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

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Message-ID: <CABCJKudQhagwdb-UfGE2JQN8H29NSpMd5PgVoftJYRp5ZwpRrA@mail.gmail.com>
Date:   Wed, 18 Aug 2021 08:06:01 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
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 7:34 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> 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 $@

Good catch, probably a leftover from an earlier iteration.

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

Please do.

Sami