[QUESTION] How to silence objtool warnings of vmlinux.o

[QUESTION] How to silence objtool warnings of vmlinux.o

[Tiezhu Yang]

When compiling with LLVM and CONFIG_LTO_CLANG is set, there exist the
following objtool warnings after silencing all of the other warnings:

   LD      vmlinux.o
vmlinux.o: warning: objtool: .head.text+0x0: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x18: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x38: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x3c: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x40: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x44: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x54: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x58: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x6c: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x84: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x94: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x9c: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0xc4: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0xf8: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0xfc: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x104: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x10c: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x11c: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x120: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x124: unreachable instruction
vmlinux.o: warning: objtool: .head.text+0x144: unreachable instruction
vmlinux.o: warning: objtool: kernel_entry+0x0: unreachable instruction
vmlinux.o: warning: objtool: smpboot_entry+0x0: unreachable instruction

All of the above instructions are in arch/loongarch/kernel/head.S,
and there is "OBJECT_FILES_NON_STANDARD_head.o := y" in Makefile
to skip objtool checking for head.o, but OBJECT_FILES_NON_STANDARD
does not work for link time validation of vmlinux.o according to
tools/objtool/Documentation/objtool.txt.

How to silence this kind of objtool warnings?
I appreciate any guidance here.

Thanks,
Tiezhu

Re: [QUESTION] How to silence objtool warnings of vmlinux.o

[Peter Zijlstra]

On Thu, Aug 14, 2025 at 10:38:07AM +0800, Tiezhu Yang wrote:
> When compiling with LLVM and CONFIG_LTO_CLANG is set, there exist the
> following objtool warnings after silencing all of the other warnings:
> 
>   LD      vmlinux.o
> vmlinux.o: warning: objtool: .head.text+0x0: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x18: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x38: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x3c: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x40: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x44: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x54: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x58: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x6c: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x84: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x94: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x9c: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0xc4: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0xf8: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0xfc: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x104: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x10c: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x11c: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x120: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x124: unreachable instruction
> vmlinux.o: warning: objtool: .head.text+0x144: unreachable instruction
> vmlinux.o: warning: objtool: kernel_entry+0x0: unreachable instruction
> vmlinux.o: warning: objtool: smpboot_entry+0x0: unreachable instruction
> 
> All of the above instructions are in arch/loongarch/kernel/head.S,
> and there is "OBJECT_FILES_NON_STANDARD_head.o := y" in Makefile
> to skip objtool checking for head.o, but OBJECT_FILES_NON_STANDARD
> does not work for link time validation of vmlinux.o according to
> tools/objtool/Documentation/objtool.txt.

Correct. OBJECT_FILES_NON_STANDARD shouldn't really be used for anything
that ends up in vmlinux.o.

> How to silence this kind of objtool warnings?
> I appreciate any guidance here.

I think for x86 we just annotated it all. Specifically, those
unreachables appear to be due to non function code symbols. You need to
give them a proper UNWIND hint.