[PATCH RESEND net-next v3 1/2] tcp: add TCP_RFC7323_TW_PAWS drop reason

Jiayuan Chen
2025-04-07T14:00:33Z

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'
2025/04/07 13:40:19 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'
2025/04/07 13:51:34 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_TW_PAWS)
'''

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
include/net/dropreason-core.h | 6 ++++++
include/net/tcp.h | 3 ++-
net/ipv4/tcp_ipv4.c | 3 ++-
net/ipv4/tcp_minisocks.c | 7 +++++--
net/ipv6/tcp_ipv6.c | 3 ++-
5 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
index e4fdc6b54cef..9701d7f936f6 100644
--- a/include/net/dropreason-core.h
+++ b/include/net/dropreason-core.h
@@ -40,6 +40,7 @@
FN(TCP_OFOMERGE) \
FN(TCP_RFC7323_PAWS) \
FN(TCP_RFC7323_PAWS_ACK) \
+ FN(TCP_RFC7323_TW_PAWS) \
FN(TCP_RFC7323_TSECR) \
FN(TCP_LISTEN_OVERFLOW) \
FN(TCP_OLD_SEQUENCE) \
@@ -283,6 +284,11 @@ enum skb_drop_reason {
* Corresponds to LINUX_MIB_PAWS_OLD_ACK.
*/
SKB_DROP_REASON_TCP_RFC7323_PAWS_ACK,
+ /**
+ * @SKB_DROP_REASON_TCP_RFC7323_TW_PAWS: PAWS check, socket is in
+ * TIME_WAIT state.
+ */
+ SKB_DROP_REASON_TCP_RFC7323_TW_PAWS,
/**
* @SKB_DROP_REASON_TCP_RFC7323_TSECR: PAWS check, invalid TSEcr.
* Corresponds to LINUX_MIB_TSECRREJECTED.
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 4450c384ef17..5078ad868fee 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 8cce0d5489da..d5b5c32115d2 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2417,7 +2417,8 @@ 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..27511bf58c0f 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_TW_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 b03c223eda4f..7dcb33f879ee 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1970,7 +1970,8 @@ 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