net: skb: use kfree_skb_reason() in tcp_v4_rcv()

Replace kfree_skb() with kfree_skb_reason() in tcp_v4_rcv(). Following
drop reasons are added:

SKB_DROP_REASON_NO_SOCKET
SKB_DROP_REASON_PKT_TOO_SMALL
SKB_DROP_REASON_TCP_CSUM
SKB_DROP_REASON_TCP_FILTER

After this patch, 'kfree_skb' event will print message like this:

$           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
$              | |         |   |||||     |         |
          <idle>-0       [000] ..s1.    36.113438: kfree_skb: skbaddr=(____ptrval____) protocol=2048 location=(____ptrval____) reason: NO_SOCKET

The reason of skb drop is printed too.
Signed-off-by: Menglong Dong <imagedong@tencent.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/skbuff.h

@@ -313,6 +313,10 @@ struct sk_buff;
  */
 enum skb_drop_reason {
 	SKB_DROP_REASON_NOT_SPECIFIED,
+	SKB_DROP_REASON_NO_SOCKET,
+	SKB_DROP_REASON_PKT_TOO_SMALL,
+	SKB_DROP_REASON_TCP_CSUM,
+	SKB_DROP_REASON_TCP_FILTER,
 	SKB_DROP_REASON_MAX,
 };
 

include/trace/events/skb.h

@@ -11,6 +11,10 @@
 
 #define TRACE_SKB_DROP_REASON					\
 	EM(SKB_DROP_REASON_NOT_SPECIFIED, NOT_SPECIFIED)	\
+	EM(SKB_DROP_REASON_NO_SOCKET, NO_SOCKET)		\
+	EM(SKB_DROP_REASON_PKT_TOO_SMALL, PKT_TOO_SMALL)	\
+	EM(SKB_DROP_REASON_TCP_CSUM, TCP_CSUM)			\
+	EM(SKB_DROP_REASON_TCP_FILTER, TCP_FILTER)		\
 	EMe(SKB_DROP_REASON_MAX, MAX)
 
 #undef EM

net/ipv4/tcp_ipv4.c

@@ -1971,8 +1971,10 @@ int tcp_v4_rcv(struct sk_buff *skb)
 	const struct tcphdr *th;
 	bool refcounted;
 	struct sock *sk;
+	int drop_reason;
 	int ret;
 
+	drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
 	if (skb->pkt_type != PACKET_HOST)
 		goto discard_it;
 
@@ -1984,8 +1986,10 @@ int tcp_v4_rcv(struct sk_buff *skb)
 
 	th = (const struct tcphdr *)skb->data;
 
-	if (unlikely(th->doff < sizeof(struct tcphdr) / 4))
+	if (unlikely(th->doff < sizeof(struct tcphdr) / 4)) {
+		drop_reason = SKB_DROP_REASON_PKT_TOO_SMALL;
 		goto bad_packet;
+	}
 	if (!pskb_may_pull(skb, th->doff * 4))
 		goto discard_it;
 
@@ -2090,8 +2094,10 @@ int tcp_v4_rcv(struct sk_buff *skb)
 
 	nf_reset_ct(skb);
 
-	if (tcp_filter(sk, skb))
+	if (tcp_filter(sk, skb)) {
+		drop_reason = SKB_DROP_REASON_TCP_FILTER;
 		goto discard_and_relse;
+	}
 	th = (const struct tcphdr *)skb->data;
 	iph = ip_hdr(skb);
 	tcp_v4_fill_cb(skb, iph, th);
@@ -2124,6 +2130,7 @@ int tcp_v4_rcv(struct sk_buff *skb)
 	return ret;
 
 no_tcp_socket:
+	drop_reason = SKB_DROP_REASON_NO_SOCKET;
 	if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb))
 		goto discard_it;
 
@@ -2131,6 +2138,7 @@ int tcp_v4_rcv(struct sk_buff *skb)
 
 	if (tcp_checksum_complete(skb)) {
 csum_error:
+		drop_reason = SKB_DROP_REASON_TCP_CSUM;
 		trace_tcp_bad_csum(skb);
 		__TCP_INC_STATS(net, TCP_MIB_CSUMERRORS);
 bad_packet:
@@ -2141,7 +2149,7 @@ int tcp_v4_rcv(struct sk_buff *skb)
 
 discard_it:
 	/* Discard frame. */
-	kfree_skb(skb);
+	kfree_skb_reason(skb, drop_reason);
 	return 0;
 
 discard_and_relse: