Re: [syzbot] [can?] WARNING: refcount bug in j1939_session_put

[Edward Adam Davis]

Fixes: c9c0ee5f20c5 ("net: skbuff: Skip early return in skb_unref when debugging")

Root cause: In commit c9c0ee5f20c5, There are following rules:
In debug builds (CONFIG_DEBUG_NET set), the reference count is always  decremented, even when it's 1

This rule will cause the reference count to be 0 after calling skc_unref,
which will affect the release of skb.

#syz test: net-next 743ff02152bc

diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index 4be73de5033c..712c0ae4f329 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c
@@ -279,6 +279,8 @@ static void j1939_session_destroy(struct j1939_session *session)
 	while ((skb = skb_dequeue(&session->skb_queue)) != NULL) {
 		/* drop ref taken in j1939_session_skb_queue() */
 		skb_unref(skb);
+
+		printk("refcnt: %d, skb: %p, %s\n", refcount_read(&skb->users), skb, __func__);
 		kfree_skb(skb);
 	}
 	__j1939_session_drop(session);
@@ -341,6 +343,7 @@ static void j1939_session_skb_drop_old(struct j1939_session *session)
 	if ((do_skcb->offset + do_skb->len) < offset_start) {
 		__skb_unlink(do_skb, &session->skb_queue);
 		/* drop ref taken in j1939_session_skb_queue() */
+		printk("refcnt: %d, skb: %p, %s\n", refcount_read(&do_skb->users), do_skb, __func__);
 		skb_unref(do_skb);
 		spin_unlock_irqrestore(&session->skb_queue.lock, flags);
 
@@ -365,6 +368,7 @@ void j1939_session_skb_queue(struct j1939_session *session,
 	skcb->flags |= J1939_ECU_LOCAL_SRC;
 
 	skb_get(skb);
+	printk("refcnt: %d, skb: %p, %s\n", refcount_read(&skb->users), skb, __func__);
 	skb_queue_tail(&session->skb_queue, skb);
 }
 
@@ -1505,7 +1509,7 @@ static struct j1939_session *j1939_session_new(struct j1939_priv *priv,
 	session->state = J1939_SESSION_NEW;
 
 	skb_queue_head_init(&session->skb_queue);
-	skb_queue_tail(&session->skb_queue, skb);
+	j1939_session_skb_queue(session, skb);
 
 	skcb = j1939_skb_to_cb(skb);
 	memcpy(&session->skcb, skcb, sizeof(session->skcb));