On Sat, 23 Jan 2021 18:24:24 +0900 Bongsu Jeon wrote:

From: Bongsu Jeon <bongsu.jeon@samsung.com>

NCI virtual device simulates a NCI device to the user. It can be used to
validate the NCI module and applications. This driver supports
communication between the virtual NCI device and NCI module.

Signed-off-by: Bongsu Jeon <bongsu.jeon@samsung.com>

+static bool virtual_ncidev_check_enabled(void)
+{
+	bool ret = true;
+
+	mutex_lock(&nci_mutex);
+	if (state != virtual_ncidev_enabled)
+		ret = false;
+	mutex_unlock(&nci_mutex);
+
+	return ret;

This can be simplified like:

	bool ret;

	mutex_lock()
	ret = state == virtual_ncidev_enabled;
	mutex_unlock()

	return ret;

+}
+
+static int virtual_nci_open(struct nci_dev *ndev)
+{
+	return 0;
+}
+
+static int virtual_nci_close(struct nci_dev *ndev)
+{
+	mutex_lock(&nci_mutex);
+	if (send_buff)
+		kfree_skb(send_buff);

kfree_skb() handles NULL, no need for the if, you can always call
kfree_skb() here

+	send_buff = NULL;
+	mutex_unlock(&nci_mutex);
+
+	return 0;
+}
+
+static int virtual_nci_send(struct nci_dev *ndev, struct sk_buff *skb)
+{
+	if (virtual_ncidev_check_enabled() == false)
+		return 0;

Shouldn't you check this _under_ the lock below? 

Otherwise there is a small window between check and use of send_buff

+	mutex_lock(&nci_mutex);
+	if (send_buff) {
+		mutex_unlock(&nci_mutex);
+		return -1;
+	}
+	send_buff = skb_copy(skb, GFP_KERNEL);
+	mutex_unlock(&nci_mutex);
+
+	return 0;
+}
+
+static struct nci_ops virtual_nci_ops = {
+	.open = virtual_nci_open,
+	.close = virtual_nci_close,
+	.send = virtual_nci_send
+};
+
+static ssize_t virtual_ncidev_read(struct file *file, char __user *buf,
+				   size_t count, loff_t *ppos)
+{
+	size_t actual_len;
+
+	mutex_lock(&nci_mutex);
+	if (!send_buff) {
+		mutex_unlock(&nci_mutex);
+		return 0;
+	}
+
+	actual_len = min_t(size_t, count, send_buff->len);
+
+	if (copy_to_user(buf, send_buff->data, actual_len)) {
+		mutex_unlock(&nci_mutex);
+		return -EFAULT;
+	}
+
+	skb_pull(send_buff, actual_len);
+	if (send_buff->len == 0) {
+		consume_skb(send_buff);
+		send_buff = NULL;
+	}
+	mutex_unlock(&nci_mutex);
+
+	return actual_len;
+}
+
+static ssize_t virtual_ncidev_write(struct file *file,
+				    const char __user *buf,
+				    size_t count, loff_t *ppos)
+{
+	struct sk_buff *skb;
+
+	skb = alloc_skb(count, GFP_KERNEL);
+	if (!skb)
+		return -ENOMEM;
+
+	if (copy_from_user(skb_put(skb, count), buf, count)) {
+		kfree_skb(skb);
+		return -EFAULT;
+	}
+
+	nci_recv_frame(ndev, skb);
+	return count;
+}
+
+static int virtual_ncidev_open(struct inode *inode, struct file *file)
+{
+	int ret = 0;
+
+	mutex_lock(&nci_mutex);
+	if (state != virtual_ncidev_disabled) {
+		mutex_unlock(&nci_mutex);
+		return -EBUSY;
+	}
+
+	ndev = nci_allocate_device(&virtual_nci_ops, VIRTUAL_NFC_PROTOCOLS,
+				   0, 0);
+	if (!ndev) {
+		mutex_unlock(&nci_mutex);
+		return -ENOMEM;
+	}
+
+	ret = nci_register_device(ndev);
+	if (ret < 0) {
+		nci_free_device(ndev);
+		mutex_unlock(&nci_mutex);
+		return ret;
+	}
+	state = virtual_ncidev_enabled;
+	mutex_unlock(&nci_mutex);
+
+	return 0;
+}
+
+static int virtual_ncidev_close(struct inode *inode, struct file *file)
+{
+	mutex_lock(&nci_mutex);
+
+	if (state == virtual_ncidev_enabled) {
+		state = virtual_ncidev_disabling;
+		mutex_unlock(&nci_mutex);
+
+		nci_unregister_device(ndev);
+		nci_free_device(ndev);
+
+		mutex_lock(&nci_mutex);
+	}
+
+	state = virtual_ncidev_disabled;
+	mutex_unlock(&nci_mutex);
+
+	return 0;
+}
+
+static long virtual_ncidev_ioctl(struct file *flip, unsigned int cmd,
+				 unsigned long arg)
+{
+	if (cmd == IOCTL_GET_NCIDEV_IDX) {
+		struct nfc_dev *nfc_dev = ndev->nfc_dev;
+		void __user *p = (void __user *)arg;
+
+		if (copy_to_user(p, &nfc_dev->idx, sizeof(nfc_dev->idx)))
+			return -EFAULT;
+	} else {
+		return -ENOTTY;
+	}
+
+	return 0;

Please flip the condition and return early. I think I suggested this
already:

{
	struct nfc_dev *nfc_dev = ndev->nfc_dev;
	void __user *p = (void __user *)arg;		

	if (cmd != IOCTL_GET_NCIDEV_IDX)
		return -ENOTTY;

	if (copy_to_user(p, &nfc_dev->idx, sizeof(nfc_dev->idx)))
		return -EFAULT;

	return 0;