tcp: add LINUX_MIB_PAWS_TW_REJECTED counter

JIRA: https://issues.redhat.com/browse/RHEL-88890
Upstream Status: net-next.git
Conflicts:\
- Context differences in the documentation due to missing upstream
  commits 54b771e ("doc: net: Fix .rst rendering of net_cachelines
  pages") and 3ba0752 ("tcp: be less liberal in TSEcr received
  while in SYN_RECV state") in c9s.

commit c449d5f
Author: Jiayuan Chen <jiayuan.chen@linux.dev>
Date:   Wed Apr 9 19:26:05 2025 +0800

    tcp: add LINUX_MIB_PAWS_TW_REJECTED counter

    When TCP is in TIME_WAIT state, PAWS verification uses
    LINUX_PAWSESTABREJECTED, which is ambiguous and cannot be distinguished
    from other PAWS verification processes.

    We added a new counter, like the existing PAWS_OLD_ACK one.

    Also we update the doc with previously missing PAWS_OLD_ACK.

    usage:
    '''
    nstat -az | grep PAWSTimewait
    TcpExtPAWSTimewait              1                  0.0
    '''

    Suggested-by: Eric Dumazet <edumazet@google.com>
    Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
    Reviewed-by: Eric Dumazet <edumazet@google.com>
    Link: https://patch.msgid.link/20250409112614.16153-3-jiayuan.chen@linux.dev
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Antoine Tenart <atenart@redhat.com>

Author: atenart

Documentation/networking/net_cachelines/snmp.rst

@@ -35,6 +35,8 @@ unsigned_long  LINUX_MIB_TIMEWAITRECYCLED
 unsigned_long  LINUX_MIB_TIMEWAITKILLED                                                      
 unsigned_long  LINUX_MIB_PAWSACTIVEREJECTED                                                  
 unsigned_long  LINUX_MIB_PAWSESTABREJECTED                                                   
+unsigned_long  LINUX_MIB_PAWS_OLD_ACK
+unsigned_long  LINUX_MIB_PAWS_TW_REJECTED
 unsigned_long  LINUX_MIB_DELAYEDACKLOST                                                      
 unsigned_long  LINUX_MIB_LISTENOVERFLOWS                                                     
 unsigned_long  LINUX_MIB_LISTENDROPS                                                         

include/net/dropreason-core.h

@@ -245,6 +245,7 @@ enum skb_drop_reason {
 	/**
 	 * @SKB_DROP_REASON_TCP_RFC7323_TW_PAWS: PAWS check, socket is in
 	 * TIME_WAIT state.
+	 * Corresponds to LINUX_MIB_PAWS_TW_REJECTED.
 	 */
 	SKB_DROP_REASON_TCP_RFC7323_TW_PAWS,
 	/**

include/uapi/linux/snmp.h

@@ -187,6 +187,7 @@ enum
 	LINUX_MIB_PAWSACTIVEREJECTED,		/* PAWSActiveRejected */
 	LINUX_MIB_PAWSESTABREJECTED,		/* PAWSEstabRejected */
 	LINUX_MIB_PAWS_OLD_ACK,			/* PAWSOldAck */
+	LINUX_MIB_PAWS_TW_REJECTED,		/* PAWSTimewait */
 	LINUX_MIB_DELAYEDACKS,			/* DelayedACKs */
 	LINUX_MIB_DELAYEDACKLOCKED,		/* DelayedACKLocked */
 	LINUX_MIB_DELAYEDACKLOST,		/* DelayedACKLost */

net/ipv4/proc.c

@@ -189,6 +189,7 @@ static const struct snmp_mib snmp4_net_list[] = {
 	SNMP_MIB_ITEM("PAWSActive", LINUX_MIB_PAWSACTIVEREJECTED),
 	SNMP_MIB_ITEM("PAWSEstab", LINUX_MIB_PAWSESTABREJECTED),
 	SNMP_MIB_ITEM("PAWSOldAck", LINUX_MIB_PAWS_OLD_ACK),
+	SNMP_MIB_ITEM("PAWSTimewait", LINUX_MIB_PAWS_TW_REJECTED),
 	SNMP_MIB_ITEM("DelayedACKs", LINUX_MIB_DELAYEDACKS),
 	SNMP_MIB_ITEM("DelayedACKLocked", LINUX_MIB_DELAYEDACKLOCKED),
 	SNMP_MIB_ITEM("DelayedACKLost", LINUX_MIB_DELAYEDACKLOST),

net/ipv4/tcp_minisocks.c

@@ -236,7 +236,7 @@ tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
 
 	if (paws_reject) {
 		*drop_reason = SKB_DROP_REASON_TCP_RFC7323_TW_PAWS;
-		__NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
+		__NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWS_TW_REJECTED);
 	}
 
 	if (!th->rst) {