x86/debug: Move historical SYSENTER junk into exc_debug_kernel()

The historical SYSENTER junk is explicitly for from-kernel, so move it
to the #DB-from-kernel handler.

It is ordered after the notifier, which is important for KGDB which uses TF
single-step and needs to consume the event before that point.

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>
Link: https://lore.kernel.org/r/20200902133201.031099736@infradead.org

Author: Peter Zijlstra

arch/x86/kernel/traps.c

@@ -783,7 +783,7 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
  *
  * May run on IST stack.
  */
-static void handle_debug(struct pt_regs *regs, unsigned long dr6)
+static bool handle_debug(struct pt_regs *regs, unsigned long *dr6)
 {
 	struct task_struct *tsk = current;
 	bool icebp;
@@ -793,15 +793,13 @@ static void handle_debug(struct pt_regs *regs, unsigned long dr6)
 	 * then it's very likely the result of an icebp/int01 trap.
 	 * User wants a sigtrap for that.
 	 */
-	icebp = !dr6;
+	icebp = !*dr6;
 
 	/* Store the virtualized DR6 value */
-	tsk->thread.debugreg6 = dr6;
+	tsk->thread.debugreg6 = *dr6;
 
-	if (notify_die(DIE_DEBUG, "debug", regs, (long)&dr6, 0,
-		       SIGTRAP) == NOTIFY_STOP) {
-		return;
-	}
+	if (notify_die(DIE_DEBUG, "debug", regs, (long)dr6, 0, SIGTRAP) == NOTIFY_STOP)
+		return true;
 
 	/* It's safe to allow irq's after DR6 has been saved */
 	cond_local_irq_enable(regs);
@@ -815,25 +813,15 @@ static void handle_debug(struct pt_regs *regs, unsigned long dr6)
 	/*
 	 * Reload dr6, the notifier might have changed it.
 	 */
-	dr6 = tsk->thread.debugreg6;
+	*dr6 = tsk->thread.debugreg6;
 
-	if (WARN_ON_ONCE((dr6 & DR_STEP) && !user_mode(regs))) {
-		/*
-		 * Historical junk that used to handle SYSENTER single-stepping.
-		 * This should be unreachable now.  If we survive for a while
-		 * without anyone hitting this warning, we'll turn this into
-		 * an oops.
-		 */
-		tsk->thread.debugreg6 &= ~DR_STEP;
-		set_tsk_thread_flag(tsk, TIF_SINGLESTEP);
-		regs->flags &= ~X86_EFLAGS_TF;
-	}
-
-	if (dr6 & (DR_STEP | DR_TRAP_BITS) || icebp)
-		send_sigtrap(regs, 0, get_si_code(dr6));
+	if (*dr6 & (DR_STEP | DR_TRAP_BITS) || icebp)
+		send_sigtrap(regs, 0, get_si_code(*dr6));
 
 out:
 	cond_local_irq_disable(regs);
+
+	return false;
 }
 
 static __always_inline void exc_debug_kernel(struct pt_regs *regs,
@@ -876,7 +864,20 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs,
 	if (!dr6)
 		goto out;
 
-	handle_debug(regs, dr6);
+	if (handle_debug(regs, &dr6))
+		goto out;
+
+	if (WARN_ON_ONCE(dr6 & DR_STEP)) {
+		/*
+		 * Historical junk that used to handle SYSENTER single-stepping.
+		 * This should be unreachable now.  If we survive for a while
+		 * without anyone hitting this warning, we'll turn this into
+		 * an oops.
+		 */
+		dr6 &= ~DR_STEP;
+		set_thread_flag(TIF_SINGLESTEP);
+		regs->flags &= ~X86_EFLAGS_TF;
+	}
 
 out:
 	instrumentation_end();
@@ -906,7 +907,7 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
 	irqentry_enter_from_user_mode(regs);
 	instrumentation_begin();
 
-	handle_debug(regs, dr6);
+	handle_debug(regs, &dr6);
 
 	instrumentation_end();
 	irqentry_exit_to_user_mode(regs);