linux-kernel - Re: [RFC PATCH 2/2] objtool/LoongArch: Fix unreachable instruction warnings about head.S

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Message-ID: <20250826082057.GE3245006@noisy.programming.kicks-ass.net>
Date: Tue, 26 Aug 2025 10:20:57 +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: [RFC PATCH 2/2] objtool/LoongArch: Fix unreachable instruction
 warnings about head.S

On Tue, Aug 26, 2025 at 02:46:31PM +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.
> 
> Just give a proper unwind hint to silence the above warnings. By the way,
> the previous instructions of kernel_entry+0xf4 and smpboot_entry+0x68 are
> the 'bl' instructions, the call destination symbols are start_kernel() and
> start_secondary() which are noreturn functions, then the 'bl' instructions
> are marked as dead end in annotate_call_site(), so actually ASM_BUG() can
> be removed due to unnecessary, otherwise there are following warnings:
> 
>   kernel_entry+0xf4: start_kernel() missing __noreturn
>   in .c/.h or NORETURN() in noreturns.h
> 
>   smpboot_entry+0x68: start_secondary() missing __noreturn
>   in .c/.h or NORETURN() in noreturns.h
> 
> Link: https://lore.kernel.org/lkml/20250814083651.GR4067720@noisy.programming.kicks-ass.net/
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> ---
>  arch/loongarch/kernel/head.S | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)

At this point you should also be able to remove that Makefile thing,
right?


> diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
> index e3865e92a917..566a1dbf5fa0 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) */
> @@ -30,6 +31,7 @@ _head:
>  	.long	pe_header - _head	/* Offset to the PE header */
>  
>  pe_header:
> +	UNWIND_HINT_UNDEFINED
>  	__EFI_PE_HEADER
>  
>  SYM_DATA(kernel_asize, .long _kernel_asize);
> @@ -42,6 +44,7 @@ SYM_DATA(kernel_fsize, .long _kernel_fsize);
>  	.align 12
>  
>  SYM_CODE_START(kernel_entry)			# kernel entry point
> +	UNWIND_HINT_UNDEFINED
>  
>  	/* Config direct window and set PG */
>  	SETUP_DMWINS	t0
> @@ -109,8 +112,6 @@ SYM_CODE_START(kernel_entry)			# kernel entry point
>  #endif
>  
>  	bl		start_kernel
> -	ASM_BUG()
> -
>  SYM_CODE_END(kernel_entry)
>  
>  #ifdef CONFIG_SMP
> @@ -120,6 +121,7 @@ SYM_CODE_END(kernel_entry)
>   * function after setting up the stack and tp registers.
>   */
>  SYM_CODE_START(smpboot_entry)
> +	UNWIND_HINT_UNDEFINED
>  
>  	SETUP_DMWINS	t0
>  	JUMP_VIRT_ADDR	t0, t1
> @@ -142,8 +144,6 @@ SYM_CODE_START(smpboot_entry)
>  	ld.d		tp, t0, CPU_BOOT_TINFO
>  
>  	bl		start_secondary
> -	ASM_BUG()
> -
>  SYM_CODE_END(smpboot_entry)
>  
>  #endif /* CONFIG_SMP */
> -- 
> 2.42.0
>