Re: [PATCH net-next v2 2/2] net: bridge: add skb drop reasons to the most common drop points

On Wed, 2024-12-18 at 19:19 +0200, Ido Schimmel wrote:

On Tue, Dec 17, 2024 at 06:07:11PM -0500, Radu Rendec wrote:

quoted

@@ -520,6 +522,16 @@ enum skb_drop_reason {

 	 * enabled.
 	 */
 	SKB_DROP_REASON_ARP_PVLAN_DISABLE,
+	/**
+	 * @SKB_DROP_REASON_MAC_IEEE_MAC_CONTROL: the destination MAC address
+	 * is an IEEE MAC Control address.
+	 */
+	SKB_DROP_REASON_MAC_IEEE_MAC_CONTROL,
+	/**
+	 * @SKB_DROP_REASON_BRIDGE_INGRESS_PORT_NFWD: the STP state of the

s/SKB_DROP_REASON_BRIDGE_INGRESS_PORT_NFWD/SKB_DROP_REASON_BRIDGE_INGRESS_STP_STATE/

Oops :) Good catch!

quoted

+	 * ingress bridge port does not allow frames to be forwarded.
+	 */
+	SKB_DROP_REASON_BRIDGE_INGRESS_STP_STATE,
 	/**
 	 * @SKB_DROP_REASON_MAX: the maximum of core drop reasons, which
 	 * shouldn't be used as a real 'reason' - only for tracing code gen

diff --git a/net/bridge/br_forward.c b/net/bridge/br_forward.c
index e19b583ff2c6d..3e9b462809b0e 100644
--- a/net/bridge/br_forward.c
+++ b/net/bridge/br_forward.c

@@ -201,6 +201,7 @@ void br_flood(struct net_bridge *br, struct sk_buff *skb,

 	      enum br_pkt_type pkt_type, bool local_rcv, bool local_orig,
 	      u16 vid)
 {
+	enum skb_drop_reason reason = SKB_DROP_REASON_NO_TX_TARGET;
 	struct net_bridge_port *prev = NULL;
 	struct net_bridge_port *p;
 

@@ -234,8 +235,11 @@ void br_flood(struct net_bridge *br, struct sk_buff *skb,

 			continue;
 
 		prev = maybe_deliver(prev, p, skb, local_orig);
-		if (IS_ERR(prev))
+		if (IS_ERR(prev)) {
+			WARN_ON_ONCE(PTR_ERR(prev) != -ENOMEM);

I don't think we want to see a stack trace just because someone forgot
to adjust the drop reason to the error code. Maybe just set it to
'NOMEM' if error code is '-ENOMEM', otherwise to 'NOT_SPECIFIED'.

Sure, that was my first choice too, but then I changed my mind. I don't
think there's a 100% clean way of doing this because maybe_deliver()
can return only -ENOMEM today, but that may change in the future. I
will change it back to what I had initially, which is essentially the
same as you suggested.

quoted

+			reason = SKB_DROP_REASON_NOMEM;
 			goto out;
+		}
 	}
 
 	if (!prev)

@@ -249,7 +253,7 @@ void br_flood(struct net_bridge *br, struct sk_buff *skb,

 
 out:
 	if (!local_rcv)
-		kfree_skb(skb);
+		kfree_skb_reason(skb, reason);
 }
 
 #ifdef CONFIG_BRIDGE_IGMP_SNOOPING

@@ -289,6 +293,7 @@ void br_multicast_flood(struct net_bridge_mdb_entry *mdst,

 			struct net_bridge_mcast *brmctx,
 			bool local_rcv, bool local_orig)
 {
+	enum skb_drop_reason reason = SKB_DROP_REASON_NO_TX_TARGET;
 	struct net_bridge_port *prev = NULL;
 	struct net_bridge_port_group *p;
 	bool allow_mode_include = true;

@@ -329,8 +334,11 @@ void br_multicast_flood(struct net_bridge_mdb_entry *mdst,

 		}
 
 		prev = maybe_deliver(prev, port, skb, local_orig);
-		if (IS_ERR(prev))
+		if (IS_ERR(prev)) {
+			WARN_ON_ONCE(PTR_ERR(prev) != -ENOMEM);

