From: Jakub Kicinski kuba@kernel.org

commit 2b446e650b418f9a9e75f99852e2f2560cabfa17 upstream.

Explain the two basic flows of struct net_device's operation.

Signed-off-by: Jakub Kicinski kuba@kernel.org --- Documentation/networking/netdevices.rst | 171 +++++++++++++++++++++++- net/core/rtnetlink.c | 2 +- 2 files changed, 166 insertions(+), 7 deletions(-)

diff --git a/Documentation/networking/netdevices.rst b/Documentation/networking/netdevices.rst index e65665c5ab50..17bdcb746dcf 100644 --- a/Documentation/networking/netdevices.rst +++ b/Documentation/networking/netdevices.rst @@ -10,18 +10,177 @@ Introduction The following is a random collection of documentation regarding network devices.

-struct net_device allocation rules -================================== +struct net_device lifetime rules +================================ Network device structures need to persist even after module is unloaded and must be allocated with alloc_netdev_mqs() and friends. If device has registered successfully, it will be freed on last use -by free_netdev(). This is required to handle the pathologic case cleanly -(example: rmmod mydriver </sys/class/net/myeth/mtu ) +by free_netdev(). This is required to handle the pathological case cleanly +(example: ``rmmod mydriver </sys/class/net/myeth/mtu``)

-alloc_netdev_mqs()/alloc_netdev() reserve extra space for driver +alloc_netdev_mqs() / alloc_netdev() reserve extra space for driver private data which gets freed when the network device is freed. If separately allocated data is attached to the network device -(netdev_priv(dev)) then it is up to the module exit handler to free that. +(netdev_priv()) then it is up to the module exit handler to free that. + +There are two groups of APIs for registering struct net_device. +First group can be used in normal contexts where ``rtnl_lock`` is not already +held: register_netdev(), unregister_netdev(). +Second group can be used when ``rtnl_lock`` is already held: +register_netdevice(), unregister_netdevice(), free_netdevice(). + +Simple drivers +-------------- + +Most drivers (especially device drivers) handle lifetime of struct net_device +in context where ``rtnl_lock`` is not held (e.g. driver probe and remove paths). + +In that case the struct net_device registration is done using +the register_netdev(), and unregister_netdev() functions: + +.. code-block:: c + + int probe() + { + struct my_device_priv *priv; + int err; + + dev = alloc_netdev_mqs(...); + if (!dev) + return -ENOMEM; + priv = netdev_priv(dev); + + /* ... do all device setup before calling register_netdev() ... + */ + + err = register_netdev(dev); + if (err) + goto err_undo; + + /* net_device is visible to the user! */ + + err_undo: + /* ... undo the device setup ... */ + free_netdev(dev); + return err; + } + + void remove() + { + unregister_netdev(dev); + free_netdev(dev); + } + +Note that after calling register_netdev() the device is visible in the system. +Users can open it and start sending / receiving traffic immediately, +or run any other callback, so all initialization must be done prior to +registration. + +unregister_netdev() closes the device and waits for all users to be done +with it. The memory of struct net_device itself may still be referenced +by sysfs but all operations on that device will fail. + +free_netdev() can be called after unregister_netdev() returns on when +register_netdev() failed. + +Device management under RTNL +---------------------------- + +Registering struct net_device while in context which already holds +the ``rtnl_lock`` requires extra care. In those scenarios most drivers +will want to make use of struct net_device's ``needs_free_netdev`` +and ``priv_destructor`` members for freeing of state. + +Example flow of netdev handling under ``rtnl_lock``: + +.. code-block:: c + + static void my_setup(struct net_device *dev) + { + dev->needs_free_netdev = true; + } + + static void my_destructor(struct net_device *dev) + { + some_obj_destroy(priv->obj); + some_uninit(priv); + } + + int create_link() + { + struct my_device_priv *priv; + int err; + + ASSERT_RTNL(); + + dev = alloc_netdev(sizeof(*priv), "net%d", NET_NAME_UNKNOWN, my_setup); + if (!dev) + return -ENOMEM; + priv = netdev_priv(dev); + + /* Implicit constructor */ + err = some_init(priv); + if (err) + goto err_free_dev; + + priv->obj = some_obj_create(); + if (!priv->obj) { + err = -ENOMEM; + goto err_some_uninit; + } + /* End of constructor, set the destructor: */ + dev->priv_destructor = my_destructor; + + err = register_netdevice(dev); + if (err) + /* register_netdevice() calls destructor on failure */ + goto err_free_dev; + + /* If anything fails now unregister_netdevice() (or unregister_netdev()) + * will take care of calling my_destructor and free_netdev(). + */ + + return 0; + + err_some_uninit: + some_uninit(priv); + err_free_dev: + free_netdev(dev); + return err; + } + +If struct net_device.priv_destructor is set it will be called by the core +some time after unregister_netdevice(), it will also be called if +register_netdevice() fails. The callback may be invoked with or without +``rtnl_lock`` held. + +There is no explicit constructor callback, driver "constructs" the private +netdev state after allocating it and before registration. + +Setting struct net_device.needs_free_netdev makes core call free_netdevice() +automatically after unregister_netdevice() when all references to the device +are gone. It only takes effect after a successful call to register_netdevice() +so if register_netdevice() fails driver is responsible for calling +free_netdev(). + +free_netdev() is safe to call on error paths right after unregister_netdevice() +or when register_netdevice() fails. Parts of netdev (de)registration process +happen after ``rtnl_lock`` is released, therefore in those cases free_netdev() +will defer some of the processing until ``rtnl_lock`` is released. + +Devices spawned from struct rtnl_link_ops should never free the +struct net_device directly. + +.ndo_init and .ndo_uninit +~~~~~~~~~~~~~~~~~~~~~~~~~ + +``.ndo_init`` and ``.ndo_uninit`` callbacks are called during net_device +registration and de-registration, under ``rtnl_lock``. Drivers can use +those e.g. when parts of their init process need to run under ``rtnl_lock``. + +``.ndo_init`` runs before device is visible in the system, ``.ndo_uninit`` +runs during de-registering after device is closed but other subsystems +may still have outstanding references to the netdevice.

