netdev - Re: [PATCH net-next 1/2] net: bridge: Flush multicast groups when snooping is disabled

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Message-ID: <aa92bbb5-c64b-4262-a382-ae5609254606@blackwall.org>
Date: Fri, 24 Oct 2025 10:42:57 +0300
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Petr Machata <petrm@...dia.com>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Cc: Simon Horman <horms@...nel.org>, Ido Schimmel <idosch@...dia.com>,
 bridge@...ts.linux.dev, mlxsw@...dia.com
Subject: Re: [PATCH net-next 1/2] net: bridge: Flush multicast groups when
 snooping is disabled

On 10/23/25 17:45, Petr Machata wrote:
> When forwarding multicast packets, the bridge takes MDB into account when
> IGMP / MLD snooping is enabled. Currently, when snooping is disabled, the
> MDB is retained, even though it is not used anymore.
> 
> At the same time, during the time that snooping is disabled, the IGMP / MLD
> control packets are obviously ignored, and after the snooping is reenabled,
> the administrator has to assume it is out of sync. In particular, missed
> join and leave messages would lead to traffic being forwarded to wrong
> interfaces.
> 
> Keeping the MDB entries around thus serves no purpose, and just takes
> memory. Note also that disabling per-VLAN snooping does actually flush the
> relevant MDB entries.
> 
> This patch flushes non-permanent MDB entries as global snooping is
> disabled.
> 
> Signed-off-by: Petr Machata <petrm@...dia.com>
> Reviewed-by: Ido Schimmel <idosch@...dia.com>
> ---
>  net/bridge/br_multicast.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
> index 22d12e545966..d55a4ab87837 100644
> --- a/net/bridge/br_multicast.c
> +++ b/net/bridge/br_multicast.c
> @@ -4649,6 +4649,14 @@ static void br_multicast_start_querier(struct net_bridge_mcast *brmctx,
>  	rcu_read_unlock();
>  }
>  
> +static void br_multicast_del_grps(struct net_bridge *br)
> +{
> +	struct net_bridge_port *port;
> +
> +	list_for_each_entry(port, &br->port_list, list)
> +		__br_multicast_disable_port_ctx(&port->multicast_ctx);
> +}
> +
>  int br_multicast_toggle(struct net_bridge *br, unsigned long val,
>  			struct netlink_ext_ack *extack)
>  {
> @@ -4669,6 +4677,7 @@ int br_multicast_toggle(struct net_bridge *br, unsigned long val,
>  	br_opt_toggle(br, BROPT_MULTICAST_ENABLED, !!val);
>  	if (!br_opt_get(br, BROPT_MULTICAST_ENABLED)) {
>  		change_snoopers = true;
> +		br_multicast_del_grps(br);
>  		goto unlock;
>  	}
>  

I've actually thought about this, disabling multicast has always been weird in the
bridge and I think this is an improvement:

Acked-by: Nikolay Aleksandrov <razor@...ckwall.org>