RDMA/core: Introduce rdma_read_gid_attr_ndev_rcu() to check GID attribute

Introduce an API rdma_read_gid_attr_ndev_rcu() to return GID attribute
netdevice which is in UP state for accessing netdevice's fields such as
net namespace and ifindex.

This is useful for users who intent to access netdevice fields under rcu
lock.
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/cache.c

@@ -1252,6 +1252,39 @@ void rdma_hold_gid_attr(const struct ib_gid_attr *attr)
 }
 EXPORT_SYMBOL(rdma_hold_gid_attr);
 
+/**
+ * rdma_read_gid_attr_ndev_rcu - Read GID attribute netdevice
+ * which must be in UP state.
+ *
+ * @attr:Pointer to the GID attribute
+ *
+ * Returns pointer to netdevice if the netdevice was attached to GID and
+ * netdevice is in UP state. Caller must hold RCU lock as this API
+ * reads the netdev flags which can change while netdevice migrates to
+ * different net namespace. Returns ERR_PTR with error code otherwise.
+ *
+ */
+struct net_device *rdma_read_gid_attr_ndev_rcu(const struct ib_gid_attr *attr)
+{
+	struct ib_gid_table_entry *entry =
+			container_of(attr, struct ib_gid_table_entry, attr);
+	struct ib_device *device = entry->attr.device;
+	struct net_device *ndev = ERR_PTR(-ENODEV);
+	u8 port_num = entry->attr.port_num;
+	struct ib_gid_table *table;
+	unsigned long flags;
+	bool valid;
+
+	table = rdma_gid_table(device, port_num);
+
+	read_lock_irqsave(&table->rwlock, flags);
+	valid = is_gid_entry_valid(table->data_vec[attr->index]);
+	if (valid && attr->ndev && (READ_ONCE(attr->ndev->flags) & IFF_UP))
+		ndev = attr->ndev;
+	read_unlock_irqrestore(&table->rwlock, flags);
+	return ndev;
+}
+
 static int config_non_roce_gid_cache(struct ib_device *device,
 				     u8 port, int gid_tbl_len)
 {

drivers/infiniband/core/core_priv.h

@@ -346,4 +346,6 @@ void rdma_copy_src_l2_addr(struct rdma_dev_addr *dev_addr,
 struct sa_path_rec;
 int roce_resolve_route_from_path(struct sa_path_rec *rec,
 				 const struct ib_gid_attr *attr);
+
+struct net_device *rdma_read_gid_attr_ndev_rcu(const struct ib_gid_attr *attr);
 #endif /* _CORE_PRIV_H */