netpoll: Ensure clean state on setup failures

Modify netpoll_setup() and __netpoll_setup() to ensure that the netpoll
structure (np) is left in a clean state if setup fails for any reason.
This prevents carrying over misconfigured fields in case of partial
setup success.

Key changes:
- np->dev is now set only after successful setup, ensuring it's always
  NULL if netpoll is not configured or if netpoll_setup() fails.
- np->local_ip is zeroed if netpoll setup doesn't complete successfully.
- Added DEBUG_NET_WARN_ON_ONCE() checks to catch unexpected states.
- Reordered some operations in __netpoll_setup() for better logical flow.

These changes improve the reliability of netpoll configuration, since it
assures that the structure is fully initialized or totally unset.
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20240822111051.179850-2-leitao@debian.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/netpoll.c

@@ -624,12 +624,9 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
 	const struct net_device_ops *ops;
 	int err;
 
-	np->dev = ndev;
-	strscpy(np->dev_name, ndev->name, IFNAMSIZ);
-
 	if (ndev->priv_flags & IFF_DISABLE_NETPOLL) {
 		np_err(np, "%s doesn't support polling, aborting\n",
-		       np->dev_name);
+		       ndev->name);
 		err = -ENOTSUPP;
 		goto out;
 	}
@@ -647,7 +644,7 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
 
 		refcount_set(&npinfo->refcnt, 1);
 
-		ops = np->dev->netdev_ops;
+		ops = ndev->netdev_ops;
 		if (ops->ndo_netpoll_setup) {
 			err = ops->ndo_netpoll_setup(ndev, npinfo);
 			if (err)
@@ -658,6 +655,8 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
 		refcount_inc(&npinfo->refcnt);
 	}
 
+	np->dev = ndev;
+	strscpy(np->dev_name, ndev->name, IFNAMSIZ);
 	npinfo->netpoll = np;
 
 	/* last thing to do is link it to the net device structure */
@@ -675,6 +674,7 @@ EXPORT_SYMBOL_GPL(__netpoll_setup);
 int netpoll_setup(struct netpoll *np)
 {
 	struct net_device *ndev = NULL;
+	bool ip_overwritten = false;
 	struct in_device *in_dev;
 	int err;
 
@@ -739,6 +739,7 @@ int netpoll_setup(struct netpoll *np)
 			}
 
 			np->local_ip.ip = ifa->ifa_local;
+			ip_overwritten = true;
 			np_info(np, "local IP %pI4\n", &np->local_ip.ip);
 		} else {
 #if IS_ENABLED(CONFIG_IPV6)
@@ -755,6 +756,7 @@ int netpoll_setup(struct netpoll *np)
 					    !!(ipv6_addr_type(&np->remote_ip.in6) & IPV6_ADDR_LINKLOCAL))
 						continue;
 					np->local_ip.in6 = ifp->addr;
+					ip_overwritten = true;
 					err = 0;
 					break;
 				}
@@ -785,6 +787,9 @@ int netpoll_setup(struct netpoll *np)
 	return 0;
 
 put:
+	DEBUG_NET_WARN_ON_ONCE(np->dev);
+	if (ip_overwritten)
+		memset(&np->local_ip, 0, sizeof(np->local_ip));
 	netdev_put(ndev, &np->dev_tracker);
 unlock:
 	rtnl_unlock();