uio: ensure class is registered before devices

When both uio and the uio drivers are built in the kernel, it is possible
for a driver to register devices before the uio class is registered.

This may result in a NULL pointer dereference later on in
get_device_parent() when accessing the class glue_dirs spinlock.

The trace looks like that:

Unable to handle kernel NULL pointer dereference at virtual address 00000140
[...]
[<ffff0000089cc234>] _raw_spin_lock+0x14/0x48
[<ffff0000084f56bc>] device_add+0x154/0x6a0
[<ffff0000084f5e48>] device_create_groups_vargs+0x120/0x128
[<ffff0000084f5edc>] device_create+0x54/0x60
[<ffff0000086e72c0>] __uio_register_device+0x120/0x4a8
[<ffff000008528b7c>] jaguar2_pci_probe+0x2d4/0x558
[<ffff0000083fc18c>] local_pci_probe+0x3c/0xb8
[<ffff0000083fd81c>] pci_device_probe+0x11c/0x180
[<ffff0000084f88bc>] driver_probe_device+0x22c/0x2d8
[<ffff0000084f8a24>] __driver_attach+0xbc/0xc0
[<ffff0000084f69fc>] bus_for_each_dev+0x4c/0x98
[<ffff0000084f81b8>] driver_attach+0x20/0x28
[<ffff0000084f7d08>] bus_add_driver+0x1b8/0x228
[<ffff0000084f93c0>] driver_register+0x60/0xf8
[<ffff0000083fb918>] __pci_register_driver+0x40/0x48

Return EPROBE_DEFER in that case so the driver can register the device
later.
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/uio/uio.c

@@ -274,6 +274,8 @@ static struct class uio_class = {
 	.dev_groups = uio_groups,
 };
 
+bool uio_class_registered;
+
 /*
  * device functions
  */
@@ -876,6 +878,9 @@ static int init_uio_class(void)
 		printk(KERN_ERR "class_register failed for uio\n");
 		goto err_class_register;
 	}
+
+	uio_class_registered = true;
+
 	return 0;
 
 err_class_register:
@@ -886,6 +891,7 @@ static int init_uio_class(void)
 
 static void release_uio_class(void)
 {
+	uio_class_registered = false;
 	class_unregister(&uio_class);
 	uio_major_cleanup();
 }
@@ -912,6 +918,9 @@ int __uio_register_device(struct module *owner,
 	struct uio_device *idev;
 	int ret = 0;
 
+	if (!uio_class_registered)
+		return -EPROBE_DEFER;
+
 	if (!parent || !info || !info->name || !info->version)
 		return -EINVAL;