ALSA: fireface: add field for the number of messages copied to user space

Current structure includes no field to express the number of messages
copied to user space, thus user space application needs to information
out of the structure to parse the content of structure.

This commit adds a field to express the number of messages copied to user
space since It is more preferable to use self-contained structure.

Kees Cook proposed an idea of annotation for bound of flexible arrays
in his future improvement for flexible-length array in kernel. The
additional field for message count is suitable to the idea as well.

Reference: https://people.kernel.org/kees/bounded-flexible-arrays-in-c
Signed-off-by: Takashi Sakamoto <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

include/uapi/sound/firewire.h

···
  *					     operating hardware knob.
  *
  * @type: Fixed to SNDRV_FIREWIRE_EVENT_FF400_MESSAGE.
+ * @message_count: The number of messages.
  * @messages.message: The messages expressing hardware knob operation.
  * @messages.tstamp: The isochronous cycle at which the request subaction of asynchronous
  *		     transaction was sent to deliver the message. It has 16 bit unsigned integer
···
  */
 struct snd_firewire_event_ff400_message {
 	unsigned int type;
+	unsigned int message_count;
 	struct {
 		__u32 message;
 		__u32 tstamp;

sound/firewire/fireface/ff-protocol-former.c

···
 
 static long ff400_copy_msg_to_user(struct snd_ff *ff, char __user *buf, long count)
 {
+	struct snd_firewire_event_ff400_message ev = {
+		.type = SNDRV_FIREWIRE_EVENT_FF400_MESSAGE,
+		.message_count = 0,
+	};
 	struct ff400_msg_parser *parser = ff->msg_parser;
-	u32 type = SNDRV_FIREWIRE_EVENT_FF400_MESSAGE;
 	long consumed = 0;
-	int ret = 0;
+	long ret = 0;
 
-	if (count < 8)
+	if (count < sizeof(ev) || parser->pull_pos == parser->push_pos)
 		return 0;
 
-	spin_unlock_irq(&ff->lock);
-	if (copy_to_user(buf, &type, sizeof(type)))
-		ret = -EFAULT;
-	spin_lock_irq(&ff->lock);
-	if (ret)
-		return ret;
-
-	count -= sizeof(type);
-	consumed += sizeof(type);
+	count -= sizeof(ev);
+	consumed += sizeof(ev);
 
 	while (count >= sizeof(*parser->msgs) && parser->pull_pos != parser->push_pos) {
 		spin_unlock_irq(&ff->lock);
···
 		++parser->pull_pos;
 		if (parser->pull_pos >= FF400_QUEUE_SIZE)
 			parser->pull_pos = 0;
+		++ev.message_count;
 		count -= sizeof(*parser->msgs);
 		consumed += sizeof(*parser->msgs);
 	}
+
+	spin_unlock_irq(&ff->lock);
+	if (copy_to_user(buf, &ev, sizeof(ev)))
+		ret = -EFAULT;
+	spin_lock_irq(&ff->lock);
+	if (ret)
+		return ret;
 
 	return consumed;
 }