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

On Thu, 2024-12-12 at 18:14 +0200, Ido Schimmel wrote:

On Sun, Dec 08, 2024 at 05:18:05PM -0500, Radu Rendec wrote:

quoted

The bridge input code may drop frames for various reasons and at various
points in the ingress handling logic. Currently kfree_skb() is used
everywhere, and therefore no drop reason is specified. Add drop reasons
to the most common drop points.

The purpose of this patch is to address the most common drop points on
the bridge ingress path. It does not exhaustively add drop reasons to
the entire bridge code. The intention here is to incrementally add drop
reasons to the rest of the bridge code in follow up patches.

Most of the skb drop points that are addressed in this patch can be
easily tested by sending crafted packets. The diagram below shows a
simple test configuration, and some examples using `packit`(*) are
also included. The bridge is set up with STP disabled.
(*) https://github.com/resurrecting-open-source-projects/packit

The following changes were *not* tested:
* SKB_DROP_REASON_BRIDGE_NO_EGRESS_PORT in br_multicast_flood(). I could
  not find an easy way to make a crafted packet get there.
* SKB_DROP_REASON_BRIDGE_INGRESS_PORT_NFWD in br_handle_frame_finish()
  when the port state is BR_STATE_DISABLED, because in that case the
  frame is already dropped in the switch/case block at the end of
  br_handle_frame().

    +---+---+
    |  br0  |
    +---+---+
        |
    +---+---+  veth pair  +-------+
    | veth0 +-------------+ xeth0 |
    +-------+             +-------+

SKB_DROP_REASON_MAC_INVALID_SOURCE - br_handle_frame()
packit -t UDP -s 192.168.0.1 -d 192.168.0.2 -S 8000 -D 8000 \
  -e 01:22:33:44:55:66 -E aa:bb:cc:dd:ee:ff -c 1 \
  -p '0x de ad be ef' -i xeth0

SKB_DROP_REASON_MAC_IEEE_MAC_CONTROL - br_handle_frame()
packit -t UDP -s 192.168.0.1 -d 192.168.0.2 -S 8000 -D 8000 \
  -e 02:22:33:44:55:66 -E 01:80:c2:00:00:01 -c 1 \
  -p '0x de ad be ef' -i xeth0

SKB_DROP_REASON_BRIDGE_INGRESS_PORT_NFWD - br_handle_frame()
bridge link set dev veth0 state 0 # disabled
packit -t UDP -s 192.168.0.1 -d 192.168.0.2 -S 8000 -D 8000 \
  -e 02:22:33:44:55:66 -E aa:bb:cc:dd:ee:ff -c 1 \
  -p '0x de ad be ef' -i xeth0

SKB_DROP_REASON_BRIDGE_INGRESS_PORT_NFWD - br_handle_frame_finish()
bridge link set dev veth0 state 2 # learning
packit -t UDP -s 192.168.0.1 -d 192.168.0.2 -S 8000 -D 8000 \
  -e 02:22:33:44:55:66 -E aa:bb:cc:dd:ee:ff -c 1 \
  -p '0x de ad be ef' -i xeth0

SKB_DROP_REASON_BRIDGE_NO_EGRESS_PORT - br_flood()
packit -t UDP -s 192.168.0.1 -d 192.168.0.2 -S 8000 -D 8000 \
  -e 02:22:33:44:55:66 -E aa:bb:cc:dd:ee:ff -c 1 \
  -p '0x de ad be ef' -i xeth0

Signed-off-by: Radu Rendec <redacted>
---
 include/net/dropreason-core.h | 18 ++++++++++++++++++
 net/bridge/br_forward.c       |  4 ++--
 net/bridge/br_input.c         | 24 +++++++++++++++---------
 3 files changed, 35 insertions(+), 11 deletions(-)

diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
index c29282fabae6..1f2ae5b387c1 100644
--- a/include/net/dropreason-core.h
+++ b/include/net/dropreason-core.h

@@ -108,6 +108,9 @@

 	FN(TUNNEL_TXINFO)		\
 	FN(LOCAL_MAC)			\
 	FN(ARP_PVLAN_DISABLE)		\
+	FN(MAC_IEEE_MAC_CONTROL)	\
+	FN(BRIDGE_INGRESS_PORT_NFWD)	\
+	FN(BRIDGE_NO_EGRESS_PORT)	\
 	FNe(MAX)
 
 /**

@@ -502,6 +505,21 @@ 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.
+	 */

IMO, dropping pause frames is not among "the most common drop points".
Are you planning on reusing this reason in other modules? If not, then I
prefer removing it. My understanding is that we should not try to
document every obscure drop with these reasons.

Fair enough. I don't have an immediate plan to reuse this reason, and
to be honest, I'm not that familiar with the networking stack to be
able to tell off hand if it's likely to be useful elsewhere.

Would you prefer to stick to not specifying a drop reason at all at
that particular drop point, or to reuse an existing reason? Two
existing reasons that could be used (although they are not entirely
accurate) are:
SKB_DROP_REASON_UNHANDLED_PROTO
SKB_DROP_REASON_MAC_INVALID_SOURCE

quoted

+	SKB_DROP_REASON_MAC_IEEE_MAC_CONTROL,
+	/**
+	 * @SKB_DROP_REASON_BRIDGE_INGRESS_PORT_NFWD: the STP state of the
+	 * ingress bridge port does not allow frames to be forwarded.
+	 */
+	SKB_DROP_REASON_BRIDGE_INGRESS_PORT_NFWD,

Are you intending on reusing this for other ingress drops (e.g., VLAN,
locked port) or is this specific to ingress STP filtering? I think it
will be useful to distinguish between the different cases, so I suggest
renaming this reason to make it clear it is about ingress STP.

No, it's specific to ingress STP filtering. I will rename it to
SKB_DROP_REASON_BRIDGE_INGRESS_STP_STATE as I said in the other thread.

quoted

+	/**
+	 * SKB_DROP_REASON_BRIDGE_NO_EGRESS_PORT: no eligible egress port was
+	 * found while attempting to flood the frame.
+	 */
+	SKB_DROP_REASON_BRIDGE_NO_EGRESS_PORT,
 	/**
 	 * @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 e19b583ff2c6..e33e2f4fc3d9 100644
--- a/net/bridge/br_forward.c
+++ b/net/bridge/br_forward.c

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

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

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

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

diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
index ceaa5a89b947..fc00e172e1e1 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_PORT_NFWD);
+			return 0;
+		}
 
 		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_PORT_NFWD);
+		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_PORT_NFWD);
 	}
 	return RX_HANDLER_CONSUMED;
 }
-- 
2.47.1

Thanks for reviewing!

-- 
Best regards,
Radu