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

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Message-ID: <20250826064631.9617-3-yangtiezhu@loongson.cn>
Date: Tue, 26 Aug 2025 14:46:31 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nathan Chancellor <nathan@...nel.org>
Cc: loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: [RFC PATCH 2/2] objtool/LoongArch: Fix unreachable instruction warnings about head.S

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(-)

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