linux-kernel - Re: [PATCH v2 3/4] objtool/LoongArch: Fix unreachable instruction warnings about entry points

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Message-ID: <CAAhV-H6yKyqU+jQ=-RoOOc0fyRgjFfdorJAk1LashV0Gt=Y=AQ@mail.gmail.com>
Date: Wed, 17 Sep 2025 15:07:17 +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 3/4] objtool/LoongArch: Fix unreachable instruction
 warnings about entry points

Hi, Tiezhu,

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: kernel_entry+0x0: unreachable instruction
>   vmlinux.o: warning: objtool: smpboot_entry+0x0: unreachable instruction
>
> kernel_entry() and smpboot_entry() are in arch/loongarch/kernel/head.S,
> there is "OBJECT_FILES_NON_STANDARD_head.o := y" to skip objtool checking
> for head.o, but the STACK_FRAME_NON_STANDARD macro does not work for link
> time validation of vmlinux.o according to objtool documentation, just give
> a proper unwind hint to fix the warnings.
>
> By the way, 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
>
> This is because the previous instructions of kernel_entry+0xf4 and
> smpboot_entry+0x68 are the 'bl' instructions, start_kernel() and
> start_secondary() are the respective call destination symbols which
> are noreturn functions, then the 'bl' instructions are already marked
> as dead end in annotate_call_site().
>
> For now, it is time to remove "OBJECT_FILES_NON_STANDARD_head.o := y"
> in arch/loongarch/kernel/Makefile.
>
> 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/Makefile | 2 --
>  arch/loongarch/kernel/head.S   | 6 ++----
>  2 files changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/arch/loongarch/kernel/Makefile b/arch/loongarch/kernel/Makefile
> index 6f5a4574a911..4302c5b0a201 100644
> --- a/arch/loongarch/kernel/Makefile
> +++ b/arch/loongarch/kernel/Makefile
> @@ -3,8 +3,6 @@
>  # Makefile for the Linux/LoongArch kernel.
>  #
>
> -OBJECT_FILES_NON_STANDARD_head.o := y
This line should be kept, othewise we get:

arch/loongarch/kernel/head.o: warning: objtool: kernel_entry+0xf4:
start_kernel() missing __noreturn in .c/.h or NORETURN() in
noreturns.h

even without LTO. This is a regression, we can only remove it after
the above warning be fixed.

Huacai
> -
>  always-$(KBUILD_BUILTIN)       := vmlinux.lds
>
>  obj-y          += head.o cpu-probe.o cacheinfo.o env.o setup.o entry.o genex.o \
> diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
> index 6ce999586757..c62dab32a06b 100644
> --- a/arch/loongarch/kernel/head.S
> +++ b/arch/loongarch/kernel/head.S
> @@ -43,6 +43,7 @@ SYM_DATA(kernel_fsize, .long _kernel_fsize);
>         .align 12
>
>  SYM_CODE_START(kernel_entry)                   # kernel entry point
> +       UNWIND_HINT_END_OF_STACK
>
>         /* Config direct window and set PG */
>         SETUP_DMWINS    t0
> @@ -110,8 +111,6 @@ SYM_CODE_START(kernel_entry)                        # kernel entry point
>  #endif
>
>         bl              start_kernel
> -       ASM_BUG()
> -
>  SYM_CODE_END(kernel_entry)
>
>  #ifdef CONFIG_SMP
> @@ -121,6 +120,7 @@ SYM_CODE_END(kernel_entry)
>   * function after setting up the stack and tp registers.
>   */
>  SYM_CODE_START(smpboot_entry)
> +       UNWIND_HINT_END_OF_STACK
>
>         SETUP_DMWINS    t0
>         JUMP_VIRT_ADDR  t0, t1
> @@ -143,8 +143,6 @@ 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
>