Messages in this thread | | | Date | Mon, 17 Dec 2018 12:16:38 -0600 | From | Josh Poimboeuf <> | Subject | Re: objtool warnings for kernel/trace/trace_selftest_dynamic.o |
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On Mon, Dec 17, 2018 at 10:04:34AM -0800, Andi Kleen wrote: > On Mon, Dec 17, 2018 at 11:39:00AM -0600, Josh Poimboeuf wrote: > > On Sun, Dec 16, 2018 at 07:33:11PM +0100, Arnd Bergmann wrote: > > > Hi Josh, > > > > > > In randconfig tests with gcc-8.1, I get this warning every > > > few hundred builds, tried it on both next/master and 4.19.y-stable: > > > > > > kernel/trace/trace_selftest_dynamic.o: warning: objtool: > > > trace_selftest_dynamic_test_func()+0x5: call without frame pointer > > > save/setup > > > kernel/trace/trace_selftest_dynamic.o: warning: objtool: > > > trace_selftest_dynamic_test_func2()+0x5: call without frame pointer > > > save/setup > > > > > > $ objdump -dr build/x86/0x90C84554_defconfig/kernel/trace/trace_selftest_dynamic.o > > > > > > build/x86/0x90C84554_defconfig/kernel/trace/trace_selftest_dynamic.o: > > > file format elf64-x86-64 > > > > > > Disassembly of section .text: > > > > > > 0000000000000000 <trace_selftest_dynamic_test_func>: > > > 0: e8 00 00 00 00 callq 5 > > > <trace_selftest_dynamic_test_func+0x5> > > > 1: R_X86_64_PC32 __fentry__-0x4 > > > 5: e8 00 00 00 00 callq a > > > <trace_selftest_dynamic_test_func+0xa> > > > 6: R_X86_64_PC32 __sanitizer_cov_trace_pc-0x4 > > > a: 31 c0 xor %eax,%eax > > > c: c3 retq > > > d: 0f 1f 00 nopl (%rax) > > > > > > 0000000000000010 <trace_selftest_dynamic_test_func2>: > > > 10: e8 00 00 00 00 callq 15 > > > <trace_selftest_dynamic_test_func2+0x5> > > > 11: R_X86_64_PC32 __fentry__-0x4 > > > 15: e8 00 00 00 00 callq 1a > > > <trace_selftest_dynamic_test_func2+0xa> > > > 16: R_X86_64_PC32 __sanitizer_cov_trace_pc-0x4 > > > 1a: 31 c0 xor %eax,%eax > > > 1c: c3 retq > > > > > > I found this reported in > > > http://kisskb.ellerman.id.au/kisskb/buildresult/13499139/, but could > > > not find an existing fix or analysis. > > > > Thanks for reporting this Arnd. > > > > The problem is that, for some reason, __noclone is preventing GCC from > > creating frame pointers for these functions. Miroslav said that > > That seems weird. > > Are you sure it's not just because they are empty? AFAIK > gcc doesn't necessarily generate frame pointers for empty functions.
I suspected that it was because they're empty, however I didn't see this warning for other leaf functions. The sancov plugin is presumably taking care of adding frame pointers where needed. Also, adding -mno-omit-leaf-frame-pointer didn't fix it.
And anyway I confirmed that it was fixed by removing __noclone.
> > __noclone is not recommended by GCC developers, and that __used can be > > used instead for the same purpose: > > > > https://lkml.kernel.org/r/alpine.LSU.2.21.1812171256390.3087@pobox.suse.cz > > > > Andi, > > > > is __noclone really needed here, since the functions aren't static? Or > > does LTO cause them to be treated like static functions? > > Yes LTO causes the to be treated like static functions. > > I guess noclone is unlikely to be really needed here because these > functions are unlikely to be cloned. > > So as a workaround it could be removed. > > But note we have other noclone functions in the tree (like in KVM) > which actually need it.
How about we just use the __used attribute then? It seems to have the same result of preventing IPA optimizations (without the weird side effect of missing frame pointers).
-- Josh
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