[NET]: Allow netdev REGISTER/CHANGENAME events to fail

This patch adds code to allow errors to be passed up from event
handlers of NETDEV_REGISTER and NETDEV_CHANGENAME.  It also adds
the notifier_from_errno/notifier_to_errnor helpers to pass the
errno value up to the notifier caller.

If an error is detected when a device is registered, it causes
that operation to fail.  A NETDEV_UNREGISTER will be sent to
all event handlers.

Similarly if NETDEV_CHANGENAME fails the original name is restored
and a new NETDEV_CHANGENAME event is sent.

As such all event handlers must be idempotent with respect to
these events.

When an event handler is registered NETDEV_REGISTER events are
sent for all devices currently registered.  Should any of them
fail, we will send NETDEV_GOING_DOWN/NETDEV_DOWN/NETDEV_UNREGISTER
events to that handler for the devices which have already been
registered with it.  The handler registration itself will fail.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 13 insertions, 0 deletions

diff --git a/include/linux/notifier.h b/include/linux/notifier.h
index be3f2bb6fcf3..fad7ff17e468 100644
--- a/include/linux/notifier.h
+++ b/include/linux/notifier.h
@@ -157,6 +157,19 @@ extern int __srcu_notifier_call_chain(struct srcu_notifier_head *nh,
  */
 #define NOTIFY_STOP		(NOTIFY_OK|NOTIFY_STOP_MASK)
 
+/* Encapsulate (negative) errno value (in particular, NOTIFY_BAD <=> EPERM). */
+static inline int notifier_from_errno(int err)
+{
+	return NOTIFY_STOP_MASK | (NOTIFY_OK - err);
+}
+
+/* Restore (negative) errno value from notify return value. */
+static inline int notifier_to_errno(int ret)
+{
+	ret &= ~NOTIFY_STOP_MASK;
+	return ret > NOTIFY_OK ? NOTIFY_OK - ret : 0;
+}
+
 /*
  *	Declared notifiers so far. I can imagine quite a few more chains
  *	over time (eg laptop power reset chains, reboot chain (to clean