net: add netdev->up protected by netdev_lock()

Some uAPI (netdev netlink) hide net_device's sub-objects while
the interface is down to ensure uniform behavior across drivers.
To remove the rtnl_lock dependency from those uAPIs we need a way
to safely tell if the device is down or up.

Add an indication of whether device is open or closed, protected
by netdev->lock. The semantics are the same as IFF_UP, but taking
netdev_lock around every write to ->flags would be a lot of code
churn.

We don't want to blanket the entire open / close path by netdev_lock,
because it will prevent us from applying it to specific structures -
core helpers won't be able to take that lock from any function
called by the drivers on open/close paths.

So the state of the flag is "pessimistic", as in it may report false
negatives, but never false positives.

Reviewed-by: Joe Damato <jdamato@fastly.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250115035319.559603-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/netdevice.h

@@ -2443,12 +2443,24 @@ struct net_device {
 	unsigned long		gro_flush_timeout;
 	u32			napi_defer_hard_irqs;
 
+	/**
+	 * @up: copy of @state's IFF_UP, but safe to read with just @lock.
+	 *	May report false negatives while the device is being opened
+	 *	or closed (@lock does not protect .ndo_open, or .ndo_close).
+	 */
+	bool			up;
+
 	/**
 	 * @lock: netdev-scope lock, protects a small selection of fields.
 	 * Should always be taken using netdev_lock() / netdev_unlock() helpers.
 	 * Drivers are free to use it for other protection.
 	 *
-	 * Protects: @reg_state, @net_shaper_hierarchy.
+	 * Protects:
+	 *	@net_shaper_hierarchy, @reg_state
+	 *
+	 * Partially protects (writers must hold both @lock and rtnl_lock):
+	 *	@up
+	 *
 	 * Ordering: take after rtnl_lock.
 	 */
 	struct mutex		lock;

net/core/dev.c

@@ -1619,7 +1619,7 @@ static int __dev_open(struct net_device *dev, struct netlink_ext_ack *extack)
 	if (ret)
 		clear_bit(__LINK_STATE_START, &dev->state);
 	else {
-		dev->flags |= IFF_UP;
+		netif_set_up(dev, true);
 		dev_set_rx_mode(dev);
 		dev_activate(dev);
 		add_device_randomness(dev->dev_addr, dev->addr_len);
@@ -1698,7 +1698,7 @@ static void __dev_close_many(struct list_head *head)
 		if (ops->ndo_stop)
 			ops->ndo_stop(dev);
 
-		dev->flags &= ~IFF_UP;
+		netif_set_up(dev, false);
 		netpoll_poll_enable(dev);
 	}
 }

net/core/dev.h

@@ -128,6 +128,18 @@ void __dev_notify_flags(struct net_device *dev, unsigned int old_flags,
 void unregister_netdevice_many_notify(struct list_head *head,
 				      u32 portid, const struct nlmsghdr *nlh);
 
+static inline void netif_set_up(struct net_device *dev, bool value)
+{
+	if (value)
+		dev->flags |= IFF_UP;
+	else
+		dev->flags &= ~IFF_UP;
+
+	netdev_lock(dev);
+	dev->up = value;
+	netdev_unlock(dev);
+}
+
 static inline void netif_set_gso_max_size(struct net_device *dev,
 					  unsigned int size)
 {