[PATCH net v2] netpoll: hold rcu read lock in __netpoll_send_skb()

[Breno Leitao]

The function __netpoll_send_skb() is being invoked without holding the
RCU read lock. This oversight triggers a warning message when
CONFIG_PROVE_RCU_LIST is enabled:

	net/core/netpoll.c:330 suspicious rcu_dereference_check() usage!

	 netpoll_send_skb
	 netpoll_send_udp
	 write_ext_msg
	 console_flush_all
	 console_unlock
	 vprintk_emit

To prevent npinfo from disappearing unexpectedly, ensure that
__netpoll_send_skb() is protected with the RCU read lock.

Fixes: 2899656b494dcd1 ("netpoll: take rcu_read_lock_bh() in netpoll_send_skb_on_dev()")
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
Changes in v2:
- Use rcu_read_lock() instead of guard() as normal people do (Jakub).
- Link to v1: https://lore.kernel.org/r/20250303-netpoll_rcu_v2-v1-1-6b34d8a01fa2@xxxxxxxxxx
---
 net/core/netpoll.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 62b4041aae1ae..0ab722d95a2df 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -319,6 +319,7 @@ static int netpoll_owner_active(struct net_device *dev)
 static netdev_tx_t __netpoll_send_skb(struct netpoll *np, struct sk_buff *skb)
 {
 	netdev_tx_t status = NETDEV_TX_BUSY;
+	netdev_tx_t ret = NET_XMIT_DROP;
 	struct net_device *dev;
 	unsigned long tries;
 	/* It is up to the caller to keep npinfo alive. */
@@ -327,11 +328,12 @@ static netdev_tx_t __netpoll_send_skb(struct netpoll *np, struct sk_buff *skb)
 	lockdep_assert_irqs_disabled();
 
 	dev = np->dev;
+	rcu_read_lock();
 	npinfo = rcu_dereference_bh(dev->npinfo);
 
 	if (!npinfo || !netif_running(dev) || !netif_device_present(dev)) {
 		dev_kfree_skb_irq(skb);
-		return NET_XMIT_DROP;
+		goto out;
 	}
 
 	/* don't get messages out of order, and no recursion */
@@ -370,7 +372,10 @@ static netdev_tx_t __netpoll_send_skb(struct netpoll *np, struct sk_buff *skb)
 		skb_queue_tail(&npinfo->txq, skb);
 		schedule_delayed_work(&npinfo->tx_work,0);
 	}
-	return NETDEV_TX_OK;
+	ret = NETDEV_TX_OK;
+out:
+	rcu_read_unlock();
+	return ret;
 }
 
 netdev_tx_t netpoll_send_skb(struct netpoll *np, struct sk_buff *skb)

---
base-commit: 848e076317446f9c663771ddec142d7c2eb4cb43
change-id: 20250303-netpoll_rcu_v2-fed72eb0cb83

Best regards,
-- 
Breno Leitao <leitao@xxxxxxxxxx>