linux-kernel - Re: [tip:x86/seves] BUILD SUCCESS WITH WARNING e6eb15c9ba3165698488ae5c34920eea20eaa38e

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Date:   Wed, 16 Sep 2020 10:46:41 +0200
From:   Marco Elver <elver@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Rong Chen <rong.a.chen@...el.com>,
        kernel test robot <lkp@...el.com>,
        "Li, Philip" <philip.li@...el.com>, x86-ml <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [tip:x86/seves] BUILD SUCCESS WITH WARNING e6eb15c9ba3165698488ae5c34920eea20eaa38e

On Wed, 16 Sep 2020 at 10:30, <peterz@...radead.org> wrote:
> On Tue, Sep 15, 2020 at 08:09:16PM +0200, Marco Elver wrote:
> > On Tue, 15 Sep 2020 at 19:40, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> > > On Tue, Sep 15, 2020 at 10:21 AM Borislav Petkov <bp@...en8.de> wrote:
>
> > > > init/calibrate.o: warning: objtool: asan.module_ctor()+0xc: call without frame pointer save/setup
> > > > init/calibrate.o: warning: objtool: asan.module_dtor()+0xc: call without frame pointer save/setup
> > > > init/version.o: warning: objtool: asan.module_ctor()+0xc: call without frame pointer save/setup
> > > > init/version.o: warning: objtool: asan.module_dtor()+0xc: call without frame pointer save/setup
> > > > certs/system_keyring.o: warning: objtool: asan.module_ctor()+0xc: call without frame pointer save/setup
> > > > certs/system_keyring.o: warning: objtool: asan.module_dtor()+0xc: call without frame pointer save/setup
> >
> > This one also appears with Clang 11. This is new I think because we
> > started emitting ASAN ctors for globals redzone initialization.
> >
> > I think we really do not care about precise stack frames in these
> > compiler-generated functions. So, would it be reasonable to make
> > objtool ignore all *san.module_ctor and *san.module_dtor functions (we
> > have them for ASAN, TSAN, MSAN)?
>
> The thing is, if objtool cannot follow, it cannot generate ORC data and
> our unwinder cannot unwind through the instrumentation, and that is a
> fail.
>
> Or am I missing something here?

They aren't about the actual instrumentation. The warnings are about
module_ctor/module_dtor functions which are compiler-generated, and
these are only called on initialization/destruction (dtors only for
modules I guess).

E.g. for KASAN it's the calls to __asan_register_globals that are
called from asan.module_ctor. For KCSAN the tsan.module_ctor is
effectively a noop (because __tsan_init() is a noop), so it really
doesn't matter much.

Is my assumption correct that the only effect would be if something
called by them fails, we just don't see the full stack trace? I think
we can live with that, there are only few central places that deal
with ctors/dtors (do_ctors(), ...?).

The "real" fix would be to teach the compilers about "frame pointer
save/setup" for generated functions, but I don't think that's
realistic.

Thanks,
-- Marco