linux-kernel - RE: [PATCH v4 00/45] x86: Kernel IBT

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Message-ID: <260c9402b6d647a39b4a5cf51024963d@AcuMS.aculab.com>
Date:   Fri, 18 Mar 2022 02:07:37 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        Masahiro Yamada <masahiroy@...nel.org>
CC:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        X86 ML <x86@...nel.org>,
        "joao@...rdrivepizza.com" <joao@...rdrivepizza.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        "alyssa.milburn@...el.com" <alyssa.milburn@...el.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>
Subject: RE: [PATCH v4 00/45] x86: Kernel IBT

From: Peter Zijlstra
> Sent: 17 March 2022 19:45
> 
> 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@...radead.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.

I don't think it should make any difference.
A quick peruse didn't show where targets-for-modules actually
ends up being used (after being added to targets).
But in a makefile, if you have:

x: a b

nothing requires make to generate 'a' before or after 'b'.
gmake might have something similar to nmake's .ORDER directive
but I don't remember seeing it defined anywhere.
You can add 'b: a' to force the order (which is how .ORDER
ends up being implemented).
But I didn't spot anything of that nature.

	David

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