x86/xen: avoid freeing static 'name' when kasprintf() fails

In case kasprintf() fails in xen_setup_timer() we assign name to the
static string "<timer kasprintf failed>". We, however, don't check
that fact before issuing kfree() in xen_teardown_timer(), kernel is
supposed to crash with 'kernel BUG at mm/slub.c:3341!'

Solve the issue by making name a fixed length string inside struct
xen_clock_event_device. 16 bytes should be enough.
Suggested-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NVitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: NDavid Vrabel <david.vrabel@citrix.com>

arch/x86/xen/time.c

@@ -391,7 +391,7 @@ static const struct clock_event_device *xen_clockevent =
 
 struct xen_clock_event_device {
 	struct clock_event_device evt;
-	char *name;
+	char name[16];
 };
 static DEFINE_PER_CPU(struct xen_clock_event_device, xen_clock_events) = { .evt.irq = -1 };
 
@@ -420,39 +420,33 @@ void xen_teardown_timer(int cpu)
 	if (evt->irq >= 0) {
 		unbind_from_irqhandler(evt->irq, NULL);
 		evt->irq = -1;
-		kfree(per_cpu(xen_clock_events, cpu).name);
-		per_cpu(xen_clock_events, cpu).name = NULL;
 	}
 }
 
 void xen_setup_timer(int cpu)
 {
-	char *name;
-	struct clock_event_device *evt;
+	struct xen_clock_event_device *xevt = &per_cpu(xen_clock_events, cpu);
+	struct clock_event_device *evt = &xevt->evt;
 	int irq;
 
-	evt = &per_cpu(xen_clock_events, cpu).evt;
 	WARN(evt->irq >= 0, "IRQ%d for CPU%d is already allocated\n", evt->irq, cpu);
 	if (evt->irq >= 0)
 		xen_teardown_timer(cpu);
 
 	printk(KERN_INFO "installing Xen timer for CPU %d\n", cpu);
 
-	name = kasprintf(GFP_KERNEL, "timer%d", cpu);
-	if (!name)
-		name = "<timer kasprintf failed>";
+	snprintf(xevt->name, sizeof(xevt->name), "timer%d", cpu);
 
 	irq = bind_virq_to_irqhandler(VIRQ_TIMER, cpu, xen_timer_interrupt,
 				      IRQF_PERCPU|IRQF_NOBALANCING|IRQF_TIMER|
 				      IRQF_FORCE_RESUME|IRQF_EARLY_RESUME,
-				      name, NULL);
+				      xevt->name, NULL);
 	(void)xen_set_irq_priority(irq, XEN_IRQ_PRIORITY_MAX);
 
 	memcpy(evt, xen_clockevent, sizeof(*evt));
 
 	evt->cpumask = cpumask_of(cpu);
 	evt->irq = irq;
-	per_cpu(xen_clock_events, cpu).name = name;
 }