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

From: Marco Elver <elver@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Rong Chen <rong.a.chen@intel.com>,
	kernel test robot <lkp@intel.com>,
	"Li, Philip" <philip.li@intel.com>, x86-ml <x86@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Kees Cook <keescook@chromium.org>,
	Masahiro Yamada <masahiroy@kernel.org>,
	kasan-dev <kasan-dev@googlegroups.com>
Subject: Re: [tip:x86/seves] BUILD SUCCESS WITH WARNING e6eb15c9ba3165698488ae5c34920eea20eaa38e
Date: Wed, 16 Sep 2020 10:46:41 +0200	[thread overview]
Message-ID: <CANpmjNOBUp0kRTODJMuSLteE=-woFZ2nUzk1=H8wqcusvi+T_g@mail.gmail.com> (raw)
In-Reply-To: <20200916083032.GL2674@hirez.programming.kicks-ass.net>

On Wed, 16 Sep 2020 at 10:30, <peterz@infradead.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@google.com> wrote:
> > > On Tue, Sep 15, 2020 at 10:21 AM Borislav Petkov <bp@alien8.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