Re: [PATCH RESEND v2] kbuild: Fix TRIM_UNUSED_KSYMS with LTO_CLANG

From: Masahiro Yamada <masahiroy@kernel.org>
To: Sami Tolvanen <samitolvanen@google.com>
Cc: Kees Cook <keescook@chromium.org>,
	Michal Marek <michal.lkml@markovi.net>,
	Alexander Lobakin <alobakin@pm.me>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RESEND v2] kbuild: Fix TRIM_UNUSED_KSYMS with LTO_CLANG
Date: Fri, 13 Aug 2021 05:01:49 +0900	[thread overview]
Message-ID: <CAK7LNAS7Hf19wxebY70jK0TsebmfUpdh5AMo5W21PEjEXOERTw@mail.gmail.com> (raw)
In-Reply-To: <20210811203035.2463343-1-samitolvanen@google.com>

On Thu, Aug 12, 2021 at 5:30 AM Sami Tolvanen <samitolvanen@google.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@google.com>
> Reviewed-by: Alexander Lobakin <alobakin@pm.me>
> Tested-by: Alexander Lobakin <alobakin@pm.me>
> ---
> Changes in v2:
> - Fixed a couple of typos.
> - Fixed objtool arguments for .lto.o to always include --module.
>
> ---
>  scripts/Makefile.build    | 24 +++++++++++++++++++++++-
>  scripts/Makefile.lib      |  7 +++++++
>  scripts/Makefile.modfinal | 21 ++-------------------
>  scripts/Makefile.modpost  | 22 +++-------------------
>  scripts/gen_autoksyms.sh  | 12 ------------
>  5 files changed, 35 insertions(+), 51 deletions(-)
>
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 02197cb8e3a7..778dabea3a89 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -271,12 +271,34 @@ $(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 $^
> +
> +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       \
> +               $(@:.ko=$(mod-prelink-ext).o)
> +endif
> +
> +$(obj)/%.lto.o: $(obj)/%.o
> +       $(call if_changed,cc_lto_link_modules)

Documentation/kbuild/makefiles.rst says:

          Note: It is a typical mistake to forget the FORCE prerequisite.

The current code in scripts/Makefile.modpost is also wrong, though.

-- 
Best Regards
Masahiro Yamada