linux-kernel - Re: Kernel build with gcc 8 a lot of warnings

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Date:   Sun, 6 May 2018 23:54:53 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     damian <damian.tometzki@...oud.com>
Cc:     peterz@...radead.org, linux-kernel@...r.kernel.org
Subject: Re: Kernel build with gcc 8 a lot of warnings

On Sat, May 05, 2018 at 09:21:12PM +0200, damian wrote:
> Helllo together,
> 
> Hello everybody,
> 
> is something special to note when kernel build with gcc 8? I receive various warnings from the objtool:
> With GCC 7 works all fine.
> 
> kernel/cgroup/cgroup.o: warning: objtool: parse_cgroup_root_flags()+0x40: sibling call from callable instruction with modified stack frame
> kernel/cgroup/cgroup.o: warning: objtool: cgroup_addrm_files()+0x2a3: sibling call from callable instruction with modified stack frame
> kernel/cgroup/cgroup.o: warning: objtool: cgroup_apply_control_enable()+0x20b: sibling call from callable instruction with modified stack frame
> kernel/cgroup/cgroup.o: warning: objtool: rebind_subsystems()+0x296: sibling call from callable instruction with modified stack frame
> kernel/cgroup/cgroup.o: warning: objtool: cgroup_sk_alloc_disable()+0x10: sibling call from callable instruction with modified stack frame
> kernel/cgroup/cgroup.o: warning: objtool: parse_cgroup_root_flags.cold.45()+0xa: call without frame pointer save/setup
> kernel/cgroup/cgroup.o: warning: objtool: cgroup_addrm_files.cold.46()+0x17: call without frame pointer save/setup
> kernel/cgroup/cgroup.o: warning: objtool: cgroup_apply_control_enable.cold.47()+0x24: call without frame pointer save/setup
> kernel/cgroup/cgroup.o: warning: objtool: rebind_subsystems.cold.48()+0x10: call without frame pointer save/setup
> kernel/cgroup/cgroup.o: warning: objtool: cgroup_sk_alloc_disable.cold.49()+0x7: call without frame pointer save/setup
>   CC      kernel/cgroup/stat.o
>   CC [M]  arch/x86/kvm/../../../virt/kvm/kvm_main.o
> arch/x86/kernel/acpi/boot.o: warning: objtool: acpi_register_lapic()+0x10: sibling call from callable instruction with modified stack frame
> arch/x86/kernel/acpi/boot.o: warning: objtool: acpi_map_cpu()+0x2c: sibling call from callable instruction with modified stack frame
> arch/x86/kernel/acpi/boot.o: warning: objtool: acpi_register_lapic.cold.6()+0x7: call without frame pointer save/setup
> arch/x86/kernel/acpi/boot.o: warning: objtool: acpi_map_cpu.cold.7()+0x7: call without frame pointer save/setup

Thanks, this is a known issue with GCC 8 that I haven't gotten a chance
to fix yet.  They're related to the fact that GCC 8 is splitting out
'unlikely' code into 'cold' subfunctions in .text.unlikely.

I've got some old patches that fix it.  Now that GCC 8 is out, I'll need
get the patches dusted off and cleaned up for merging.

-- 
Josh