net: qualcomm: rmnet: Fix warning seen with fill_info

When the last rmnet device attached to a real device is removed, the
real device is unregistered from rmnet. As a result, the real device
lookup fails resulting in a warning when the fill_info handler is
called as part of the rmnet device unregistration.

Fix this by returning the rmnet flags as 0 when no real device is
present.

WARNING: CPU: 0 PID: 1779 at net/core/rtnetlink.c:3254
rtmsg_ifinfo_build_skb+0xca/0x10d
Modules linked in:
CPU: 0 PID: 1779 Comm: ip Not tainted 4.16.0-11872-g7ce23672 #1
Stack:
 7fe655f0 60371ea3 00000000 00000000
 60282bc6 6006b116 7fe65600 60371ee8
 7fe65660 6003a68c 00000000 900000000
Call Trace:
 [<6006b116>] ? printk+0x0/0x94
 [<6001f375>] show_stack+0xfe/0x158
 [<60371ea3>] ? dump_stack_print_info+0xe8/0xf1
 [<60282bc6>] ? rtmsg_ifinfo_build_skb+0xca/0x10d
 [<6006b116>] ? printk+0x0/0x94
 [<60371ee8>] dump_stack+0x2a/0x2c
 [<6003a68c>] __warn+0x10e/0x13e
 [<6003a82c>] warn_slowpath_null+0x48/0x4f
 [<60282bc6>] rtmsg_ifinfo_build_skb+0xca/0x10d
 [<60282c4d>] rtmsg_ifinfo_event.part.37+0x1e/0x43
 [<60282c2f>] ? rtmsg_ifinfo_event.part.37+0x0/0x43
 [<60282d03>] rtmsg_ifinfo+0x24/0x28
 [<60264e86>] dev_close_many+0xba/0x119
 [<60282cdf>] ? rtmsg_ifinfo+0x0/0x28
 [<6027c225>] ? rtnl_is_locked+0x0/0x1c
 [<6026ca67>] rollback_registered_many+0x1ae/0x4ae
 [<600314be>] ? unblock_signals+0x0/0xae
 [<6026cdc0>] ? unregister_netdevice_queue+0x19/0xec
 [<6026ceec>] unregister_netdevice_many+0x21/0xa1
 [<6027c765>] rtnl_delete_link+0x3e/0x4e
 [<60280ecb>] rtnl_dellink+0x262/0x29c
 [<6027c241>] ? rtnl_get_link+0x0/0x3e
 [<6027f867>] rtnetlink_rcv_msg+0x235/0x274

Fixes: be81a85f ("net: qualcomm: rmnet: Implement fill_info")
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/qualcomm/rmnet/rmnet_config.c

@@ -350,15 +350,16 @@ static int rmnet_fill_info(struct sk_buff *skb, const struct net_device *dev)
 
 	real_dev = priv->real_dev;
 
-	if (!rmnet_is_real_dev_registered(real_dev))
-		return -ENODEV;
-
 	if (nla_put_u16(skb, IFLA_RMNET_MUX_ID, priv->mux_id))
 		goto nla_put_failure;
 
-	port = rmnet_get_port_rtnl(real_dev);
+	if (rmnet_is_real_dev_registered(real_dev)) {
+		port = rmnet_get_port_rtnl(real_dev);
+		f.flags = port->data_format;
+	} else {
+		f.flags = 0;
+	}
 
-	f.flags = port->data_format;
 	f.mask  = ~0;
 
 	if (nla_put(skb, IFLA_RMNET_FLAGS, sizeof(f), &f))