Re: [PATCH net-next] net: dsa: unsync host addresses when destroying user port

[Vladimir Oltean <olteanv@gmail.com>]

On Fri, Jul 24, 2026 at 01:57:35AM +0300, Vladimir Oltean wrote:
> On Wed, Jul 22, 2026 at 01:12:47AM +0100, Daniel Golle wrote:
> > When a user port is destroyed while addresses are still synced to it,
> > e.g. multicast addresses synced by a bridge the port is a member of,
> > the host FDB/MDB entries these addresses installed on the CPU port are
> > never removed: the only removal path is dsa_user_unsync_uc()/_mc() via
> > ndo_set_rx_mode, and __dev_set_rx_mode() does not call the ndo on a
> > device which is down. By the time the bridge unsyncs its addresses in
> > del_nbp() during unregistration, the netdev has already been closed,
> > so the unsync never reaches DSA and the entries linger until
> > dsa_switch_release_ports() reports them:
> > 
> >   Cleaning up multicast address 33:33:00:00:00:01 vid 0 from port 9
> > 
> > This happens on every unbind of a DSA driver supporting host address
> > filtering while its ports are up.
> > 
> > Unsync the host addresses in dsa_user_destroy() before unregistering
> > the netdev, at a point where the driver can still process the
> > deletion, just like dsa_user_change_conduit() already does when
> > migrating host addresses to a new conduit.
> > 
> > Signed-off-by: Daniel Golle <daniel@makrotopia.org>
> > ---
> >  net/dsa/user.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/net/dsa/user.c b/net/dsa/user.c
> > index 03c7af6abe18..a7dabb645036 100644
> > --- a/net/dsa/user.c
> > +++ b/net/dsa/user.c
> > @@ -2885,6 +2885,7 @@ void dsa_user_destroy(struct net_device *user_dev)
> >  
> >  	netif_carrier_off(user_dev);
> >  	rtnl_lock();
> > +	dsa_user_unsync_ha(user_dev);
> >  	netdev_upper_dev_unlink(conduit, user_dev);
> >  	unregister_netdevice(user_dev);
> >  	phylink_disconnect_phy(dp->pl);
> > -- 
> > 2.55.0
> 
> Sorry, I noticed this patch late. Something doesn't add up - I don't
> understand what makes the unregistration path unique, since according to
> all you've said, it should be enough to remove the user port from the
> bridge while administratively down, and it should lead to the same
> effect (no unsync event triggered). In that case, maybe the
> dsa_user_unsync_ha() belongs somewhere in dsa_user_close(), near
> dsa_user_host_uc_uninstall().
> 
> I will return tomorrow with more comments after I do some testing.

Back with some more comments.

Your statement "multicast addresses synced by a bridge the port is a
member of" is not correct. The bridge does not call dev_mc_add().
The multicast addresses come from a different place - likely from
net/ipv6/mcast.c instead.

Therefore, the part of the explanation that ties del_nbp() to the chain
of events truly has no relationship and should be dropped. The
host-joined multicast groups for the bridge are all synced to hardware
through the SWITCHDEV_OBJ_ID_HOST_MDB mechanism.

The minimal reproducer for the problem you observed should be:

$ ip link set swp0 up
$ ip link set swp0 down
$ echo <device-name> > /path/to/driver/unbind

and the correct fix is to put the dsa_user_unsync_ha() call where I
suggested earlier - in dsa_user_close().

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