linux-kernel - Re: [PATCH v1 2/3] objtool/LoongArch: Fix unreachable instruction warnings about EFISTUB

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Message-ID: <20250901081904.GB4067720@noisy.programming.kicks-ass.net>
Date: Mon, 1 Sep 2025 10:19:04 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Huacai Chen <chenhuacai@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>, loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/3] objtool/LoongArch: Fix unreachable instruction
 warnings about EFISTUB

On Mon, Sep 01, 2025 at 03:21:55PM +0800, Tiezhu Yang 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
> 
> The instructions in the .head.text section are related with EFISTUB,
> they are image header and can be ignored by objtool, so just check the
> section name in ignore_unreachable_insn() to ignore it.

I am confused; why do the efi/libstub functions generate this error?

Is this zboot-header.S perhaps? Why can't we properly annotate that
file?