linux-kernel - Re: [PATCH v4] kbuild: Fix TRIM_UNUSED_KSYMS with LTO

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Date:   Wed, 18 Aug 2021 11:33:43 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Alexander Lobakin <alobakin@...me>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] kbuild: Fix TRIM_UNUSED_KSYMS with LTO_CLANG

On Tue, Aug 17, 2021 at 3:05 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
>
> With CONFIG_LTO_CLANG, we currently link modules into native
> code just before modpost, which means with TRIM_UNUSED_KSYMS
> enabled, we still look at the LLVM bitcode in the .o files when
> generating the list of used symbols. As the bitcode doesn't
> yet have calls to compiler intrinsics and llvm-nm doesn't see
> function references that only exist in function-level inline
> assembly, we currently need a whitelist for TRIM_UNUSED_KSYMS to
> work with LTO.
>
> This change moves module LTO linking to happen earlier, and
> thus avoids the issue with LLVM bitcode and TRIM_UNUSED_KSYMS
> entirely, allowing us to also drop the whitelist from
> gen_autoksyms.sh.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1369
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> Reviewed-by: Alexander Lobakin <alobakin@...me>
> Tested-by: Alexander Lobakin <alobakin@...me>
> ---
> Changes in v4:
> - Added .lto.o to targets to actually fix the use of if_changed.
> - Replaced an unnecessary mod-prelink-ext with .lto.
>
> Changes in v3:
> - Added missing FORCE.
>
> Changes in v2:
> - Fixed a couple of typos.
> - Fixed objtool arguments for .lto.o to always include --module.
>
> ---
>  scripts/Makefile.build    | 28 +++++++++++++++++++++++++++-
>  scripts/Makefile.lib      |  7 +++++++
>  scripts/Makefile.modfinal | 21 ++-------------------
>  scripts/Makefile.modpost  | 22 +++-------------------
>  scripts/gen_autoksyms.sh  | 12 ------------
>  5 files changed, 39 insertions(+), 51 deletions(-)
>
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 02197cb8e3a7..a6f43afd09bb 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -88,6 +88,10 @@ endif
>
>  targets-for-modules := $(patsubst %.o, %.mod, $(filter %.o, $(obj-m)))
>
> +ifdef CONFIG_LTO_CLANG
> +targets-for-modules += $(patsubst %.o, %.lto.o, $(filter %.o, $(obj-m)))
> +endif
> +
>  ifdef need-modorder
>  targets-for-modules += $(obj)/modules.order
>  endif
> @@ -271,12 +275,34 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) $$(objtool_dep) FORCE
>         $(call if_changed_rule,cc_o_c)
>         $(call cmd,force_checksrc)
>
> +ifdef CONFIG_LTO_CLANG
> +# Module .o files may contain LLVM bitcode, compile them into native code
> +# before ELF processing
> +quiet_cmd_cc_lto_link_modules = LTO [M] $@
> +cmd_cc_lto_link_modules =                                              \
> +       $(LD) $(ld_flags) -r -o $@                                      \
> +               $(shell [ -s $(@:.lto.o=.o.symversions) ] &&            \
> +                       echo -T $(@:.lto.o=.o.symversions))             \
> +               --whole-archive $(filter-out FORCE,$^)
> +
> +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 += ;                                           \
> +       $(objtree)/tools/objtool/objtool $(objtool_args) --module       \
> +               $(@:.ko=.lto.o)


What is this "$(@:.ko=.lto.o)" doing ?

The target is already suffixed with .lto.o
so $(@:.ko=.lto.o) should be the same as $@


Shall I fix it locally unless
I find more questionable code?





> +endif
> +
> +$(obj)/%.lto.o: $(obj)/%.o FORCE
> +       $(call if_changed,cc_lto_link_modules)
> +endif
> +


-- 
Best Regards
Masahiro Yamada