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Date:   Thu, 17 Mar 2022 20:44:47 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Kumar Kartikeya Dwivedi <memxor@gmail.com>,
        X86 ML <x86@kernel.org>, joao@overdrivepizza.com,
        "H . J . Lu" <hjl.tools@gmail.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Kees Cook <keescook@chromium.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Mark Rutland <mark.rutland@arm.com>, alyssa.milburn@intel.com,
        Miroslav Benes <mbenes@suse.cz>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>, bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH v4 00/45] x86: Kernel IBT
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On Wed, Mar 16, 2022 at 01:28:08AM +0900, Masahiro Yamada wrote:
> On Tue, Mar 15, 2022 at 5:15 PM Peter Zijlstra <peterz@infradead.org> wrote:

> > Index: linux-2.6/scripts/Makefile.build
> > ===================================================================
> > --- linux-2.6.orig/scripts/Makefile.build
> > +++ linux-2.6/scripts/Makefile.build
> > @@ -86,12 +86,18 @@ ifdef need-builtin
> >  targets-for-builtin += $(obj)/built-in.a
> >  endif
> >
> > -targets-for-modules := $(patsubst %.o, %.mod, $(filter %.o, $(obj-m)))
> > +targets-for-modules :=
> 
> 
> Why do you need to change this line?
> 
> 
> 
> >
> >  ifdef CONFIG_LTO_CLANG
> >  targets-for-modules += $(patsubst %.o, %.lto.o, $(filter %.o, $(obj-m)))
> >  endif
> >
> > +ifdef CONFIG_X86_KERNEL_IBT
> > +targets-for-modules += $(patsubst %.o, %.objtool, $(filter %.o, $(obj-m)))
> > +endif
> > +
> > +targets-for-modules += $(patsubst %.o, %.mod, $(filter %.o, $(obj-m)))
> > +
> >  ifdef need-modorder
> >  targets-for-modules += $(obj)/modules.order
> >  endif

The thinking was that by having the .objtool rule before the .mod rule,
objtool runs first. If mod runs before objtool, objtool will change the
timestamp and then mod will get remade, even if nothing's changed.