linux-kernel - Re: [PATCH 06/13] kbuild: merge vmlinux_link() between the ordinary link and Clang LTO

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Message-ID: <202108181940.896CA4311@keescook>
Date:   Wed, 18 Aug 2021 19:42:18 -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>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH 06/13] kbuild: merge vmlinux_link() between the ordinary
 link and Clang LTO

On Thu, Aug 19, 2021 at 09:57:37AM +0900, Masahiro Yamada wrote:
> When Clang LTO is enabled, vmlinux_link() reuses vmlinux.o instead of
> linking ${KBUILD_VMLINUX_OBJS} and ${KBUILD_VMLINUX_LIBS} again.
> 
> That is the only difference here, so merge the similar code.

Oh excellent! I had tried to get this merged before and was not happy
with anything I constructed. This is much cleaner. Nice! (I think I
didn't realize there could be an empty --start-group/--end-group with
no side-effects.)

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> 
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> ---
> 
>  scripts/link-vmlinux.sh | 30 ++++++++++++++----------------
>  1 file changed, 14 insertions(+), 16 deletions(-)
> 
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index 36ef7b37fc5d..a6c4d0bce3ba 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -154,12 +154,23 @@ vmlinux_link()
>  	local objects
>  	local strip_debug
>  	local map_option
> +	local objs
> +	local libs
>  
>  	info LD ${output}
>  
>  	# skip output file argument
>  	shift
>  
> +	if [ -n "${CONFIG_LTO_CLANG}" ]; then
> +		# Use vmlinux.o instead of performing the slow LTO link again.
> +		objs=vmlinux.o
> +		libs=
> +	else
> +		objs="${KBUILD_VMLINUX_OBJS}"
> +		libs="${KBUILD_VMLINUX_LIBS}"
> +	fi
> +
>  	# The kallsyms linking does not need debug symbols included.
>  	if [ "$output" != "${output#.tmp_vmlinux.kallsyms}" ] ; then
>  		strip_debug=-Wl,--strip-debug
> @@ -170,22 +181,9 @@ vmlinux_link()
>  	fi
>  
>  	if [ "${SRCARCH}" != "um" ]; then
> -		if [ -n "${CONFIG_LTO_CLANG}" ]; then
> -			# Use vmlinux.o instead of performing the slow LTO
> -			# link again.
> -			objects="--whole-archive		\
> -				vmlinux.o 			\
> -				--no-whole-archive		\
> -				${@}"
> -		else
> -			objects="--whole-archive		\
> -				${KBUILD_VMLINUX_OBJS}		\
> -				--no-whole-archive		\
> -				--start-group			\
> -				${KBUILD_VMLINUX_LIBS}		\
> -				--end-group			\
> -				${@}"
> -		fi
> +		objects="--whole-archive ${objs} --no-whole-archive	\
> +			 --start-group ${libs} --end-group		\
> +			 $@"
>  
>  		${LD} ${KBUILD_LDFLAGS} ${LDFLAGS_vmlinux}	\
>  			${strip_debug#-Wl,}			\
> -- 
> 2.30.2
> 

-- 
Kees Cook