Date:   Wed, 16 Sep 2020 20:51:34 +0200
From:   Marco Elver <elver@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        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>,
        Daniel Kiss <daniel.kiss@....com>, momchil.velikov@....com,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [tip:x86/seves] BUILD SUCCESS WITH WARNING e6eb15c9ba3165698488ae5c34920eea20eaa38e

On Wed, 16 Sep 2020 at 20:22, 'Nick Desaulniers' via kasan-dev
<kasan-dev@...glegroups.com> wrote:
>
> On Wed, Sep 16, 2020 at 1:46 AM Marco Elver <elver@...gle.com> wrote:
> >
> > 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.
>
> So this has come up before, specifically in the context of gcov:
> https://github.com/ClangBuiltLinux/linux/issues/955.
>
> I looked into this a bit, and IIRC, the issue was that compiler
> generated functions aren't very good about keeping track of whether
> they should or should not emit framepointer setup/teardown
> prolog/epilogs.  In LLVM's IR, -fno-omit-frame-pointer gets attached
> to every function as a function level attribute.
> https://godbolt.org/z/fcn9c6 ("frame-pointer"="all").
>
> There were some recent LLVM patches for BTI (arm64) that made some BTI
> related command line flags module level attributes, which I thought
> was interesting; I was wondering last night if -fno-omit-frame-pointer
> and maybe even the level of stack protector should be?  I guess LTO
> would complicate things; not sure it would be good to merge modules
> with different attributes; I'm not sure how that's handled today in
> LLVM.
>
> Basically, when the compiler is synthesizing a new function
> definition, it should check whether a frame pointer should be emitted
> or not.  We could do that today by maybe scanning all other function
> definitions for the presence of "frame-pointer"="all" fn attr,
> breaking early if we find one, and emitting the frame pointer setup in
> that case.  Though I guess it's "frame-pointer"="none" otherwise, so
> maybe checking any other fn def would be fine; I don't see any C fn
> attr's that allow you to keep frame pointers or not.  What's tricky is
> that the front end flag was resolved much earlier than where this code
> gets generated, so it would need to look for traces that the flag ever
> existed, which sounds brittle on paper to me.

Thanks for the summary -- yeah, that was my suspicion, that some
attribute was being lost somewhere. And I think if we generalize this,
and don't just try to attach "frame-pointer" attr to the function, we
probably also solve the BTI issue that Mark still pointed out with
these module_ctor/dtors.

I was trying to see if there was a generic way to attach all the
common attributes to the function generated here:
https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/Utils/ModuleUtils.cpp#L122
-- but we probably can't attach all attributes, and need to remove a
bunch of them again like the sanitizers (or alternatively just select
the ones we need). But, I'm still digging for the function that
attaches all the common attributes...

Thanks,
-- Marco

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