Attempt to resume a previously deactivated target when the associated
interface comes back (NETDEV_REGISTER) or when it changes name
(NETDEV_CHANGENAME) by calling netpoll_setup on the device.

Depending on how the target was setup (by mac or interface name), the
corresponding field is compared with the device being brought up. Targets
that match the incoming device, are scheduled for resume on resume_wq, so
that netpoll_setup is able to force the device UP.

Target transitions to STATE_DISABLED in case of failures resuming it to
avoid retrying the same target indefinitely.

Signed-off-by: Andre Carvalho <asantostc@gmail.com>
---
 drivers/net/netconsole.c | 90 ++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 84 insertions(+), 6 deletions(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 02a3463e8d24..489fef5c3407 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -39,6 +39,7 @@
 #include <linux/u64_stats_sync.h>
 #include <linux/utsname.h>
 #include <linux/rtnetlink.h>
+#include <linux/workqueue.h>
 
 MODULE_AUTHOR("Matt Mackall <mpm@selenic.com>");
 MODULE_DESCRIPTION("Console driver for network interfaces");
@@ -138,10 +139,14 @@ enum target_state {
  * @stats:	Packet send stats for the target. Used for debugging.
  * @state:	State of the target.
  *		Visible from userspace (read-write).
- *		We maintain a strict 1:1 correspondence between this and
- *		whether the corresponding netpoll is active or inactive.
+ *		From a userspace perspective, the target is either enabled or
+ *		disabled. Internally, although both STATE_DISABLED and
+ *		STATE_DEACTIVATED correspond to inactive targets, the latter is
+ *		due to automatic interface state changes and will try
+ *		recover automatically, if the interface comes back
+ *		online.
  *		Also, other parameters of a target may be modified at
- *		runtime only when it is disabled (state == STATE_DISABLED).
+ *		runtime only when it is disabled (state != STATE_ENABLED).
  * @extended:	Denotes whether console is extended or not.
  * @release:	Denotes whether kernel release version should be prepended
  *		to the message. Depends on extended console.
@@ -155,6 +160,7 @@ enum target_state {
  *		local_mac	(read-only)
  *		remote_mac	(read-write)
  * @buf:	The buffer used to send the full msg to the network stack
+ * @resume_wq:	Workqueue to resume deactivated target
  */
 struct netconsole_target {
 	struct list_head	list;
@@ -177,6 +183,7 @@ struct netconsole_target {
 	struct netpoll		np;
 	/* protected by target_list_lock */
 	char			buf[MAX_PRINT_CHUNK];
+	struct work_struct	resume_wq;
 };
 
 #ifdef	CONFIG_NETCONSOLE_DYNAMIC
@@ -248,6 +255,67 @@ static bool bound_by_mac(struct netconsole_target *nt)
 	return is_valid_ether_addr(nt->np.dev_mac);
 }
 
+/* Attempts to resume logging to a deactivated target. */
+static void resume_target(struct netconsole_target *nt)
+{
+	int ret;
+
+	/* check if target is still deactivated as it may have been disabled
+	 * while resume was being scheduled.
+	 */
+	if (nt->state != STATE_DEACTIVATED)
+		return;
+
+	ret = netpoll_setup(&nt->np);
+	if (ret) {
+		/* netpoll fails setup once, do not try again. */
+		nt->state = STATE_DISABLED;
+		return;
+	}
+
+	nt->state = STATE_ENABLED;
+	pr_info("network logging resumed on interface %s\n", nt->np.dev_name);
+}
+
+/* Checks if a deactivated target matches a device. */
+static bool deactivated_target_match(struct netconsole_target *nt,
+				     struct net_device *ndev)
+{
+	if (nt->state != STATE_DEACTIVATED)
+		return false;
+
+	if (bound_by_mac(nt))
+		return !memcmp(nt->np.dev_mac, ndev->dev_addr, ETH_ALEN);
+	return !strncmp(nt->np.dev_name, ndev->name, IFNAMSIZ);
+}
+
+/* Process work scheduled for target resume. */
+static void process_resume_target(struct work_struct *work)
+{
+	struct netconsole_target *nt =
+		container_of(work, struct netconsole_target, resume_wq);
+	unsigned long flags;
+
+	mutex_lock(&dynamic_netconsole_mutex);
+	/* resume_target is IRQ unsafe, remove target from
+	 * target_list in order to resume it with IRQ enabled.
+	 */
+	spin_lock_irqsave(&target_list_lock, flags);
+	list_del_init(&nt->list);
+	spin_unlock_irqrestore(&target_list_lock, flags);
+
+	resume_target(nt);
+
+	/* At this point the target is either enabled or disabled and
+	 * was cleaned up before getting deactivated. Either way, add it
+	 * back to target list.
+	 */
+	spin_lock_irqsave(&target_list_lock, flags);
+	list_add(&nt->list, &target_list);
+	spin_unlock_irqrestore(&target_list_lock, flags);
+	mutex_unlock(&dynamic_netconsole_mutex);
+}
+
 /* Allocate and initialize with defaults.
  * Note that these targets get their config_item fields zeroed-out.
  */
@@ -270,6 +338,7 @@ static struct netconsole_target *alloc_and_init(void)
 	nt->np.remote_port = 6666;
 	eth_broadcast_addr(nt->np.remote_mac);
 	nt->state = STATE_DISABLED;
+	INIT_WORK(&nt->resume_wq, process_resume_target);
 
