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

[Tiezhu Yang]

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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
Link: https://lore.kernel.org/lkml/CAAhV-H6A_swQmqpWHp6ryAEvc96CAMOMd2ZGyJEVNMsJfLkz6w@xxxxxxxxxxxxxx/
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Suggested-by: Huacai Chen <chenhuacai@xxxxxxxxxx>
Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
---
 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