[PATCH] kbuild: fix Clang LTO with CONFIG_OBJTOOL=n

[PATCH] kbuild: fix Clang LTO with CONFIG_OBJTOOL=n

[Masahiro Yamada]

Since commit bede169618c6 ("kbuild: enable objtool for *.mod.o and
additional kernel objects"), Clang LTO builds do not perform any
optimizations when CONFIG_OBJTOOL is disable (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: bede169618c6 ("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>
---

 scripts/Makefile.build |  2 ++
 scripts/Makefile.lib   | 10 ++++++----
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 81d9dacad03c..993708d11874 100644
--- a/scripts/Makefile.build
+++ b/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)))
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 7395200538da..2e280a02e9e6 100644
--- a/scripts/Makefile.lib
+++ b/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)  $@
-- 
2.43.0

Re: [PATCH] kbuild: fix Clang LTO with CONFIG_OBJTOOL=n

[Yonghong Song]

On 1/31/25 6:04 AM, Masahiro Yamada wrote:
> Since commit bede169618c6 ("kbuild: enable objtool for *.mod.o and
> additional kernel objects"), Clang LTO builds do not perform any
> optimizations when CONFIG_OBJTOOL is disable (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: bede169618c6 ("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>

Thanks for the fix!

I tested x86_64 and arm64 for with and without lto, everything works fine. So

Tested-by: Yonghong Song <yonghong.song@linux.dev>

Re: [PATCH] kbuild: fix Clang LTO with CONFIG_OBJTOOL=n

[Song Liu]

On Fri, Jan 31, 2025 at 6:04 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> Since commit bede169618c6 ("kbuild: enable objtool for *.mod.o and
> additional kernel objects"), Clang LTO builds do not perform any
> optimizations when CONFIG_OBJTOOL is disable (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: bede169618c6 ("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: Song Liu <song@kernel.org>