 	return nt;
 }
@@ -1442,13 +1511,14 @@ static int prepare_sysdata(struct netconsole_target *nt)
 static int netconsole_netdev_event(struct notifier_block *this,
 				   unsigned long event, void *ptr)
 {
-	unsigned long flags;
-	struct netconsole_target *nt, *tmp;
 	struct net_device *dev = netdev_notifier_info_to_dev(ptr);
+	struct netconsole_target *nt, *tmp;
 	bool stopped = false;
+	unsigned long flags;
 
 	if (!(event == NETDEV_CHANGENAME || event == NETDEV_UNREGISTER ||
-	      event == NETDEV_RELEASE || event == NETDEV_JOIN))
+	      event == NETDEV_RELEASE || event == NETDEV_JOIN ||
+	      event == NETDEV_REGISTER))
 		goto done;
 
 	mutex_lock(&target_cleanup_list_lock);
@@ -1477,6 +1547,13 @@ static int netconsole_netdev_event(struct notifier_block *this,
 				stopped = true;
 			}
 		}
+		if ((event == NETDEV_REGISTER || event == NETDEV_CHANGENAME) &&
+		    deactivated_target_match(nt, dev))
+			/* Schedule resume on a workqueue as it will attempt
+			 * to UP the device, which can't be done as part of this
+			 * notifier.
+			 */
+			schedule_work(&nt->resume_wq);
 		netconsole_target_put(nt);
 	}
 	spin_unlock_irqrestore(&target_list_lock, flags);
@@ -1945,6 +2022,7 @@ static struct netconsole_target *alloc_param_target(char *target_config,
 /* Cleanup netpoll for given target (from boot/module param) and free it */
 static void free_param_target(struct netconsole_target *nt)
 {
+	cancel_work_sync(&nt->resume_wq);
 	netpoll_cleanup(&nt->np);
 #ifdef	CONFIG_NETCONSOLE_DYNAMIC
 	kfree(nt->userdata);

-- 
2.52.0

On Sun, Jan 04, 2026 at 06:41:15PM +0000, Andre Carvalho wrote:
> +static void process_resume_target(struct work_struct *work)
> +{
> +	struct netconsole_target *nt =
> +		container_of(work, struct netconsole_target, resume_wq);
> +	unsigned long flags;
> +
> +	mutex_lock(&dynamic_netconsole_mutex);

This ended up causing build failures in CI as it needs to be guarded by
ifdef for CONFIG_NETCONSOLE_DYNAMIC. Unfortunately, this was always set on my
local tests - will fix that as well.

Sorry for the noise in the CI. Will fix this in v10.

> @@ -1945,6 +2022,7 @@ static struct netconsole_target *alloc_param_target(char *target_config,
>  /* Cleanup netpoll for given target (from boot/module param) and free it */
>  static void free_param_target(struct netconsole_target *nt)
>  {
> +	cancel_work_sync(&nt->resume_wq);
>  	netpoll_cleanup(&nt->np);

Will also address the AI Review[1], which seems to indicate a potential use-after-free
when a dynamic target gets removes (and disabled) while resume_wq has some pending
work. I think this might be a true positive and I'll see if also canceling the work
on netconsole_target_release() makes sense.

[1] https://netdev-ai.bots.linux.dev/ai-review.html?id=ca5cba91-a1a6-4240-bf10-e4da9c5bc58a

-- 
Andre Carvalho