Re: can/j1939: hung inside rtnl_dellink()

[Oleksij Rempel]

Hello Tetsuo,

On Sat, Aug 16, 2025 at 03:51:54PM +0900, Tetsuo Handa wrote:
> Hello.
> 
> I made a minimized C reproducer for
> 
>   unregister_netdevice: waiting for vcan0 to become free. Usage count = 2
> 
> problem at https://syzkaller.appspot.com/bug?extid=881d65229ca4f9ae8c84 , and
> obtained some data using debug printk() patch. It seems that the cause is
> net/can/j1939/ does not handle NETDEV_UNREGISTER notification
> while net/can/j1939/ can directly call rtnl_dellink() via sendmsg().

Sorry for long delay and than you for your investigation!

> The minimized C reproducer is shown below.
....

> Therefore, I guess that either
> 
>   j1939_netdev_notify() is handling NETDEV_UNREGISTER notification
> 
> or
> 
>   rtnl_dellink() can be called via sendmsg() despite the j1939 socket
>   are in use
> 
> is wrong. How to fix this problem?

I assume the first variant is correct. Can you please test following change:
--- a/net/can/j1939/main.c
+++ b/net/can/j1939/main.c
@@ -370,6 +370,7 @@
 		goto notify_done;
 
 	switch (msg) {
+	case NETDEV_UNREGISTER:
 	case NETDEV_DOWN:
 		j1939_cancel_active_session(priv, NULL);
 		j1939_sk_netdev_event_netdown(priv);

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