Re: [PATCH v3 net-next 08/12] net: dsa: replay port and host-joined mdb entries when joining the bridge

[Nikolay Aleksandrov]

On 21/03/2021 00:34, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@xxxxxxx>
> 
> I have udhcpcd in my system and this is configured to bring interfaces
> up as soon as they are created.
> 
> I create a bridge as follows:
> 
> ip link add br0 type bridge
> 
> As soon as I create the bridge and udhcpcd brings it up, I also have
> avahi which automatically starts sending IPv6 packets to advertise some
> local services, and because of that, the br0 bridge joins the following
> IPv6 groups due to the code path detailed below:
> 
> 33:33:ff:6d:c1:9c vid 0
> 33:33:00:00:00:6a vid 0
> 33:33:00:00:00:fb vid 0
> 
> br_dev_xmit
> -> br_multicast_rcv
>    -> br_ip6_multicast_add_group
>       -> __br_multicast_add_group
>          -> br_multicast_host_join
>             -> br_mdb_notify
> 
> This is all fine, but inside br_mdb_notify we have br_mdb_switchdev_host
> hooked up, and switchdev will attempt to offload the host joined groups
> to an empty list of ports. Of course nobody offloads them.
> 
> Then when we add a port to br0:
> 
> ip link set swp0 master br0
> 
> the bridge doesn't replay the host-joined MDB entries from br_add_if,
> and eventually the host joined addresses expire, and a switchdev
> notification for deleting it is emitted, but surprise, the original
> addition was already completely missed.
> 
> The strategy to address this problem is to replay the MDB entries (both
> the port ones and the host joined ones) when the new port joins the
> bridge, similar to what vxlan_fdb_replay does (in that case, its FDB can
> be populated and only then attached to a bridge that you offload).
> However there are 2 possibilities: the addresses can be 'pushed' by the
> bridge into the port, or the port can 'pull' them from the bridge.
> 
> Considering that in the general case, the new port can be really late to
> the party, and there may have been many other switchdev ports that
> already received the initial notification, we would like to avoid
> delivering duplicate events to them, since they might misbehave. And
> currently, the bridge calls the entire switchdev notifier chain, whereas
> for replaying it should just call the notifier block of the new guy.
> But the bridge doesn't know what is the new guy's notifier block, it
> just knows where the switchdev notifier chain is. So for simplification,
> we make this a driver-initiated pull for now, and the notifier block is
> passed as an argument.
> 
> To emulate the calling context for mdb objects (deferred and put on the
> blocking notifier chain), we must iterate under RCU protection through
> the bridge's mdb entries, queue them, and only call them once we're out
> of the RCU read-side critical section.
> 
> Suggested-by: Ido Schimmel <idosch@xxxxxxxxxx>
> Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>
> ---
> Changes in v3:
> - Removed the implication that avahi is crap from the commit message.
> - Made the br_mdb_replay shim return -EOPNOTSUPP.
> 
>  include/linux/if_bridge.h |  9 +++++
>  net/bridge/br_mdb.c       | 84 +++++++++++++++++++++++++++++++++++++++
>  net/dsa/dsa_priv.h        |  2 +
>  net/dsa/port.c            |  6 +++
>  net/dsa/slave.c           |  2 +-
>  5 files changed, 102 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/if_bridge.h b/include/linux/if_bridge.h
> index ebd16495459c..f6472969bb44 100644
> --- a/include/linux/if_bridge.h
> +++ b/include/linux/if_bridge.h
> @@ -69,6 +69,8 @@ bool br_multicast_has_querier_anywhere(struct net_device *dev, int proto);
>  bool br_multicast_has_querier_adjacent(struct net_device *dev, int proto);
>  bool br_multicast_enabled(const struct net_device *dev);
>  bool br_multicast_router(const struct net_device *dev);
> +int br_mdb_replay(struct net_device *br_dev, struct net_device *dev,
> +		  struct notifier_block *nb, struct netlink_ext_ack *extack);
>  #else
>  static inline int br_multicast_list_adjacent(struct net_device *dev,
>  					     struct list_head *br_ip_list)
> @@ -93,6 +95,13 @@ static inline bool br_multicast_router(const struct net_device *dev)
>  {
>  	return false;
>  }
> +static inline int br_mdb_replay(struct net_device *br_dev,
> +				struct net_device *dev,
> +				struct notifier_block *nb,
> +				struct netlink_ext_ack *extack)
> +{
> +	return -EOPNOTSUPP;
> +}
>  #endif
>  
>  #if IS_ENABLED(CONFIG_BRIDGE) && IS_ENABLED(CONFIG_BRIDGE_VLAN_FILTERING)
> diff --git a/net/bridge/br_mdb.c b/net/bridge/br_mdb.c
> index 8846c5bcd075..23973186094c 100644
> --- a/net/bridge/br_mdb.c
> +++ b/net/bridge/br_mdb.c
> @@ -506,6 +506,90 @@ static void br_mdb_complete(struct net_device *dev, int err, void *priv)
>  	kfree(priv);
>  }
>  
> +static int br_mdb_replay_one(struct notifier_block *nb, struct net_device *dev,
> +			     struct net_bridge_mdb_entry *mp, int obj_id,
> +			     struct net_device *orig_dev,
> +			     struct netlink_ext_ack *extack)
> +{
> +	struct switchdev_notifier_port_obj_info obj_info = {
> +		.info = {
> +			.dev = dev,
> +			.extack = extack,
> +		},
> +	};
> +	struct switchdev_obj_port_mdb mdb = {
> +		.obj = {
> +			.orig_dev = orig_dev,
> +			.id = obj_id,
> +		},
> +		.vid = mp->addr.vid,
> +	};
> +	int err;
> +
> +	if (mp->addr.proto == htons(ETH_P_IP))
> +		ip_eth_mc_map(mp->addr.dst.ip4, mdb.addr);
> +#if IS_ENABLED(CONFIG_IPV6)
> +	else if (mp->addr.proto == htons(ETH_P_IPV6))
> +		ipv6_eth_mc_map(&mp->addr.dst.ip6, mdb.addr);
> +#endif
> +	else
> +		ether_addr_copy(mdb.addr, mp->addr.dst.mac_addr);
> +
> +	obj_info.obj = &mdb.obj;
> +
> +	err = nb->notifier_call(nb, SWITCHDEV_PORT_OBJ_ADD, &obj_info);
> +	return notifier_to_errno(err);
> +}
> +
> +int br_mdb_replay(struct net_device *br_dev, struct net_device *dev,
> +		  struct notifier_block *nb, struct netlink_ext_ack *extack)
> +{
> +	struct net_bridge_mdb_entry *mp;
> +	struct list_head mdb_list;

