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From:   Marco Elver <elver@google.com>
Date:   Wed, 16 Sep 2020 11:33:41 +0200
Message-ID: <CANpmjNPnnkfkRetEHWNwafP43qjbKypsWxLrBVidrzjrTOCFaQ@mail.gmail.com>
Subject: Re: [tip:x86/seves] BUILD SUCCESS WITH WARNING e6eb15c9ba3165698488ae5c34920eea20eaa38e
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>
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On Wed, 16 Sep 2020 at 11:06, <peterz@infradead.org> wrote:
> On Wed, Sep 16, 2020 at 10:46:41AM +0200, Marco Elver wrote:
> > 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(), ...?).
>
> Not only fails, lockdep for example likes to store stack traces of
> various callsites etc.. Also perf (NMI) likes to think it can unwind at
> all times.

That's fair, and I would also prefer a proper fix. :-)

> > 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.
>
> How is that unrealistic? If you build with framepointers enabled, the
> compiler is supposed to know about this stuff.

If it's a bug in current compilers, it'll be hard to get the fix into
those. My suspicion is there's a bug somewhere. We can try to make new
compiler versions do the right thing. Or maybe we're just missing some
flags, which would be nice. I'll investigate some more.

Thanks,
-- Marco