x86/extable: Annotate ex_handler_msr_mce() as a dead end

Fix

  vmlinux.o: warning: objtool: fixup_exception+0x2d6: unreachable instruction
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220520192729.23969-1-bp@alien8.de

arch/x86/include/asm/extable.h

@@ -42,9 +42,13 @@ extern int ex_get_fixup_type(unsigned long ip);
 extern void early_fixup_exception(struct pt_regs *regs, int trapnr);
 
 #ifdef CONFIG_X86_MCE
-extern void ex_handler_msr_mce(struct pt_regs *regs, bool wrmsr);
+extern void __noreturn ex_handler_msr_mce(struct pt_regs *regs, bool wrmsr);
 #else
-static inline void ex_handler_msr_mce(struct pt_regs *regs, bool wrmsr) { }
+static inline void __noreturn ex_handler_msr_mce(struct pt_regs *regs, bool wrmsr)
+{
+	for (;;)
+		cpu_relax();
+}
 #endif
 
 #if defined(CONFIG_BPF_JIT) && defined(CONFIG_X86_64)

tools/objtool/check.c

@@ -187,6 +187,7 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
 		"__invalid_creds",
 		"cpu_startup_entry",
 		"__ubsan_handle_builtin_unreachable",
+		"ex_handler_msr_mce",
 	};
 
 	if (!func)