[PATCH RFC] net: bridge: don't flood known multicast traffic when snooping is enabled

[Nikolay Aleksandrov <nikolay@cumulusnetworks.com>]

The behaviour since b00589af3b04 ("bridge: disable snooping if there is
no querier") is wrong, we shouldn't be flooding multicast traffic when
there is an mdb entry and we know where it should be forwarded to when
multicast snooping is enabled. This patch changes the behaviour to not
flood known unicast traffic. I'll give two obviously broken cases:
 - most obvious: static mdb created by the user with snooping enabled
 - user-space daemon controlling the mdb table (e.g. MLAG)

Every user would expect to have traffic forwarded only to the configured
mdb destination when snooping is enabled, instead now to get that one
needs to enable both snooping and querier. Enabling querier on all
switches could be problematic and is not a good solution, for example
as summarized by our multicast experts:
"every switch would send an IGMP query for any random multicast traffic it
received across the entire domain and it would send it forever as long as a
host exists wanting that stream even if it has no downstream/directly
connected receivers"

Sending as an RFC to get the discussion going, but I'm strongly for
removing this behaviour and would like to send this patch officially.

We could make this behaviour possible via a knob if necessary, but
it really should not be the default.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
---
 net/bridge/br_device.c | 3 +--
 net/bridge/br_input.c  | 3 +--
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/net/bridge/br_device.c b/net/bridge/br_device.c
index 013323b6dbe4..2aa8a6509924 100644
--- a/net/bridge/br_device.c
+++ b/net/bridge/br_device.c
@@ -96,8 +96,7 @@ netdev_tx_t br_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 		}
 
 		mdst = br_mdb_get(br, skb, vid);
-		if ((mdst || BR_INPUT_SKB_CB_MROUTERS_ONLY(skb)) &&
-		    br_multicast_querier_exists(br, eth_hdr(skb)))
+		if (mdst || BR_INPUT_SKB_CB_MROUTERS_ONLY(skb))
 			br_multicast_flood(mdst, skb, false, true);
 		else
 			br_flood(br, skb, BR_PKT_MULTICAST, false, true);
diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
index 5ea7e56119c1..aae78095cf67 100644
--- a/net/bridge/br_input.c
+++ b/net/bridge/br_input.c
@@ -136,8 +136,7 @@ int br_handle_frame_finish(struct net *net, struct sock *sk, struct sk_buff *skb
 	switch (pkt_type) {
 	case BR_PKT_MULTICAST:
 		mdst = br_mdb_get(br, skb, vid);
-		if ((mdst || BR_INPUT_SKB_CB_MROUTERS_ONLY(skb)) &&
-		    br_multicast_querier_exists(br, eth_hdr(skb))) {
+		if (mdst || BR_INPUT_SKB_CB_MROUTERS_ONLY(skb)) {
 			if ((mdst && mdst->host_joined) ||
 			    br_multicast_is_router(br)) {
 				local_rcv = true;
-- 
2.17.2