x86/debug: Move kprobe_debug_handler() into exc_debug_kernel()

Kprobes are on kernel text, and thus only matter for #DB-from-kernel.
Kprobes are ordered before the generic notifier, preserve that order.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20200902133200.847465360@infradead.org

Author: Peter Zijlstra

arch/x86/include/asm/kprobes.h

@@ -106,5 +106,9 @@ extern int kprobe_exceptions_notify(struct notifier_block *self,
 extern int kprobe_int3_handler(struct pt_regs *regs);
 extern int kprobe_debug_handler(struct pt_regs *regs);
 
+#else
+
+static inline int kprobe_debug_handler(struct pt_regs *regs) { return 0; }
+
 #endif /* CONFIG_KPROBES */
 #endif /* _ASM_X86_KPROBES_H */

arch/x86/kernel/traps.c

@@ -806,12 +806,6 @@ static void handle_debug(struct pt_regs *regs, unsigned long dr6, bool user)
 	/* Store the virtualized DR6 value */
 	tsk->thread.debugreg6 = dr6;
 
-#ifdef CONFIG_KPROBES
-	if (kprobe_debug_handler(regs)) {
-		return;
-	}
-#endif
-
 	if (notify_die(DIE_DEBUG, "debug", regs, (long)&dr6, 0,
 		       SIGTRAP) == NOTIFY_STOP) {
 		return;
@@ -877,8 +871,12 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs,
 	if ((dr6 & DR_STEP) && is_sysenter_singlestep(regs))
 		dr6 &= ~DR_STEP;
 
+	if (kprobe_debug_handler(regs))
+		goto out;
+
 	handle_debug(regs, dr6, false);
 
+out:
 	instrumentation_end();
 	idtentry_exit_nmi(regs, irq_state);