kbuild: fix Clang LTO with CONFIG_OBJTOOL=n

Since commit bede169 ("kbuild: enable objtool for *.mod.o and
additional kernel objects"), Clang LTO builds do not perform any
optimizations when CONFIG_OBJTOOL is disabled (e.g., for ARCH=arm64).
This is because every LLVM bitcode file is immediately converted to
ELF format before the object files are linked together.

This commit fixes the breakage.

Fixes: bede169 ("kbuild: enable objtool for *.mod.o and additional kernel objects")
Reported-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Yonghong Song <yonghong.song@linux.dev>

Author: masahir0y

scripts/Makefile.build

@@ -194,7 +194,9 @@ endif # CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT
 
 is-standard-object = $(if $(filter-out y%, $(OBJECT_FILES_NON_STANDARD_$(target-stem).o)$(OBJECT_FILES_NON_STANDARD)n),$(is-kernel-object))
 
+ifdef CONFIG_OBJTOOL
 $(obj)/%.o: private objtool-enabled = $(if $(is-standard-object),$(if $(delay-objtool),$(is-single-obj-m),y))
+endif
 
 ifneq ($(findstring 1, $(KBUILD_EXTRA_WARN)),)
 cmd_warn_shared_object = $(if $(word 2, $(modname-multi)),$(warning $(kbuild-file): $*.o is added to multiple modules: $(modname-multi)))

scripts/Makefile.lib

@@ -287,6 +287,8 @@ delay-objtool := $(or $(CONFIG_LTO_CLANG),$(CONFIG_X86_KERNEL_IBT))
 cmd_objtool = $(if $(objtool-enabled), ; $(objtool) $(objtool-args) $@)
 cmd_gen_objtooldep = $(if $(objtool-enabled), { echo ; echo '$@: $$(wildcard $(objtool))' ; } >> $(dot-target).cmd)
 
+objtool-enabled := y
+
 endif # CONFIG_OBJTOOL
 
 # Useful for describing the dependency of composite objects
@@ -302,11 +304,11 @@ endef
 # ===========================================================================
 # These are shared by some Makefile.* files.
 
-objtool-enabled := y
-
 ifdef CONFIG_LTO_CLANG
-# objtool cannot process LLVM IR. Make $(LD) covert LLVM IR to ELF here.
-cmd_ld_single = $(if $(objtool-enabled), ; $(LD) $(ld_flags) -r -o $(tmp-target) $@; mv $(tmp-target) $@)
+# Run $(LD) here to covert LLVM IR to ELF in the following cases:
+#  - when this object needs objtool processing, as objtool cannot process LLVM IR
+#  - when this is a single-object module, as modpost cannot process LLVM IR
+cmd_ld_single = $(if $(objtool-enabled)$(is-single-obj-m), ; $(LD) $(ld_flags) -r -o $(tmp-target) $@; mv $(tmp-target) $@)
 endif
 
 quiet_cmd_cc_o_c = CC $(quiet_modtag)  $@