x86/boot: Mark start_secondary() with __noendbr

The handoff between the boot stubs and start_secondary() are before IBT is
enabled and is definitely not subject to kCFI. As such, suppress all that for
this function.

Notably when the ENDBR poison would become fatal (ud1 instead of nop) this will
trigger a tripple fault because we haven't set up the IDT to handle #UD yet.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20250207122546.509520369@infradead.org

Author: Peter Zijlstra

arch/x86/kernel/smpboot.c

@@ -229,7 +229,7 @@ static void ap_calibrate_delay(void)
 /*
  * Activate a secondary processor.
  */
-static void notrace start_secondary(void *unused)
+static void notrace __noendbr start_secondary(void *unused)
 {
 	/*
 	 * Don't put *anything* except direct CPU state initialization
@@ -314,6 +314,7 @@ static void notrace start_secondary(void *unused)
 	wmb();
 	cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
 }
+ANNOTATE_NOENDBR_SYM(start_secondary);
 
 /*
  * The bootstrap kernel entry code has set these up. Save them for

include/linux/objtool.h

@@ -128,7 +128,7 @@
 #define UNWIND_HINT(type, sp_reg, sp_offset, signal) "\n\t"
 #define STACK_FRAME_NON_STANDARD(func)
 #define STACK_FRAME_NON_STANDARD_FP(func)
-#define __ASM_ANNOTATE(label, type)
+#define __ASM_ANNOTATE(label, type) ""
 #define ASM_ANNOTATE(type)
 #else
 .macro UNWIND_HINT type:req sp_reg=0 sp_offset=0 signal=0
@@ -147,6 +147,8 @@
  * these relocations will never be used for indirect calls.
  */
 #define ANNOTATE_NOENDBR		ASM_ANNOTATE(ANNOTYPE_NOENDBR)
+#define ANNOTATE_NOENDBR_SYM(sym)	asm(__ASM_ANNOTATE(sym, ANNOTYPE_NOENDBR))
+
 /*
  * This should be used immediately before an indirect jump/call. It tells
  * objtool the subsequent indirect jump/call is vouched safe for retpoline