MTU === diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c index bb0596c41b3e..79f514afb17d 100644 --- a/net/core/rtnetlink.c +++ b/net/core/rtnetlink.c @@ -3441,7 +3441,7 @@ static int __rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,

if (ops->newlink) { err = ops->newlink(link_net ? : net, dev, tb, data, extack); - /* Drivers should call free_netdev() in ->destructor + /* Drivers should set dev->needs_free_netdev * and unregister it on failure after registration * so that device could be finally freed in rtnl_unlock. */

From: Jakub Kicinski kuba@kernel.org

commit 766b0515d5bec4b780750773ed3009b148df8c0a upstream.

If register_netdevice() fails at the very last stage - the notifier call - some subsystems may have already seen it and grabbed a reference. struct net_device can't be freed right away without calling netdev_wait_all_refs().

Now that we have a clean interface in form of dev->needs_free_netdev and lenient free_netdev() we can undo what commit 93ee31f14f6f ("[NET]: Fix free_netdev on register_netdev failure.") has done and complete the unregistration path by bringing the net_set_todo() call back.

After registration fails user is still expected to explicitly free the net_device, so make sure ->needs_free_netdev is cleared, otherwise rolling back the registration will cause the old double free for callers who release rtnl_lock before the free.

This also solves the problem of priv_destructor not being called on notifier error.

net_set_todo() will be moved back into unregister_netdevice_queue() in a follow up.

Reported-by: Hulk Robot hulkci@huawei.com Reported-by: Yang Yingliang yangyingliang@huawei.com Signed-off-by: Jakub Kicinski kuba@kernel.org --- net/core/dev.c | 14 ++++---------- 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c index adde93cbca9f..0071a11a6dc3 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -10077,17 +10077,11 @@ int register_netdevice(struct net_device *dev) ret = call_netdevice_notifiers(NETDEV_REGISTER, dev); ret = notifier_to_errno(ret); if (ret) { + /* Expect explicit free_netdev() on failure */ + dev->needs_free_netdev = false; rollback_registered(dev); - rcu_barrier(); - - dev->reg_state = NETREG_UNREGISTERED; - /* We should put the kobject that hold in - * netdev_unregister_kobject(), otherwise - * the net device cannot be freed when - * driver calls free_netdev(), because the - * kobject is being hold. - */ - kobject_put(&dev->dev.kobj); + net_set_todo(dev); + goto out; } /* * Prevent userspace races by waiting until the network