linux-kernel - Re: [PATCH v2 06/13] kbuild: reuse $(cmd_objtool) for cmd_cc_lto_link

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Date:   Tue, 31 Aug 2021 10:35:29 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org,
        Michal Marek <michal.lkml@...kovi.net>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 06/13] kbuild: reuse $(cmd_objtool) for
 cmd_cc_lto_link_modules

On Tue, Aug 31, 2021 at 04:39:57PM +0900, Masahiro Yamada wrote:
> For CONFIG_LTO_CLANG=y, the objtool processing is not possible at the
> compilation, hence postponed by the link time.
> 
> Reuse $(cmd_objtool) for CONFIG_LTO_CLANG=y by defining objtool-enabled
> properly.
> 
> For CONFIG_LTO_CLANG=y:
> 
>   objtool-enabled is off for %.o compilation
>   objtool-enabled is on  for %.lto link
> 
> For CONFIG_LTO_CLANG=n:
> 
>   objtool-enabled is on for %.o compilation
>       (but, it depends on OBJECT_FILE_NON_STANDARD)
> 
> Set part-of-module := y for %.lto.o to avoid repeating --module.
> 
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> ---
> 
>  scripts/Makefile.build | 28 +++++++++++++++++-----------
>  1 file changed, 17 insertions(+), 11 deletions(-)
> 
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 21b55f37a23f..afc906cd7256 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -236,20 +236,26 @@ objtool_args =								\
>  	$(if $(CONFIG_X86_SMAP), --uaccess)				\
>  	$(if $(CONFIG_FTRACE_MCOUNT_USE_OBJTOOL), --mcount)
>  
> -ifndef CONFIG_LTO_CLANG
> +cmd_objtool = $(if $(objtool-enabled), ; $(objtool) $(objtool_args) $@)
> +cmd_gen_objtooldep = $(if $(objtool-enabled), { echo ; echo '$@: $$(wildcard $(objtool))' ; } >> $(dot-target).cmd)
> +
> +endif # CONFIG_STACK_VALIDATION
> +
> +ifdef CONFIG_LTO_CLANG
> +
> +# Skip objtool for LLVM bitcode
> +$(obj)/%o: objtool-enabled :=

Is this intentionally "%o" instead of "%.o"? (And it later overridden by
the "%.lto.o" rule?

> +
> +else
>  
>  # 'OBJECT_FILES_NON_STANDARD := y': skip objtool checking for a directory
>  # 'OBJECT_FILES_NON_STANDARD_foo.o := 'y': skip objtool checking for a file
>  # 'OBJECT_FILES_NON_STANDARD_foo.o := 'n': override directory skip for a file
>  
> -objtool-enabled = $(if $(filter-out y%, \
> +$(obj)/%o: objtool-enabled = $(if $(filter-out y%, \
>  	$(OBJECT_FILES_NON_STANDARD_$(basetarget).o)$(OBJECT_FILES_NON_STANDARD)n),y)
>  
> -cmd_objtool = $(if $(objtool-enabled), ; $(objtool) $(objtool_args) $@)
> -cmd_gen_objtooldep = $(if $(objtool-enabled), { echo ; echo '$@: $$(wildcard $(objtool))' ; } >> $(dot-target).cmd)
> -
> -endif # CONFIG_LTO_CLANG
> -endif # CONFIG_STACK_VALIDATION
> +endif
>  
>  ifdef CONFIG_TRIM_UNUSED_KSYMS
>  cmd_gen_ksymdeps = \
> @@ -289,13 +295,13 @@ cmd_cc_lto_link_modules =						\
>  	$(LD) $(ld_flags) -r -o $@					\
>  		$(shell [ -s $(@:.lto.o=.o.symversions) ] &&		\
>  			echo -T $(@:.lto.o=.o.symversions))		\
> -		--whole-archive $(filter-out FORCE,$^)
> +		--whole-archive $(filter-out FORCE,$^)			\
> +		$(cmd_objtool)
>  
> -ifdef CONFIG_STACK_VALIDATION
>  # objtool was skipped for LLVM bitcode, run it now that we have compiled
>  # modules into native code
> -cmd_cc_lto_link_modules += ; $(objtool) $(objtool_args) --module $@
> -endif
> +$(obj)/%.lto.o: objtool-enabled = y
> +$(obj)/%.lto.o: part-of-module := y
>  
>  $(obj)/%.lto.o: $(obj)/%.o FORCE
>  	$(call if_changed,cc_lto_link_modules)
> -- 
> 2.30.2
> 

-- 
Kees Cook