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

Fangrui Song
2020-09-17T04:18:09Z

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@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

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"