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

[Ido Schimmel <idosch@idosch.org>]

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

> +	 * 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'.

> +			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

> +			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.

Same in br_handle_frame().

>  
>  		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
>