[PATCH v5 4/6] netdev: add netdev_is_upper_master

[Alexis Bauvin <abauvin@scaleway.com>]

In preparation of next patch, this function allows to check if a device
is a master, be it direct or indirect, of another one. It walks up the
master chain until it finds the device, or there is no more master.

This allows to check e.g. if br-blue is a master of eth0:

   +----------+
   | vrf-blue |
   +----+-----+
        |
   +----+----+
   | br-blue |
   +----+----+
        |
    +---+---+
    | bond0 |
    +--+-+--+
       | |
    +--+ +--+
    |       |
+---+--+ +--+---+
| eth0 | | eth1 |
+------+ +------+

Signed-off-by: Alexis Bauvin <abauvin@scaleway.com>
Reviewed-by: Amine Kherbouche <akherbouche@scaleway.com>
Tested-by: Amine Kherbouche <akherbouche@scaleway.com>
---
 include/linux/netdevice.h |  1 +
 net/core/dev.c            | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index d837dad24b4c..102f79337d7c 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -4212,6 +4212,7 @@ void *netdev_lower_dev_get_private(struct net_device *dev,
 				   struct net_device *lower_dev);
 void netdev_lower_state_changed(struct net_device *lower_dev,
 				void *lower_state_info);
+bool netdev_is_upper_master(struct net_device *dev, struct net_device *master);
 
 /* RSS keys are 40 or 52 bytes long */
 #define NETDEV_RSS_KEY_LEN 52
diff --git a/net/core/dev.c b/net/core/dev.c
index 93243479085f..9222434a5332 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -7225,6 +7225,23 @@ void netdev_lower_state_changed(struct net_device *lower_dev,
 }
 EXPORT_SYMBOL(netdev_lower_state_changed);
 
+/**
+ * netdev_is_upper_master - Test if a device is a master, direct or indirect,
+ *                          of another one.
+ * @dev: device to start looking from
+ * @master: device to test if master of dev
+ */
+bool netdev_is_upper_master(struct net_device *dev, struct net_device *master)
+{
+	if (!dev)
+		return false;
+
+	if (dev == master)
+		return true;
+	return netdev_is_upper_master(netdev_master_upper_dev_get(dev), master);
+}
+EXPORT_SYMBOL(netdev_is_upper_master);
+
 static void dev_change_rx_flags(struct net_device *dev, int flags)
 {
 	const struct net_device_ops *ops = dev->netdev_ops;
--