Messages in this thread | | | Date | Wed, 16 Sep 2020 21:11:50 -0700 | From | Fangrui Song <> | Subject | Re: [tip:x86/seves] BUILD SUCCESS WITH WARNING e6eb15c9ba3165698488ae5c34920eea20eaa38e |
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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"
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