# \[PATCH net v5 4/5\] amt: make pre-query report drops visible - From: Omar Ramadan \ - Date: 2026-11-05 12:50:20 - List: netdev - Page: https://ratatoskr.run/netdev/2026/11/17870463 - Thread: https://ratatoskr.run/netdev/2026/11/17870459/t.md ```text A gateway cannot forward an IGMP or MLD report until it has received the relay's Membership Query for that family. The Query supplies the response MAC the Update echoes, so dropping an early report is required, but doing so silently leaves operators with a dark multicast path and no indication why the join never happened. Emit a family-specific debug message before taking the drop path. tx_dropped could not show these drops either: amt_dev_xmit() reaches the same tx_dropped increment after amt_send_membership_update() has handed the report to the tunnel, so in gateway mode every forwarded report was also counted as a drop. Return NETDEV_TX_OK directly once the report has been sent, so tx_dropped counts only reports that were discarded. The device is LLTX, so amt_dev_xmit() can run on several CPUs at once; use DEV_STATS_INC() for its tx_dropped and tx_errors updates so concurrent increments are not lost. Found by code inspection while debugging an IPv6-in-IPv6 forwarding failure in a selftest for the AMT IPv6 outer transport. This drop can leave one address family dark, so it was a suspect, but that was not confirmed: the failure was later traced to the selftest, which sent a single datagram without waiting for the relay's multicast routes to converge. The drop has not been observed in production, and there is no oops or trace. Review of v3 found the tx_dropped miscount and the non-atomic counter updates. Fixes: cbc21dc1cfe9 ("amt: add data plane of amt interface") Assisted-by: LLM Signed-off-by: Omar Ramadan --- drivers/net/amt.c | 42 +++++++++++++++++++++++++----------------- 1 file changed, 25 insertions(+), 17 deletions(-) diff --git a/drivers/net/amt.c b/drivers/net/amt.c index 68f08f5f6..26a942c2a 100644 --- a/drivers/net/amt.c +++ b/drivers/net/amt.c @@ -701,7 +701,7 @@ static void amt_send_discovery(struct amt_dev *amt) IPPROTO_UDP, 0, amt->stream_dev->ifindex); if (IS_ERR(rt)) { - amt->dev->stats.tx_errors++; + DEV_STATS_INC(amt->dev, tx_errors); goto out; } @@ -711,7 +711,7 @@ static void amt_send_discovery(struct amt_dev *amt) skb = netdev_alloc_skb_ip_align(amt->dev, len); if (!skb) { ip_rt_put(rt); - amt->dev->stats.tx_errors++; + DEV_STATS_INC(amt->dev, tx_errors); goto out; } @@ -756,7 +756,7 @@ static void amt_send_discovery(struct amt_dev *amt) ip_send_check(iph); err = ip_local_out(amt->net, sk, skb); if (unlikely(net_xmit_eval(err))) - amt->dev->stats.tx_errors++; + DEV_STATS_INC(amt->dev, tx_errors); amt_update_gw_status(amt, AMT_STATUS_SENT_DISCOVERY, true); out: @@ -792,7 +792,7 @@ static void amt_send_request(struct amt_dev *amt, bool v6) IPPROTO_UDP, 0, amt->stream_dev->ifindex); if (IS_ERR(rt)) { - amt->dev->stats.tx_errors++; + DEV_STATS_INC(amt->dev, tx_errors); goto out; } @@ -802,7 +802,7 @@ static void amt_send_request(struct amt_dev *amt, bool v6) skb = netdev_alloc_skb_ip_align(amt->dev, len); if (!skb) { ip_rt_put(rt); - amt->dev->stats.tx_errors++; + DEV_STATS_INC(amt->dev, tx_errors); goto