linux-kernel - Re: [PATCH 03/13] kbuild: detect objtool changes correctly and remove .SECONDEXPANSION

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Message-ID: <202108181952.14AEEED@keescook>
Date:   Wed, 18 Aug 2021 19:55:37 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org,
        Sami Tolvanen <samitolvanen@...gle.com>,
        linux-kernel@...r.kernel.org,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH 03/13] kbuild: detect objtool changes correctly and
 remove .SECONDEXPANSION

On Thu, Aug 19, 2021 at 09:57:34AM +0900, Masahiro Yamada wrote:
> This reverts commit 8852c5524029 ("kbuild: Fix objtool dependency for
> 'OBJECT_FILES_NON_STANDARD_<obj> := n'"), and fix the dependency in a
> cleaner way.
> 
> Using .SECONDEXPANSION is expensive since Makefile.build is parsed
> twice every time, and the escaping dollars makes the code unreadable.
> 
> Adding include/config/* as dependency is not maintainable either because
> objtool_args is dependent on more CONFIG options.
> 
> A better fix is to include the objtool command in *.cmd files so any
> command change is naturally detected by if_change.
> 
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> ---
> 
>  scripts/Makefile.build | 28 ++++++++++------------------
>  1 file changed, 10 insertions(+), 18 deletions(-)
> 
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 31154e44c251..3e4cd1439cd4 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -155,7 +155,7 @@ $(obj)/%.ll: $(src)/%.c FORCE
>  # (See cmd_cc_o_c + relevant part of rule_cc_o_c)
>  
>  quiet_cmd_cc_o_c = CC $(quiet_modtag)  $@
> -      cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $<
> +      cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $< $(cmd_objtool)
>  
>  ifdef CONFIG_MODVERSIONS
>  # When module versioning is enabled the following steps are executed:
> @@ -223,6 +223,8 @@ endif # CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT
>  
>  ifdef CONFIG_STACK_VALIDATION
>  
> +objtool := $(objtree)/tools/objtool/objtool
> +
>  # Objtool arguments are also needed for modfinal with LTO, so we define
>  # then here to avoid duplication.
>  objtool_args =								\
> @@ -237,26 +239,19 @@ objtool_args =								\
>  
>  ifndef CONFIG_LTO_CLANG
>  
> -__objtool_obj := $(objtree)/tools/objtool/objtool
> -
>  # '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
>  cmd_objtool = $(if $(patsubst y%,, \
>  	$(OBJECT_FILES_NON_STANDARD_$(basetarget).o)$(OBJECT_FILES_NON_STANDARD)n), \
> -	$(__objtool_obj) $(objtool_args) $@)
> -objtool_obj = $(if $(patsubst y%,, \
> -	$(OBJECT_FILES_NON_STANDARD_$(basetarget).o)$(OBJECT_FILES_NON_STANDARD)n), \
> -	$(__objtool_obj))
> +	; $(objtool) $(objtool_args) $@)

This is extremely clever -- pasting commands together. :)

Does this correctly propagate failures in the first half of the command?

For example, now we'd have:
	gcc flags..... -c -o out in ; objtool...

But I think objtool will run even on gcc failure, and "make" won't see
the failure from gcc? I need to go test this.

-Kees

> +
> +# Rebuild all objects when objtool is updated
> +objtool_dep = $(objtool)
>  
>  endif # CONFIG_LTO_CLANG
>  endif # CONFIG_STACK_VALIDATION
>  
> -# Rebuild all objects when objtool changes, or is enabled/disabled.
> -objtool_dep = $(objtool_obj)					\
> -	      $(wildcard include/config/ORC_UNWINDER		\
> -			 include/config/STACK_VALIDATION)
> -
>  ifdef CONFIG_TRIM_UNUSED_KSYMS
>  cmd_gen_ksymdeps = \
>  	$(CONFIG_SHELL) $(srctree)/scripts/gen_ksymdeps.sh $@ >> $(dot-target).cmd
> @@ -270,7 +265,6 @@ define rule_cc_o_c
>  	$(call cmd,gen_ksymdeps)
>  	$(call cmd,checksrc)
>  	$(call cmd,checkdoc)
> -	$(call cmd,objtool)
>  	$(call cmd,modversions_c)
>  	$(call cmd,record_mcount)
>  endef
> @@ -278,13 +272,11 @@ endef
>  define rule_as_o_S
>  	$(call cmd_and_fixdep,as_o_S)
>  	$(call cmd,gen_ksymdeps)
> -	$(call cmd,objtool)
>  	$(call cmd,modversions_S)
>  endef
>  
>  # Built-in and composite module parts
> -.SECONDEXPANSION:
> -$(obj)/%.o: $(src)/%.c $(recordmcount_source) $$(objtool_dep) FORCE
> +$(obj)/%.o: $(src)/%.c $(recordmcount_source) $(objtool_dep) FORCE
>  	$(call if_changed_rule,cc_o_c)
>  	$(call cmd,force_checksrc)
>  
> @@ -367,7 +359,7 @@ $(obj)/%.s: $(src)/%.S FORCE
>  	$(call if_changed_dep,cpp_s_S)
>  
>  quiet_cmd_as_o_S = AS $(quiet_modtag)  $@
> -      cmd_as_o_S = $(CC) $(a_flags) -c -o $@ $<
> +      cmd_as_o_S = $(CC) $(a_flags) -c -o $@ $< $(cmd_objtool)
>  
>  ifdef CONFIG_ASM_MODVERSIONS
>  
> @@ -386,7 +378,7 @@ cmd_modversions_S =								\
>  	fi
>  endif
>  
> -$(obj)/%.o: $(src)/%.S $$(objtool_dep) FORCE
> +$(obj)/%.o: $(src)/%.S $(objtool_dep) FORCE
>  	$(call if_changed_rule,as_o_S)
>  
>  targets += $(filter-out $(subdir-builtin), $(real-obj-y))
> -- 
> 2.30.2
> 

-- 
Kees Cook