init) $(vmlinux-main) $(vmlinux-lds) FORCE ,$^)
+define rule_vmlinux-modpost
+	:
+	+$(call cmd,vmlinux-modpost)
+	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost $@
+	$(Q)echo 'cmd_$@ := $(cmd_vmlinux-modpost)' > $(dot-target).cmd
+endef
+
 # vmlinux image - including updated kernel symbols
-vmlinux: $(vmlinux-lds) $(vmlinux-init) $(vmlinux-main) $(kallsyms.o) FORCE
+vmlinux: $(vmlinux-lds) $(vmlinux-init) $(vmlinux-main) $(kallsyms.o) vmlinux.o FORCE
 ifdef CONFIG_HEADERS_CHECK
 	$(Q)$(MAKE) -f $(srctree)/Makefile headers_check
 endif
+	$(call vmlinux-modpost)
 	$(call if_changed_rule,vmlinux__)
-	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost $@
 	$(Q)rm -f .old_version
 
+vmlinux.o: $(vmlinux-lds) $(vmlinux-init) $(vmlinux-main) $(kallsyms.o) FORCE
+	$(call if_changed_rule,vmlinux-modpost)
+
 # The actual objects are generated when descending, 
 # make sure no implicit rule kicks in
 $(sort $(vmlinux-init) $(vmlinux-main)) $(vmlinux-lds): $(vmlinux-dirs) ;

scripts/Makefile.modpost

@@ -70,10 +70,10 @@ __modpost: $(modules:.ko=.o) FORCE
 	$(call cmd,modpost) $(wildcard vmlinux) $(filter-out FORCE,$^)
 
 quiet_cmd_kernel-mod = MODPOST $@
-      cmd_kernel-mod = $(cmd_modpost) $(KBUILD_VMLINUX_OBJS)
+      cmd_kernel-mod = $(cmd_modpost) $@
 
 PHONY += vmlinux
-vmlinux: FORCE
+vmlinux.o: FORCE
 	$(call cmd,kernel-mod)
 
 # Declare generated files as targets for modpost

scripts/mod/modpost.c

@@ -75,7 +75,8 @@ static int is_vmlinux(const char *modname)
 	else
 		myname = modname;
 
-	return strcmp(myname, "vmlinux") == 0;
+	return (strcmp(myname, "vmlinux") == 0) ||
+	       (strcmp(myname, "vmlinux.o") == 0);
 }
 
 void *do_nofail(void *ptr, const char *expr)