[PATCH net-next] net: create a dummy net_device allocator

[Breno Leitao]

It is impossible to use init_dummy_netdev together with alloc_netdev()
as the 'setup' argument.

This is because alloc_netdev() initializes some fields in the net_device
structure, and later init_dummy_netdev() memzero them all. This causes
some problems as reported here:

	https://lore.kernel.org/all/20240322082336.49f110cc@xxxxxxxxxx/

Split the init_dummy_netdev() function in two. Create a new function called
init_dummy_netdev_core() that does not memzero the net_device structure.
Then have init_dummy_netdev() memzero-ing and calling
init_dummy_netdev_core(), keeping the old behaviour.

init_dummy_netdev_core() is the new function that could be called as an
argument for alloc_netdev().

Also, create a helper to allocate and initialize dummy net devices,
leveraging init_dummy_netdev_core() as the setup argument. This function
basically simplify the allocation of dummy devices, by allocating and
initializing it. Freeing the device continue to be done through
free_netdev()

Suggested-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 include/linux/netdevice.h |  3 +++
 net/core/dev.c            | 55 ++++++++++++++++++++++++++-------------
 2 files changed, 40 insertions(+), 18 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index c6f6ac779b34..f4226a99a146 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -4545,6 +4545,9 @@ static inline void netif_addr_unlock_bh(struct net_device *dev)
 
 void ether_setup(struct net_device *dev);
 
+/* Allocate dummy net_device */
+struct net_device *alloc_netdev_dummy(int sizeof_priv, const char *name);
+
 /* Support for loadable net-drivers */
 struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
 				    unsigned char name_assign_type,
diff --git a/net/core/dev.c b/net/core/dev.c
index 0766a245816b..df2484bbc041 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10340,25 +10340,12 @@ int register_netdevice(struct net_device *dev)
 }
 EXPORT_SYMBOL(register_netdevice);
 
-/**
- *	init_dummy_netdev	- init a dummy network device for NAPI
- *	@dev: device to init
- *
- *	This takes a network device structure and initialize the minimum
- *	amount of fields so it can be used to schedule NAPI polls without
- *	registering a full blown interface. This is to be used by drivers
- *	that need to tie several hardware interfaces to a single NAPI
- *	poll scheduler due to HW limitations.
+/* Initialize the core of a dummy net device.
+ * This is useful if you are calling this function after alloc_netdev(),
+ * since it does not memset the net_device fields.
  */
-void init_dummy_netdev(struct net_device *dev)
+static void init_dummy_netdev_core(struct net_device *dev)
 {
-	/* Clear everything. Note we don't initialize spinlocks
-	 * are they aren't supposed to be taken by any of the
-	 * NAPI code and this dummy netdev is supposed to be
-	 * only ever used for NAPI polls
-	 */
-	memset(dev, 0, sizeof(struct net_device));
-
 	/* make sure we BUG if trying to hit standard
 	 * register/unregister code path
 	 */
@@ -10379,8 +10366,28 @@ void init_dummy_netdev(struct net_device *dev)
 	 * its refcount.
 	 */
 }
-EXPORT_SYMBOL_GPL(init_dummy_netdev);
 
+/**
+ *	init_dummy_netdev	- init a dummy network device for NAPI
+ *	@dev: device to init
+ *
+ *	This takes a network device structure and initialize the minimum
+ *	amount of fields so it can be used to schedule NAPI polls without
+ *	registering a full blown interface. This is to be used by drivers
+ *	that need to tie several hardware interfaces to a single NAPI
+ *	poll scheduler due to HW limitations.
+ */
+void init_dummy_netdev(struct net_device *dev)
+{
+	/* Clear everything. Note we don't initialize spinlocks
+	 * are they aren't supposed to be taken by any of the
+	 * NAPI code and this dummy netdev is supposed to be
+	 * only ever used for NAPI polls
+	 */
+	memset(dev, 0, sizeof(struct net_device));
+	init_dummy_netdev_core(dev);
+}
+EXPORT_SYMBOL_GPL(init_dummy_netdev);
 
 /**
  *	register_netdev	- register a network device
@@ -10991,6 +10998,18 @@ void free_netdev(struct net_device *dev)
 }
 EXPORT_SYMBOL(free_netdev);
 
+/**
+ * alloc_netdev_dummy - Allocate and initialize a dummy net device.
+ * @sizeof_priv: size of private data to allocate space for
+ * @name: device name format string
+ */
+struct net_device *alloc_netdev_dummy(int sizeof_priv, const char *name)
+{
+	return alloc_netdev(sizeof_priv, name, NET_NAME_UNKNOWN,
+			    init_dummy_netdev_core);
+}
+EXPORT_SYMBOL_GPL(alloc_netdev_dummy);
+
 /**
  *	synchronize_net -  Synchronize with packet receive processing
  *
-- 
2.43.0