Bluetooth: l2cap: add l2cap_user sub-modules

Several sub-modules like HIDP, rfcomm, ... need to track l2cap
connections. The l2cap_conn->hcon->dev object is used as parent for sysfs
devices so the sub-modules need to be notified when the hci_conn object is
removed from sysfs.

As submodules normally use the l2cap layer, the l2cap_user objects are
registered there instead of on the underlying hci_conn object. This avoids
any direct dependency on the HCI layer and lets the l2cap core handle any
specifics.

This patch introduces l2cap_user objects which contain a "probe" and
"remove" callback. You can register them on any l2cap_conn object and if
it is active, the "probe" callback will get called. Otherwise, an error is
returned.

The l2cap_conn object will call your "remove" callback directly before it
is removed from user-space. This allows you to remove your submodules
_before_ the parent l2cap_conn and hci_conn object is removed.

At any time you can asynchronously unregister your l2cap_user object if
your submodule vanishes before the l2cap_conn object does.

There is no way around l2cap_user. If we want wire-protocols in the
kernel, we always want the hci_conn object as parent in the sysfs tree. We
cannot use a channel here since we might need multiple channels for a
single protocol.
But the problem is, we _must_ get notified when an l2cap_conn object is
removed. We cannot use reference-counting for object-removal! This is not
how it works. If a hardware is removed, we should immediately remove the
object from sysfs. Any other behavior would be inconsistent with the rest
of the system. Also note that device_del() might sleep, but it doesn't
wait for user-space or block very long. It only _unlinks_ the object from
sysfs and the whole device-tree. Everything else is handled by ref-counts!
This is exactly what the other sub-modules must do: unlink their devices
when the "remove" l2cap_user callback is called. They should not do any
cleanup or synchronous shutdowns.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>

1 files changed, 86 insertions, 0 deletions

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index be9ad89339cd..eae1d9f90b68 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -1446,6 +1446,89 @@ static void l2cap_info_timeout(struct work_struct *work)
         l2cap_conn_start(conn);
 }
 
+/*
+ * l2cap_user
+ * External modules can register l2cap_user objects on l2cap_conn. The ->probe
+ * callback is called during registration. The ->remove callback is called
+ * during unregistration.
+ * An l2cap_user object can either be explicitly unregistered or when the
+ * underlying l2cap_conn object is deleted. This guarantees that l2cap->hcon,
+ * l2cap->hchan, .. are valid as long as the remove callback hasn't been called.
+ * External modules must own a reference to the l2cap_conn object if they intend
+ * to call l2cap_unregister_user(). The l2cap_conn object might get destroyed at
+ * any time if they don't.
+ */
+
+int l2cap_register_user(struct l2cap_conn *conn, struct l2cap_user *user)
+{
+        struct hci_dev *hdev = conn->hcon->hdev;
+        int ret;
+
+        /* We need to check whether l2cap_conn is registered. If it is not, we
+         * must not register the l2cap_user. l2cap_conn_del() is unregisters
+         * l2cap_conn objects, but doesn't provide its own locking. Instead, it
+         * relies on the parent hci_conn object to be locked. This itself relies
+         * on the hci_dev object to be locked. So we must lock the hci device
+         * here, too. */
+
+        hci_dev_lock(hdev);
+
+        if (user->list.next || user->list.prev) {
+                ret = -EINVAL;
+                goto out_unlock;
+        }
+
+        /* conn->hchan is NULL after l2cap_conn_del() was called */
+        if (!conn->hchan) {
+                ret = -ENODEV;
+                goto out_unlock;
+        }
+
+        ret = user->probe(conn, user);
+        if (ret)
+                goto out_unlock;
+
+        list_add(&user->list, &conn->users);
+        ret = 0;
+
+out_unlock:
+        hci_dev_unlock(hdev);
+        return ret;
+}
+EXPORT_SYMBOL(l2cap_register_user);
+
+void l2cap_unregister_user(struct l2cap_conn *conn, struct l2cap_user *user)
+{
+        struct hci_dev *hdev = conn->hcon->hdev;
+
+        hci_dev_lock(hdev);
+
+        if (!user->list.next || !user->list.prev)
+                goto out_unlock;
+
+        list_del(&user->list);
+        user->list.next = NULL;
+        user->list.prev = NULL;
+        user->remove(conn, user);
+
+out_unlock:
+        hci_dev_unlock(hdev);
+}
+EXPORT_SYMBOL(l2cap_unregister_user);
+
+static void l2cap_unregister_all_users(struct l2cap_conn *conn)
+{
+        struct l2cap_user *user;
+
+        while (!list_empty(&conn->users)) {
+                user = list_first_entry(&conn->users, struct l2cap_user, list);
+                list_del(&user->list);
+                user->list.next = NULL;
+                user->list.prev = NULL;
+                user->remove(conn, user);
+        }
+}
+
 static void l2cap_conn_del(struct hci_conn *hcon, int err)
 {
         struct l2cap_conn *conn = hcon->l2cap_data;
@@ -1458,6 +1541,8 @@ static void l2cap_conn_del(struct hci_conn *hcon, int err)
 
         kfree_skb(conn->rx_skb);
 
+        l2cap_unregister_all_users(conn);
+
         mutex_lock(&conn->chan_lock);
 
         /* Kill channels */
@@ -1550,6 +1635,7 @@ static struct l2cap_conn *l2cap_conn_add(struct hci_conn *hcon)
         mutex_init(&conn->chan_lock);
 
         INIT_LIST_HEAD(&conn->chan_l);
+        INIT_LIST_HEAD(&conn->users);
 
         if (hcon->type == LE_LINK)
                 INIT_DELAYED_WORK(&conn->security_timer, security_timeout);