From: Marco Elver <elver@google.com>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Borislav Petkov <bp@alien8.de>, 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>,
Daniel Kiss <daniel.kiss@arm.com>,
momchil.velikov@arm.com, Mark Rutland <mark.rutland@arm.com>
Subject: Re: [tip:x86/seves] BUILD SUCCESS WITH WARNING e6eb15c9ba3165698488ae5c34920eea20eaa38e
Date: Wed, 16 Sep 2020 20:51:34 +0200 [thread overview]
Message-ID: <CANpmjNPGZnwJVN6ZuBiRUocGPp8c3rnx1v7iGfYna9t8c3ty0w@mail.gmail.com> (raw)
In-Reply-To: <CAKwvOd=T3w1eqwBkpa8_dJjbOLMTTDshfevT3EuQD4aNn4e_ZQ@mail.gmail.com>
On Wed, 16 Sep 2020 at 20:22, 'Nick Desaulniers' via kasan-dev
<kasan-dev@googlegroups.com> wrote:
>
> On Wed, Sep 16, 2020 at 1:46 AM Marco Elver <elver@google.com> 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(), ...?).
> >
> > 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
next prev parent reply other threads:[~2020-09-16 18:52 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-15 13:42 [tip:x86/seves] BUILD SUCCESS WITH WARNING e6eb15c9ba3165698488ae5c34920eea20eaa38e kernel test robot
2020-09-15 13:55 ` Borislav Petkov
2020-09-15 14:18 ` Rong Chen
2020-09-15 14:41 ` Borislav Petkov
2020-09-15 16:05 ` Borislav Petkov
2020-09-15 17:02 ` Josh Poimboeuf
2020-09-15 17:21 ` Borislav Petkov
2020-09-15 17:34 ` Borislav Petkov
2020-09-15 17:41 ` Nick Desaulniers
2020-09-15 18:01 ` Borislav Petkov
2020-09-15 18:04 ` Nick Desaulniers
2020-09-15 17:40 ` Nick Desaulniers
2020-09-15 18:09 ` Marco Elver
2020-09-16 8:30 ` peterz
2020-09-16 8:46 ` Marco Elver
2020-09-16 9:06 ` peterz
2020-09-16 9:33 ` Marco Elver
2020-09-16 18:22 ` Nick Desaulniers
2020-09-16 18:51 ` Marco Elver [this message]
2020-09-17 4:11 ` Fangrui Song
[not found] ` <333D40A0-4550-4309-9693-1ABA4AC75399@arm.com>
2020-09-17 11:04 ` Mark Rutland
2020-09-17 11:16 ` Daniel Kiss
2020-09-17 18:39 ` Josh Poimboeuf
2020-09-15 17:44 ` Nick Desaulniers
2020-09-15 20:12 ` Nick Desaulniers
2020-09-15 20:49 ` Borislav Petkov
2020-09-15 21:02 ` Josh Poimboeuf
2020-09-15 21:14 ` Borislav Petkov
2020-09-15 22:34 ` Nick Desaulniers
2020-09-16 7:03 ` Ilie Halip
2020-09-16 8:59 ` Marco Elver
2020-09-21 16:51 ` [tip: objtool/core] objtool: Ignore unreachable trap after call to noreturn functions tip-bot2 for Ilie Halip
2020-09-15 21:50 ` [tip:x86/seves] BUILD SUCCESS WITH WARNING e6eb15c9ba3165698488ae5c34920eea20eaa38e Arvind Sankar
2020-09-15 21:59 ` Nick Desaulniers
2020-09-15 22:44 ` Arvind Sankar
2020-09-16 11:34 ` Borislav Petkov
2020-09-16 18:28 ` Nick Desaulniers
2020-09-16 18:48 ` Borislav Petkov
2020-09-15 21:13 ` Nick Desaulniers
2020-09-15 21:28 ` Josh Poimboeuf
2020-09-15 23:35 ` Marco Elver
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