linux-kernel - Re: [PATCH 1/4] kbuild: fix UNUSED_KSYMS

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Date:   Thu, 25 Feb 2021 09:45:58 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild <linux-kbuild@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jessica Yu <jeyu@...nel.org>, Nicolas Pitre <nico@...xnic.net>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Arnd Bergmann <arnd@...nel.org>
Subject: Re: [PATCH 1/4] kbuild: fix UNUSED_KSYMS_WHITELIST for Clang LTO

Hi Masahiro,

On Thu, Feb 25, 2021 at 8:03 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> Commit fbe078d397b4 ("kbuild: lto: add a default list of used symbols")
> does not work as expected if the .config file has already specified
> CONFIG_UNUSED_KSYMS_WHITELIST="my/own/white/list" before enabling
> CONFIG_LTO_CLANG.
>
> So, the user-supplied whitelist and LTO-specific white list must be
> independent of each other.
>
> I refactored the shell script so CONFIG_MODVERSIONS and CONFIG_CLANG_LTO
> handle whitelists in the same way.
>
> Fixes: fbe078d397b4 ("kbuild: lto: add a default list of used symbols")
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> ---
>
>  init/Kconfig                    |  1 -
>  scripts/gen_autoksyms.sh        | 33 ++++++++++++++++++++++++---------
>  scripts/lto-used-symbollist.txt |  5 -----
>  3 files changed, 24 insertions(+), 15 deletions(-)
>  delete mode 100644 scripts/lto-used-symbollist.txt

> +
> +ksym_wl=
>  if [ -n "$CONFIG_UNUSED_KSYMS_WHITELIST" ]; then
>         # Use 'eval' to expand the whitelist path and check if it is relative
>         eval ksym_wl="$CONFIG_UNUSED_KSYMS_WHITELIST"
> @@ -40,16 +57,14 @@ cat > "$output_file" << EOT
>  EOT
>
>  [ -f modules.order ] && modlist=modules.order || modlist=/dev/null
> -sed 's/ko$/mod/' $modlist |
> -xargs -n1 sed -n -e '2{s/ /\n/g;/^$/!p;}' -- |
> -cat - "$ksym_wl" |
> +
> +{
> +       sed 's/ko$/mod/' $modlist | xargs -n1 sed -n -e '2p'
> +       echo "$needed_symbols"
> +       [ -n "$ksym_wl" ] && cat "$ksym_wl"
> +} | sed -e 's/ /\n/g' | sed -n -e '/^$/!p' |
>  # Remove the dot prefix for ppc64; symbol names with a dot (.) hold entry
>  # point addresses.
>  sed -e 's/^\.//' |
>  sort -u |
>  sed -e 's/\(.*\)/#define __KSYM_\1 1/' >> "$output_file"
> -
> -# Special case for modversions (see modpost.c)
> -if [ -n "$CONFIG_MODVERSIONS" ]; then
> -       echo "#define __KSYM_module_layout 1" >> "$output_file"
> -fi
> diff --git a/scripts/lto-used-symbollist.txt b/scripts/lto-used-symbollist.txt
> deleted file mode 100644
> index 38e7bb9ebaae..000000000000
> --- a/scripts/lto-used-symbollist.txt
> +++ /dev/null
> @@ -1,5 +0,0 @@
> -memcpy
> -memmove
> -memset
> -__stack_chk_fail
> -__stack_chk_guard
> --
> 2.27.0
>
>
> diff --git a/init/Kconfig b/init/Kconfig
> index 0bf5b340b80e..351161326e3c 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -2277,7 +2277,6 @@ config TRIM_UNUSED_KSYMS
>  config UNUSED_KSYMS_WHITELIST
>         string "Whitelist of symbols to keep in ksymtab"
>         depends on TRIM_UNUSED_KSYMS
> -       default "scripts/lto-used-symbollist.txt" if LTO_CLANG
>         help
>           By default, all unused exported symbols will be un-exported from the
>           build when TRIM_UNUSED_KSYMS is selected.
> diff --git a/scripts/gen_autoksyms.sh b/scripts/gen_autoksyms.sh
> index d54dfba15bf2..b74d5949fea6 100755
> --- a/scripts/gen_autoksyms.sh
> +++ b/scripts/gen_autoksyms.sh
> @@ -19,7 +19,24 @@ esac
>  # We need access to CONFIG_ symbols
>  . include/config/auto.conf
>
> -ksym_wl=/dev/null
> +needed_symbols=
> +
> +# Special case for modversions (see modpost.c)
> +if [ -n "$CONFIG_MODVERSIONS" ]; then
> +       needed_symbols="$needed_symbols module_layout"
> +fi
> +
> +# With CONFIG_LTO_CLANG, LLVM bitcode has not yet been compiled into a binary
> +# when the .mod files are generated, which means they don't yet contain
> +# references to certain symbols that will be present in the final binaries.
> +if [ -n "$CONFIG_LTO_CLANG" ]; then
> +       # intrinsic functions
> +       needed_symbols="$needed_symbols memcpy memmove memset"
> +       # stack protector symbols
> +       needed_symbols="$needed_symbols __stack_chk_fail __stack_chk_guard"
> +fi

Thank you for the patch!

Arnd just reported that _mcount is also needed with some
configurations. Would you mind including that in the next version?

https://lore.kernel.org/r/20210225143456.3829513-1-arnd@kernel.org/

Sami