[PATCH net-next v1] tcp: Support skb PAWS drop reason when TIME-WAIT

From: Jiayuan Chen
Date: Mon Mar 24 2025 - 09:18:59 EST


When the server actively closes a connection and enters the TIME_WAIT
state, if PAWS validation fails, the discarded skb previously lacked a
corresponding reason identifier. This commit assigns the existing
SKB_DROP_REASON_TCP_RFC7323_PAWS as the reason for such drops.

We use 'pwru' to capture drop reason.

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@xxxxxxxxx>

---
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 | 2 +-
net/ipv4/tcp_ipv4.c | 2 +-
net/ipv4/tcp_minisocks.c | 6 ++++--
net/ipv6/tcp_ipv6.c | 2 +-
4 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 2d08473a6dc0..16e6efef939a 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -388,7 +388,7 @@ 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);
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 2632844d2c35..ad168a3a73bb 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2406,7 +2406,7 @@ int tcp_v4_rcv(struct sk_buff *skb)
inet_twsk_put(inet_twsk(sk));
goto csum_error;
}
- switch (tcp_timewait_state_process(inet_twsk(sk), skb, th, &isn)) {
+ switch (tcp_timewait_state_process(inet_twsk(sk), skb, th, &isn, &drop_reason)) {
case TCP_TW_SYN: {
struct sock *sk2 = inet_lookup_listener(net,
net->ipv4.tcp_death_row.hashinfo,
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index b089b08e9617..39e5b55041f1 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -97,7 +97,7 @@ 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 +245,10 @@ 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 2debdf085a3b..66d30df95f7a 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1965,7 +1965,7 @@ INDIRECT_CALLABLE_SCOPE int tcp_v6_rcv(struct sk_buff *skb)
goto csum_error;
}

- switch (tcp_timewait_state_process(inet_twsk(sk), skb, th, &isn)) {
+ switch (tcp_timewait_state_process(inet_twsk(sk), skb, th, &isn, &drop_reason)) {
case TCP_TW_SYN:
{
struct sock *sk2;
--
2.47.1