Likewise

quoted

+			reason = SKB_DROP_REASON_NOMEM;
 			goto out;
+		}
 delivered:
 		if ((unsigned long)lport >= (unsigned long)port)
 			p = rcu_dereference(p->next);

@@ -349,6 +357,6 @@ void br_multicast_flood(struct net_bridge_mdb_entry *mdst,

 
 out:
 	if (!local_rcv)
-		kfree_skb(skb);
+		kfree_skb_reason(skb, reason);
 }
 #endif

diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
index ceaa5a89b947f..0adad3986c77d 100644
--- a/net/bridge/br_input.c
+++ b/net/bridge/br_input.c

@@ -96,8 +96,10 @@ int br_handle_frame_finish(struct net *net, struct sock *sk, struct sk_buff *skb

 	if (br_mst_is_enabled(br)) {
 		state = BR_STATE_FORWARDING;
 	} else {
-		if (p->state == BR_STATE_DISABLED)
-			goto drop;
+		if (p->state == BR_STATE_DISABLED) {
+			kfree_skb_reason(skb, SKB_DROP_REASON_BRIDGE_INGRESS_STP_STATE);
+			return 0;
+		}

It would be good to keep the error path consolidated with 'goto drop' in
case we ever want to increment a drop counter or do something else that
is common to all the drops.

Did you consider adding a 'reason' variable that is initialized to
'SKB_DROP_REASON_NOT_SPECIFIED' and setting it to the appropriate reason
before 'goto drop'? Seems like a common pattern.

I did not consider it because I didn't realize there was an intention
to keep the error path consolidated. I did that for the two "flood"
functions though. And now that you explained it, I can see why you'd
want to do it here. I will refactor it and post v3 soon if I don't see
any new comments on v2.

Same in br_handle_frame().

quoted

 
 		state = p->state;
 	}

@@ -155,8 +157,10 @@ int br_handle_frame_finish(struct net *net,

struct sock *sk, struct sk_buff *skb
 		}
 	}
 
-	if (state == BR_STATE_LEARNING)
-		goto drop;
+	if (state == BR_STATE_LEARNING) {
+		kfree_skb_reason(skb,
SKB_DROP_REASON_BRIDGE_INGRESS_STP_STATE);
+		return 0;
+	}
 
 	BR_INPUT_SKB_CB(skb)->brdev = br->dev;
 	BR_INPUT_SKB_CB(skb)->src_port_isolated = !!(p->flags &
BR_ISOLATED);

@@ -331,8 +335,10 @@ static rx_handler_result_t

br_handle_frame(struct sk_buff **pskb)
 	if (unlikely(skb->pkt_type == PACKET_LOOPBACK))
 		return RX_HANDLER_PASS;
 
-	if (!is_valid_ether_addr(eth_hdr(skb)->h_source))
-		goto drop;
+	if (!is_valid_ether_addr(eth_hdr(skb)->h_source)) {
+		kfree_skb_reason(skb,
SKB_DROP_REASON_MAC_INVALID_SOURCE);
+		return RX_HANDLER_CONSUMED;
+	}
 
 	skb = skb_share_check(skb, GFP_ATOMIC);
 	if (!skb)

@@ -374,7 +380,8 @@ static rx_handler_result_t

br_handle_frame(struct sk_buff **pskb)
 			return RX_HANDLER_PASS;
 
 		case 0x01:	/* IEEE MAC (Pause) */
-			goto drop;
+			kfree_skb_reason(skb,
SKB_DROP_REASON_MAC_IEEE_MAC_CONTROL);
+			return RX_HANDLER_CONSUMED;
 
 		case 0x0E:	/* 802.1AB LLDP */
 			fwd_mask |= p->br->group_fwd_mask;

@@ -423,8 +430,7 @@ static rx_handler_result_t

br_handle_frame(struct sk_buff **pskb)
 
 		return nf_hook_bridge_pre(skb, pskb);
 	default:
-drop:
-		kfree_skb(skb);
+		kfree_skb_reason(skb,
SKB_DROP_REASON_BRIDGE_INGRESS_STP_STATE);
 	}
 	return RX_HANDLER_CONSUMED;
 }
-- 
2.47.1