linux-kernel - Re: [PATCH v2 06/13] kbuild: reuse $(cmd_objtool) for cmd_cc_lto_link

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Date:   Thu, 2 Sep 2021 18:49:07 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 06/13] kbuild: reuse $(cmd_objtool) for
 cmd_cc_lto_link_modules

On Fri, Sep 03, 2021 at 09:39:14AM +0900, Masahiro Yamada wrote:
> On Wed, Sep 1, 2021 at 2:35 AM Kees Cook <keescook@...omium.org> wrote:
> >
> > On Tue, Aug 31, 2021 at 04:39:57PM +0900, Masahiro Yamada wrote:
> > > For CONFIG_LTO_CLANG=y, the objtool processing is not possible at the
> > > compilation, hence postponed by the link time.
> > >
> > > Reuse $(cmd_objtool) for CONFIG_LTO_CLANG=y by defining objtool-enabled
> > > properly.
> > >
> > > For CONFIG_LTO_CLANG=y:
> > >
> > >   objtool-enabled is off for %.o compilation
> > >   objtool-enabled is on  for %.lto link
> > >
> > > For CONFIG_LTO_CLANG=n:
> > >
> > >   objtool-enabled is on for %.o compilation
> > >       (but, it depends on OBJECT_FILE_NON_STANDARD)
> > >
> > > Set part-of-module := y for %.lto.o to avoid repeating --module.
> > >
> > > Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> > > ---
> > >
> > >  scripts/Makefile.build | 28 +++++++++++++++++-----------
> > >  1 file changed, 17 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> > > index 21b55f37a23f..afc906cd7256 100644
> > > --- a/scripts/Makefile.build
> > > +++ b/scripts/Makefile.build
> > > @@ -236,20 +236,26 @@ objtool_args =                                                          \
> > >       $(if $(CONFIG_X86_SMAP), --uaccess)                             \
> > >       $(if $(CONFIG_FTRACE_MCOUNT_USE_OBJTOOL), --mcount)
> > >
> > > -ifndef CONFIG_LTO_CLANG
> > > +cmd_objtool = $(if $(objtool-enabled), ; $(objtool) $(objtool_args) $@)
> > > +cmd_gen_objtooldep = $(if $(objtool-enabled), { echo ; echo '$@: $$(wildcard $(objtool))' ; } >> $(dot-target).cmd)
> > > +
> > > +endif # CONFIG_STACK_VALIDATION
> > > +
> > > +ifdef CONFIG_LTO_CLANG
> > > +
> > > +# Skip objtool for LLVM bitcode
> > > +$(obj)/%o: objtool-enabled :=
> >
> > Is this intentionally "%o" instead of "%.o"?
> 
> Good catch.
> 
> No, it is not intentional.
> 
> I will fix "%o" to "%.o"

Ah-ha, okay, excellent. :) With that:

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

Thanks!

-Kees

> 
> 
> > (And it later overridden by the "%.lto.o" rule?
> 
> No, opposite.
> 
> While building %.lto.o, we want to set objtool-enabled.
> But, we want to cancel it for %.o
> 
> 
> 
> 
> -- 
> Best Regards
> Masahiro Yamada
> 
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-- 
Kees Cook