[PATCH net-next v2] tcp: Support skb PAWS drop reason when TIME-WAIT
From: Jiayuan Chen
Date: Tue Mar 25 2025 - 07:04:52 EST
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@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 | 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