[PATCH net-next] packet: add a generic drop reason for receive

[Yan Zhai]

Commit da37845fdce2 ("packet: uses kfree_skb() for errors.") switches
from consume_skb to kfree_skb to improve error handling. However, this
could bring a lot of noises when we monitor real packet drops in
kfree_skb[1], because in tpacket_rcv or packet_rcv only packet clones
can be freed, not actual packets.

Adding a generic drop reason to allow distinguish these "clone drops".

[1]: https://lore.kernel.org/netdev/CABWYdi00L+O30Q=Zah28QwZ_5RU-xcxLFUK2Zj08A8MrLk9jzg@xxxxxxxxxxxxxx/
Fixes: da37845fdce2 ("packet: uses kfree_skb() for errors.")
Signed-off-by: Yan Zhai <yan@xxxxxxxxxxxxxx>
---
 include/net/dropreason-core.h |  6 ++++++
 net/packet/af_packet.c        | 16 +++++++++++++---
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
index 845dce805de7..6ff543fe8a8b 100644
--- a/include/net/dropreason-core.h
+++ b/include/net/dropreason-core.h
@@ -81,6 +81,7 @@
 	FN(IPV6_NDISC_NS_OTHERHOST)	\
 	FN(QUEUE_PURGE)			\
 	FN(TC_ERROR)			\
+	FN(PACKET_SOCK_ERROR)		\
 	FNe(MAX)
 
 /**
@@ -348,6 +349,11 @@ enum skb_drop_reason {
 	SKB_DROP_REASON_QUEUE_PURGE,
 	/** @SKB_DROP_REASON_TC_ERROR: generic internal tc error. */
 	SKB_DROP_REASON_TC_ERROR,
+	/**
+	 * @SKB_DROP_REASON_PACKET_SOCK_ERROR: generic packet socket errors
+	 * after its filter matches an incoming packet.
+	 */
+	SKB_DROP_REASON_PACKET_SOCK_ERROR,
 	/**
 	 * @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/packet/af_packet.c b/net/packet/af_packet.c
index a84e00b5904b..94b8a9d8e038 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2128,6 +2128,7 @@ static int packet_rcv(struct sk_buff *skb, struct net_device *dev,
 	int skb_len = skb->len;
 	unsigned int snaplen, res;
 	bool is_drop_n_account = false;
+	enum skb_drop_reason drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
 
 	if (skb->pkt_type == PACKET_LOOPBACK)
 		goto drop;
@@ -2161,6 +2162,10 @@ static int packet_rcv(struct sk_buff *skb, struct net_device *dev,
 	res = run_filter(skb, sk, snaplen);
 	if (!res)
 		goto drop_n_restore;
+
+	/* skb will only be "consumed" not "dropped" before this */
+	drop_reason = SKB_DROP_REASON_PACKET_SOCK_ERROR;
+
 	if (snaplen > res)
 		snaplen = res;
 
@@ -2230,7 +2235,7 @@ static int packet_rcv(struct sk_buff *skb, struct net_device *dev,
 	if (!is_drop_n_account)
 		consume_skb(skb);
 	else
-		kfree_skb(skb);
+		kfree_skb_reason(skb, drop_reason);
 	return 0;
 }
 
@@ -2253,6 +2258,7 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 	bool is_drop_n_account = false;
 	unsigned int slot_id = 0;
 	int vnet_hdr_sz = 0;
+	enum skb_drop_reason drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
 
 	/* struct tpacket{2,3}_hdr is aligned to a multiple of TPACKET_ALIGNMENT.
 	 * We may add members to them until current aligned size without forcing
@@ -2355,6 +2361,10 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 			vnet_hdr_sz = 0;
 		}
 	}
+
+	/* skb will only be "consumed" not "dropped" before this */
+	drop_reason = SKB_DROP_REASON_PACKET_SOCK_ERROR;
+
 	spin_lock(&sk->sk_receive_queue.lock);
 	h.raw = packet_current_rx_frame(po, skb,
 					TP_STATUS_KERNEL, (macoff+snaplen));
@@ -2501,7 +2511,7 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 	if (!is_drop_n_account)
 		consume_skb(skb);
 	else
-		kfree_skb(skb);
+		kfree_skb_reason(skb, drop_reason);
 	return 0;
 
 drop_n_account:
@@ -2510,7 +2520,7 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 	is_drop_n_account = true;
 
 	sk->sk_data_ready(sk);
-	kfree_skb(copy_skb);
+	kfree_skb_reason(copy_skb, drop_reason);
 	goto drop_n_restore;
 }
 
-- 
2.30.2