[PATCH v3 net-next] packet: add a generic drop reason for receive

[Yan Zhai <yan@cloudflare.com>]

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@mail.gmail.com/
Fixes: da37845fdce2 ("packet: uses kfree_skb() for errors.")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Yan Zhai <yan@cloudflare.com>
---
v2->v3: removed an unused variable
v1->v2: fixups suggested by Eric Dumazet
v2: https://lore.kernel.org/netdev/ZWobMUp22oTpP3FW@debian.debian/
v1: https://lore.kernel.org/netdev/ZU3EZKQ3dyLE6T8z@debian.debian/
---
 include/net/dropreason-core.h |  6 ++++++
 net/packet/af_packet.c        | 26 +++++++++++++-------------
 2 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
index 3c70ad53a49c..278e4c7d465c 100644
--- a/include/net/dropreason-core.h
+++ b/include/net/dropreason-core.h
@@ -86,6 +86,7 @@
 	FN(IPV6_NDISC_NS_OTHERHOST)	\
 	FN(QUEUE_PURGE)			\
 	FN(TC_ERROR)			\
+	FN(PACKET_SOCK_ERROR)		\
 	FNe(MAX)
 
 /**
@@ -378,6 +379,11 @@ 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..933fdfaacc44 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2127,7 +2127,7 @@ static int packet_rcv(struct sk_buff *skb, struct net_device *dev,
 	u8 *skb_head = skb->data;
 	int skb_len = skb->len;
 	unsigned int snaplen, res;
-	bool is_drop_n_account = false;
+	enum skb_drop_reason drop_reason = SKB_CONSUMED;
 
 	if (skb->pkt_type == PACKET_LOOPBACK)
 		goto drop;
@@ -2161,6 +2161,10 @@ 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;
 
@@ -2217,7 +2221,6 @@ static int packet_rcv(struct sk_buff *skb, struct net_device *dev,
 	return 0;
 
 drop_n_acct:
-	is_drop_n_account = true;
 	atomic_inc(&po->tp_drops);
 	atomic_inc(&sk->sk_drops);
 
@@ -2227,10 +2230,7 @@ static int packet_rcv(struct sk_buff *skb, struct net_device *dev,
 		skb->len = skb_len;
 	}
 drop:
-	if (!is_drop_n_account)
-		consume_skb(skb);
-	else
-		kfree_skb(skb);
+	kfree_skb_reason(skb, drop_reason);
 	return 0;
 }
 
@@ -2250,9 +2250,9 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 	struct sk_buff *copy_skb = NULL;
 	struct timespec64 ts;
 	__u32 ts_status;
-	bool is_drop_n_account = false;
 	unsigned int slot_id = 0;
 	int vnet_hdr_sz = 0;
+	enum skb_drop_reason drop_reason = SKB_CONSUMED;
 
 	/* 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 +2355,10 @@ 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));
@@ -2498,19 +2502,15 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 		skb->len = skb_len;
 	}
 drop:
-	if (!is_drop_n_account)
-		consume_skb(skb);
-	else
-		kfree_skb(skb);
+	kfree_skb_reason(skb, drop_reason);
 	return 0;
 
 drop_n_account:
 	spin_unlock(&sk->sk_receive_queue.lock);
 	atomic_inc(&po->tp_drops);
-	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