out; } @@ -849,7 +849,7 @@ static void amt_send_request(struct amt_dev *amt, bool v6) ip_send_check(iph); err = ip_local_out(amt->net, sk, skb); if (unlikely(net_xmit_eval(err))) - amt->dev->stats.tx_errors++; + DEV_STATS_INC(amt->dev, tx_errors); out: rcu_read_unlock(); @@ -871,7 +871,7 @@ static void amt_send_igmp_gq(struct amt_dev *amt, skb_pull(skb, sizeof(struct ethhdr)); if (amt_send_membership_query(amt, skb, tunnel, false)) { - amt->dev->stats.tx_dropped++; + DEV_STATS_INC(amt->dev, tx_dropped); kfree_skb(skb); } } @@ -915,7 +915,7 @@ static struct sk_buff *amt_build_mld_gq(struct amt_dev *amt) if (ipv6_dev_get_saddr(amt->net, amt->dev, &ip6h->daddr, 0, &ip6h->saddr)) { - amt->dev->stats.tx_errors++; + DEV_STATS_INC(amt->dev, tx_errors); kfree_skb(skb); return NULL; } @@ -959,7 +959,7 @@ static void amt_send_mld_gq(struct amt_dev *amt, struct amt_tunnel_list *tunnel) skb_pull(skb, sizeof(struct ethhdr)); if (amt_send_membership_query(amt, skb, tunnel, true)) { - amt->dev->stats.tx_dropped++; + DEV_STATS_INC(amt->dev, tx_dropped); kfree_skb(skb); } } @@ -1319,7 +1319,7 @@ static netdev_tx_t amt_dev_xmit(struct sk_buff *skb, struct net_device *dev) group.ip6 = ipv6_hdr(skb)->daddr; #endif } else { - dev->stats.tx_errors++; + DEV_STATS_INC(dev, tx_errors); goto free; } @@ -1332,12 +1332,21 @@ static netdev_tx_t amt_dev_xmit(struct sk_buff *skb, struct net_device *dev) /* Gateway only passes IGMP/MLD packets */ if (!report) goto free; + /* A validated report can only be forwarded after the relay's + * family-specific Membership Query supplies the state echoed + * by the Membership Update. Log this readiness failure before + * the shared drop path accounts it. + */ if ((!v6 && !READ_ONCE(amt->ready4)) || - (v6 && !READ_ONCE(amt->ready6))) + (v6 && !READ_ONCE(amt->ready6))) { + netdev_dbg(dev, "drop %s report: waiting for Membership Query\n", + v6 ? "MLD" : "IGMP"); goto free; + } if (amt_send_membership_update(amt, skb, v6)) goto free; - goto unlock; + /* Sent: the tunnel consumed the skb. Not a drop. */ + return NETDEV_TX_OK; } else if (amt->mode == AMT_MODE_RELAY) { if (!data) goto free; @@ -1366,8 +1375,7 @@ static netdev_tx_t amt_dev_xmit(struct sk_buff *skb, struct net_device *dev) return NETDEV_TX_OK; free: dev_kfree_skb(skb); -unlock: - dev->stats.tx_dropped++; + DEV_STATS_INC(dev, tx_dropped); return NETDEV_TX_OK; } @@ -2730,7 +2738,7 @@ static void amt_send_advertisement(struct amt_dev *amt, __be32 nonce, IPPROTO_UDP, 0, amt->stream_dev->ifindex); if (IS_ERR(rt)) { - amt->dev->stats.tx_errors++; + DEV_STATS_INC(amt->dev, tx_errors); goto out; } @@ -2740,7 +2748,7 @@ static void amt_send_advertisement(struct amt_dev *amt, __be32 nonce, skb = netdev_alloc_skb_ip_align(amt->dev, len); if (!skb) { ip_rt_put(rt); - amt->dev->stats.tx_errors++; + DEV_STATS_INC(amt->dev, tx_errors); goto out; } @@ -2786,7 +2794,7 @@ static void amt_send_advertisement(struct amt_dev *amt, __be32 nonce, ip_send_check(iph); err = ip_local_out(amt->net, sk, skb); if (unlikely(net_xmit_eval(err))) - amt->dev->stats.tx_errors++; + DEV_STATS_INC(amt->dev, tx_errors); out: rcu_read_unlock(); -- 2.47.3 ```