Re: objtool warnings for kernel/trace/trace_selftest_dynamic.o

[Josh Poimboeuf]

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
__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@xxxxxxxxxxxxx

Andi,

is __noclone really needed here, since the functions aren't static?  Or
does LTO cause them to be treated like static functions?

Or if it is really needed, would __used be sufficient instead?

noinline __noclone int DYN_FTRACE_TEST_NAME(void)
{
	/* used to call mcount */
	return 0;
}

noinline __noclone int DYN_FTRACE_TEST_NAME2(void)
{
	/* used to call mcount */
	return 0;
}

-- 
Josh