linux-kernel - Re: [PATCH v2 2/4] objtool/LoongArch: Fix unreachable instruction warnings about image header

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Message-ID: <CAAhV-H6L6nSuDxn_8cpexmSdLjO6DLLG0+9faPdhuD9iFMfdVw@mail.gmail.com>
Date: Wed, 17 Sep 2025 15:04:04 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, Peter Zijlstra <peterz@...radead.org>, loongarch@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/4] objtool/LoongArch: Fix unreachable instruction
 warnings about image header

Hi, Tiezhu,

Patch-2 and Patch-3 can be squashed.

Huacai

On Wed, Sep 17, 2025 at 9:10 AM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>
> When compiling with LLVM and CONFIG_LTO_CLANG is set, there exist the
> following objtool warnings:
>
>   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
>
> 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.
>
> After many discussions, it is not proper to ignore .head.text section
> in objtool or put them from text section to data section, so just give
> a unwind hint to fix the above warnings.
>
> Link: https://lore.kernel.org/lkml/20250814083651.GR4067720@noisy.programming.kicks-ass.net/
> Link: https://lore.kernel.org/lkml/CAAhV-H6A_swQmqpWHp6ryAEvc96CAMOMd2ZGyJEVNMsJfLkz6w@mail.gmail.com/
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Suggested-by: Huacai Chen <chenhuacai@...nel.org>
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> ---
>  arch/loongarch/kernel/head.S | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
> index e3865e92a917..6ce999586757 100644
> --- a/arch/loongarch/kernel/head.S
> +++ b/arch/loongarch/kernel/head.S
> @@ -20,6 +20,7 @@
>         __HEAD
>
>  _head:
> +       UNWIND_HINT_UNDEFINED
>         .word   IMAGE_DOS_SIGNATURE     /* "MZ", MS-DOS header */
>         .org    0x8
>         .dword  _kernel_entry           /* Kernel entry point (physical address) */
> --
> 2.42.0
>