Re: [PATCH net] net: bridge: switchdev: don't notify FDB entries with "master dynamic"

[Ido Schimmel <idosch@nvidia.com>]

On Mon, Apr 10, 2023 at 11:49:51PM +0300, Vladimir Oltean wrote:
> There is a structural problem in switchdev, where the flag bits in
> struct switchdev_notifier_fdb_info (added_by_user, is_local etc) only
> represent a simplified / denatured view of what's in struct
> net_bridge_fdb_entry :: flags (BR_FDB_ADDED_BY_USER, BR_FDB_LOCAL etc).
> Each time we want to pass more information about struct
> net_bridge_fdb_entry :: flags to struct switchdev_notifier_fdb_info
> (here, BR_FDB_STATIC), we find that FDB entries were already notified to
> switchdev with no regard to this flag, and thus, switchdev drivers had
> no indication whether the notified entries were static or not.
> 
> For example, this command:
> 
> ip link add br0 type bridge && ip link set swp0 master br0
> bridge fdb add dev swp0 00:01:02:03:04:05 master dynamic
> 
> causes a struct net_bridge_fdb_entry to be passed to
> br_switchdev_fdb_notify() which has a single flag set:
> BR_FDB_ADDED_BY_USER.
> 
> This is further passed to the switchdev notifier chain, where interested
> drivers have no choice but to assume this is a static FDB entry.
> So currently, all drivers offload it to hardware as such.
> 
> bridge fdb get 00:01:02:03:04:05 dev swp0 master
> 00:01:02:03:04:05 dev swp0 offload master br0
> 
> The software FDB entry expires after the $ageing_time and the bridge
> notifies its deletion as well, so it eventually disappears from hardware
> too.
> 
> This is a problem, because it is actually desirable to start offloading
> "master dynamic" FDB entries correctly, and this is how the current
> incorrect behavior was discovered.
> 
> To see why the current behavior of "here's a static FDB entry when you
> asked for a dynamic one" is incorrect, it is possible to imagine a
> scenario like below, where this decision could lead to packet loss:
> 
> Step 1: management prepares FDB entries like this:
> 
> bridge fdb add dev swp0 ${MAC_A} master dynamic
> bridge fdb add dev swp2 ${MAC_B} master dynamic
> 
>         br0
>       /  |  \
>      /   |   \
>   swp0  swp1  swp2
>    |           |
>    A           B
> 
> Step 2: station A migrates to swp1 (assume that swp0's link doesn't flap
> during that time so that the port isn't flushed, for example station A
> was behind an intermediary switch):
> 
>         br0
>       /  |  \
>      /   |   \
>   swp0  swp1  swp2
>    |     |     |
>          A     B
> 
> Whenever A wants to ping B, its packets will be autonomously forwarded
> by the switch (because ${MAC_B} is known). So the software will never
> see packets from ${MAC_A} as source address, and will never know it
> needs to invalidate the dynamic FDB entry towards swp0. As for the
> hardware FDB entry, that's static, it doesn't move when the station
> roams.
> 
> So when B wants to reply to A's pings, the switch will forward those
> replies to swp0 until the software bridge ages out its dynamic entry,
> and that can cause connectivity loss for up to 5 minutes after roaming.
> 
> With a correctly offloaded dynamic FDB entry, the switch would update
> its entry for ${MAC_A} to be towards swp1 as soon as it sees packets
> from it (no need for CPU intervention).
> 
> Looking at tools/testing/selftests/net/forwarding/, there is no valid
> use of the "bridge fdb add ... master dynamic" command there, so I am
> fairly confident that no one used to rely on this behavior.

Yes, but there are tests that use "extern_learn". If you post a v2 that
takes "BR_FDB_ADDED_BY_EXT_LEARN" into account, then I can ask Petr to
run it through our regression and report back (not sure we will make it
to this week's PR though).

Thanks

> 
> With the change in place, these FDB entries are no longer offloaded:
> 
> bridge fdb get 00:01:02:03:04:05 dev swp0 master
> 00:01:02:03:04:05 dev swp0 master br0
> 
> and this also constitutes a better way (assuming a backport to stable
> kernels) for user space to determine whether the switchdev driver did
> actually act upon the dynamic FDB entry or not.
> 
> Fixes: 6b26b51b1d13 ("net: bridge: Add support for notifying devices about FDB add/del")
> Link: https://lore.kernel.org/netdev/20230327115206.jk5q5l753aoelwus@skbuf/
> Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
> ---
>  net/bridge/br_switchdev.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/bridge/br_switchdev.c b/net/bridge/br_switchdev.c
> index de18e9c1d7a7..0ec3d5e5e77d 100644
> --- a/net/bridge/br_switchdev.c
> +++ b/net/bridge/br_switchdev.c
> @@ -148,6 +148,10 @@ br_switchdev_fdb_notify(struct net_bridge *br,
>  	if (test_bit(BR_FDB_LOCKED, &fdb->flags))
>  		return;
>  
> +	if (test_bit(BR_FDB_ADDED_BY_USER, &fdb->flags) &&
> +	    !test_bit(BR_FDB_STATIC, &fdb->flags))
> +		return;
> +
>  	br_switchdev_fdb_populate(br, &item, fdb, NULL);
>  
>  	switch (type) {
> -- 
> 2.34.1
>