[PATCH net-next v2] tcp: Support skb PAWS drop reason when TIME-WAIT

[Jiayuan Chen <jiayuan.chen@linux.dev>]

PAWS is a long-standing issue, especially when there are upstream network
devices, making it more prone to occur.

Currently, packet loss statistics for PAWS can only be viewed through MIB,
which is a global metric and cannot be precisely obtained through tracing
to get the specific 4-tuple of the dropped packet. In the past, we had to
use kprobe ret to retrieve relevant skb information from
tcp_timewait_state_process().

We add a drop_reason pointer, similar to what previous commit does:
commit e34100c2ecbb ("tcp: add a drop_reason pointer to tcp_check_req()")

This commit addresses the PAWSESTABREJECTED case and also sets the
corresponding drop reason.

We use 'pwru' to test.

Before this commit:
''''
./pwru 'port 9999'
2025/03/24 13:46:03 Listening for events..
TUPLE                                        FUNC
172.31.75.115:12345->172.31.75.114:9999(tcp) sk_skb_reason_drop(SKB_DROP_REASON_NOT_SPECIFIED)
'''

After this commit:
'''
./pwru 'port 9999'
2025/03/24 16:06:59 Listening for events..
TUPLE                                        FUNC
172.31.75.115:12345->172.31.75.114:9999(tcp) sk_skb_reason_drop(SKB_DROP_REASON_TCP_RFC7323_PAWS)
'''

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
My apologize.
I struggled for a long time to get packetdrill to fix the client port, but
ultimately failed to do so, which is why I couldn't provide a packetdrill
script.
Instead, I wrote my own program to trigger PAWS, which can be found at
https://github.com/mrpre/nettrigger/tree/main
---
 include/net/tcp.h        | 3 ++-
 net/ipv4/tcp_ipv4.c      | 2 +-
 net/ipv4/tcp_minisocks.c | 7 +++++--
 net/ipv6/tcp_ipv6.c      | 2 +-
 4 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index f8efe56bbccb..e1574e804530 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -427,7 +427,8 @@ enum tcp_tw_status {
 enum tcp_tw_status tcp_timewait_state_process(struct inet_timewait_sock *tw,
 					      struct sk_buff *skb,
 					      const struct tcphdr *th,
-					      u32 *tw_isn);
+					      u32 *tw_isn,
+					      enum skb_drop_reason *drop_reason);
 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 			   struct request_sock *req, bool fastopen,
 			   bool *lost_race, enum skb_drop_reason *drop_reason);
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 1cd0938d47e0..a9dde473a23f 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2417,7 +2417,7 @@ int tcp_v4_rcv(struct sk_buff *skb)
 		goto csum_error;
 	}
 
-	tw_status = tcp_timewait_state_process(inet_twsk(sk), skb, th, &isn);
+	tw_status = tcp_timewait_state_process(inet_twsk(sk), skb, th, &isn, &drop_reason);
 	switch (tw_status) {
 	case TCP_TW_SYN: {
 		struct sock *sk2 = inet_lookup_listener(net,
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index fb9349be36b8..d16dfd41397e 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -97,7 +97,8 @@ static void twsk_rcv_nxt_update(struct tcp_timewait_sock *tcptw, u32 seq,
  */
 enum tcp_tw_status
 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
-			   const struct tcphdr *th, u32 *tw_isn)
+			   const struct tcphdr *th, u32 *tw_isn,
+			   enum skb_drop_reason *drop_reason)
 {
 	struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
 	u32 rcv_nxt = READ_ONCE(tcptw->tw_rcv_nxt);
@@ -245,8 +246,10 @@ tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
 		return TCP_TW_SYN;
 	}
 
-	if (paws_reject)
+	if (paws_reject) {
+		*drop_reason = SKB_DROP_REASON_TCP_RFC7323_PAWS;
 		__NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
+	}
 
 	if (!th->rst) {
 		/* In this case we must reset the TIMEWAIT timer.
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index c134cf1a603a..8862e315edc5 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1970,7 +1970,7 @@ INDIRECT_CALLABLE_SCOPE int tcp_v6_rcv(struct sk_buff *skb)
 		goto csum_error;
 	}
 
-	tw_status = tcp_timewait_state_process(inet_twsk(sk), skb, th, &isn);
+	tw_status = tcp_timewait_state_process(inet_twsk(sk), skb, th, &isn, &drop_reason);
 	switch (tw_status) {
 	case TCP_TW_SYN:
 	{
-- 
2.47.1