If you use LIST_HEAD(mdb_list)...

> +	struct net_bridge *br;
> +	int err = 0;
> +
> +	ASSERT_RTNL();
> +
> +	INIT_LIST_HEAD(&mdb_list);

... you can drop this one.

> +
> +	if (!netif_is_bridge_master(br_dev) || !netif_is_bridge_port(dev))
> +		return -EINVAL;
> +
> +	br = netdev_priv(br_dev);
> +
> +	if (!br_opt_get(br, BROPT_MULTICAST_ENABLED))
> +		return 0;
> +
> +	hlist_for_each_entry(mp, &br->mdb_list, mdb_node) {

You cannot walk over these lists without the multicast lock or RCU. RTNL is not
enough because of various timers and leave messages that can alter both the mdb_list
and the port group lists. I'd prefer RCU to avoid blocking the bridge mcast.

> +		struct net_bridge_port_group __rcu **pp;
> +		struct net_bridge_port_group *p;
> +
> +		if (mp->host_joined) {
> +			err = br_mdb_replay_one(nb, dev, mp,
> +						SWITCHDEV_OBJ_ID_HOST_MDB,
> +						br_dev, extack);
> +			if (err)
> +				return err;
> +		}
> +
> +		for (pp = &mp->ports; (p = rtnl_dereference(*pp)) != NULL;
> +		     pp = &p->next) {
> +			if (p->key.port->dev != dev)
> +				continue;
> +
> +			err = br_mdb_replay_one(nb, dev, mp,
> +						SWITCHDEV_OBJ_ID_PORT_MDB,
> +						dev, extack);
> +			if (err)
> +				return err;> +		}
> +	}
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(br_mdb_replay);

EXPORT_SYMBOL_GPL

> +
>  static void br_mdb_switchdev_host_port(struct net_device *dev,
>  				       struct net_device *lower_dev,
>  				       struct net_bridge_mdb_entry *mp,
> diff --git a/net/dsa/dsa_priv.h b/net/dsa/dsa_priv.h
> index b8778c5d8529..b14c43cb88bb 100644
> --- a/net/dsa/dsa_priv.h
> +++ b/net/dsa/dsa_priv.h
> @@ -262,6 +262,8 @@ static inline bool dsa_tree_offloads_bridge_port(struct dsa_switch_tree *dst,
>  
>  /* slave.c */
>  extern const struct dsa_device_ops notag_netdev_ops;
> +extern struct notifier_block dsa_slave_switchdev_blocking_notifier;
> +
>  void dsa_slave_mii_bus_init(struct dsa_switch *ds);
>  int dsa_slave_create(struct dsa_port *dp);
>  void dsa_slave_destroy(struct net_device *slave_dev);
> diff --git a/net/dsa/port.c b/net/dsa/port.c
> index 95e6f2861290..3e61e9e6675c 100644
> --- a/net/dsa/port.c
> +++ b/net/dsa/port.c
> @@ -199,6 +199,12 @@ static int dsa_port_switchdev_sync(struct dsa_port *dp,
>  	if (err && err != -EOPNOTSUPP)
>  		return err;
>  
> +	err = br_mdb_replay(br, brport_dev,
> +			    &dsa_slave_switchdev_blocking_notifier,
> +			    extack);
> +	if (err && err != -EOPNOTSUPP)
> +		return err;
> +
>  	return 0;
>  }
>  
> diff --git a/net/dsa/slave.c b/net/dsa/slave.c
> index 1ff48be476bb..b974d8f84a2e 100644
> --- a/net/dsa/slave.c
> +++ b/net/dsa/slave.c
> @@ -2396,7 +2396,7 @@ static struct notifier_block dsa_slave_switchdev_notifier = {
>  	.notifier_call = dsa_slave_switchdev_event,
>  };
>  
> -static struct notifier_block dsa_slave_switchdev_blocking_notifier = {
> +struct notifier_block dsa_slave_switchdev_blocking_notifier = {
>  	.notifier_call = dsa_slave_switchdev_blocking_event,
>  };
>  
>