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

[Fangrui Song]

On 2020-09-16, 'Marco Elver' via Clang Built Linux wrote:
> On Wed, 16 Sep 2020 at 20:22, 'Nick Desaulniers' via kasan-dev
> <kasan-dev@xxxxxxxxxxxxxxxx> wrote:
> > On Wed, Sep 16, 2020 at 1:46 AM Marco Elver <elver@xxxxxxxxxx> wrote:
> > >
> > > On Wed, 16 Sep 2020 at 10:30, <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxx> wrote:
> > > > > > On Tue, Sep 15, 2020 at 10:21 AM Borislav Petkov <bp@xxxxxxxxx> 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

Speaking of gcov, do people know whether frame pointers in
kernel's libgcov implementation help?

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94394 "random kernel panic during collecting kernel code coverage"