PAWS 在tcp协议栈中的实现

0.1. PAWS 检查：

1. tcp_timewait_state_process 中使用，也就是在syn 报文明中了 tw 状态的 socket 山海，才使用。 不是所有syn报文都做检查

 91 enum tcp_tw_status
 92 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
 93                            const struct tcphdr *th)
 94 {
 95         struct tcp_options_received tmp_opt;
 96         struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
 97         bool paws_reject = false;
 98
 99         tmp_opt.saw_tstamp = 0;
100         if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
101                 tcp_parse_options(skb, &tmp_opt, 0, NULL);
102
103                 if (tmp_opt.saw_tstamp) {
104                         tmp_opt.rcv_tsecr       -= tcptw->tw_ts_offset;
105                         tmp_opt.ts_recent       = tcptw->tw_ts_recent;
106                         tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp;
107                         paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
108                 }
109         }

1142
1143 static inline bool tcp_paws_check(const struct tcp_options_received *rx_opt,
1144                                   int paws_win)
1145 {
1146         if ((s32)(rx_opt->ts_recent - rx_opt->rcv_tsval) <= paws_win)
1147                 return true;
1148         if (unlikely(get_seconds() >= rx_opt->ts_recent_stamp + TCP_PAWS_24DAYS))
1149                 return true;
1150         /*
1151          * Some OSes send SYN and SYNACK messages with tsval=0 tsecr=0,
1152          * then following tcp messages have valid values. Ignore 0 value,
1153          * or else 'negative' tsval might forbid us to accept their packets.
1154          */
1155         if (!rx_opt->ts_recent)
1156                 return true;
1157         return false;
1158 }

1160 static inline bool tcp_paws_reject(const struct tcp_options_received *rx_opt,
1161                                    int rst)
1162 {
1163         if (tcp_paws_check(rx_opt, 0))
1164                 return false;
1165
1166         /* RST segments are not recommended to carry timestamp,
1167            and, if they do, it is recommended to ignore PAWS because
1168            "their cleanup function should take precedence over timestamps."
1169            Certainly, it is mistake. It is necessary to understand the reasons
1170            of this constraint to relax it: if peer reboots, clock may go
1171            out-of-sync and half-open connections will not be reset.
1172            Actually, the problem would be not existing if all
1173            the implementations followed draft about maintaining clock
1174            via reboots. Linux-2.2 DOES NOT!
1175
1176            However, we can relax time bounds for RST segments to MSL.
1177          */
1178         if (rst && get_seconds() >= rx_opt->ts_recent_stamp + TCP_PAWS_MSL)
1179                 return false;
1180         return true;
1181 }