[PATCH net 1/4] drop_monitor: synchronize tracepoint unregistration on error path

If register_trace_napi_poll() fails in net_dm_trace_on_set(),
unregister_trace_kfree_skb() is called to roll back the kfree_skb
tracepoint registration.

However, tracepoint_synchronize_unregister() is omitted before calling
cancel_work_sync() and module_put(). An in-flight probe executing
concurrently on another CPU could call schedule_work() after
cancel_work_sync() has already returned, leaving a pending work item
scheduled after the module reference is dropped. If the module is then
unloaded, executing the work item triggers a kernel panic.

Add tracepoint_synchronize_unregister() after unregister_trace_kfree_skb()
in the error path, matching net_dm_trace_off_set() and
net_dm_hw_probe_unregister().

Fixes: 7c747838a558 ("drop_monitor: Split tracing enable / disable to different functions")
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/core/drop_monitor.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/core/drop_monitor.c b/net/core/drop_monitor.c
index abaf108ac4db8c762c23f1d9c708c687ee36c7c9..018d19e3a71de0be50bcc5753ba65f715f678aaa 100644
--- a/net/core/drop_monitor.c
+++ b/net/core/drop_monitor.c

@@ -1173,6 +1173,7 @@ static int net_dm_trace_on_set(struct netlink_ext_ack *extack)
 
 err_unregister_trace:
 	unregister_trace_kfree_skb(ops->kfree_skb_probe, NULL);
+	tracepoint_synchronize_unregister();
 err_module_put:
 	for_each_possible_cpu(cpu) {
 		struct per_cpu_dm_data *data = &per_cpu(dm_cpu_data, cpu);

-- 
2.55.0.1007.g17ff1f9808-goog