On Wed, 16 Sep 2020 at 20:22, 'Nick Desaulniers' via kasan-dev <kasan-...@googlegroups.com> wrote: > > On Wed, Sep 16, 2020 at 1:46 AM Marco Elver <el...@google.com> wrote: > > > > On Wed, 16 Sep 2020 at 10:30, <pet...@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 > > > > <ndesaulni...@google.com> wrote: > > > > > On Tue, Sep 15, 2020 at 10:21 AM Borislav Petkov <b...@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
