tcp: A better fix for the previously attempted fix of the ack-war iss…

…ue with tcp.

So it turns out that my fix before was not correct. It ended with us failing
some of the "improved" SYN tests, since we are not in the correct states.
With more digging I have figured out the root of the problem is that when
we receive a SYN|FIN the reassembly code made it so we create a segq entry
to hold the FIN. In the established state where we were not in order this
would be correct i.e. a 0 len with a FIN would need to be accepted. But
if you are in a front state we need to strip the FIN so we correctly handle
the ACK but ignore the FIN. This gets us into the proper states
and avoids the previous ack war.

I back out some of the previous changes but then add a new change
here in tcp_reass() that fixes the root cause of the issue. We still
leave the rack panic fixes in place however.

Reviewed by: mtuexen
Sponsored by: Netflix Inc
Differential Revision:	https://reviews.freebsd.org/D30627

sys/netinet/tcp_input.c

@@ -3191,15 +3191,9 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
 			 * when trimming from the head.
 			 */
 			tcp_seq temp = save_start;
-			if (tlen || (th->th_seq != tp->rcv_nxt)) {
-				/*
-				 * We add the th_seq != rcv_nxt to
-				 * catch the case of a stand alone out
-				 * of order FIN.
-				 */
-				thflags = tcp_reass(tp, th, &temp, &tlen, m);
-				tp->t_flags |= TF_ACKNOW;
-			}
+
+			thflags = tcp_reass(tp, th, &temp, &tlen, m);
+			tp->t_flags |= TF_ACKNOW;
 		}
 		if ((tp->t_flags & TF_SACK_PERMIT) &&
 		    (save_tlen > 0) &&

sys/netinet/tcp_reass.c

@@ -571,6 +571,7 @@ tcp_reass(struct tcpcb *tp, struct tcphdr *th, tcp_seq *seq_start,
 		 * the rcv_nxt <-> rcv_wnd but thats
 		 * already done for us by the caller.
 		 */
+strip_fin:
 #ifdef TCP_REASS_COUNTERS
 		counter_u64_add(tcp_zero_input, 1);
 #endif
@@ -579,6 +580,19 @@ tcp_reass(struct tcpcb *tp, struct tcphdr *th, tcp_seq *seq_start,
 		tcp_reass_log_dump(tp);
 #endif
 		return (0);
+	} else if ((*tlenp == 0) &&
+		   (th->th_flags & TH_FIN) &&
+		   !TCPS_HAVEESTABLISHED(tp->t_state)) {
+		/*
+		 * We have not established, and we
+		 * have a FIN and no data. Lets treat
+		 * this as the same as if the FIN were
+		 * not present. We don't want to save
+		 * the FIN bit in a reassembly buffer
+		 * we want to get established first before
+		 * we do that (the peer will retransmit).
+		 */
+		goto strip_fin;
 	}
 	/*
 	 * Will it fit?

sys/netinet/tcp_stacks/bbr.c

@@ -8320,15 +8320,9 @@ bbr_process_data(struct mbuf *m, struct tcphdr *th, struct socket *so,
 			 * trimming from the head.
 			 */
 			tcp_seq temp = save_start;
-			if (tlen || (th->th_seq != tp->rcv_nxt)) {
-				/*
-				 * We add the th_seq != rcv_nxt to
-				 * catch the case of a stand alone out
-				 * of order FIN.
-				 */
-				thflags = tcp_reass(tp, th, &temp, &tlen, m);
-				tp->t_flags |= TF_ACKNOW;
-			}
+
+			thflags = tcp_reass(tp, th, &temp, &tlen, m);
+			tp->t_flags |= TF_ACKNOW;
 		}
 		if ((tp->t_flags & TF_SACK_PERMIT) &&
 		    (save_tlen > 0) &&

sys/netinet/tcp_stacks/rack.c

@@ -10235,15 +10235,10 @@ rack_process_data(struct mbuf *m, struct tcphdr *th, struct socket *so,
 			 * trimming from the head.
 			 */
 			tcp_seq temp = save_start;
-			if (tlen || (th->th_seq != tp->rcv_nxt)) {
-				/*
-				 * We add the th_seq != rcv_nxt to
-				 * catch the case of a stand alone out
-				 * of order FIN.
-				 */
-				thflags = tcp_reass(tp, th, &temp, &tlen, m);
-				tp->t_flags |= TF_ACKNOW;
-			}
+
+			thflags = tcp_reass(tp, th, &temp, &tlen, m);
+			tp->t_flags |= TF_ACKNOW;
+
 		}
 		if ((tp->t_flags & TF_SACK_PERMIT) &&
 		    (save_tlen > 0) &&

sys/netinet/tcp_usrreq.c

@@ -2647,22 +2647,6 @@ tcp_usrclosed(struct tcpcb *tp)
 		tcp_state_change(tp, TCPS_LAST_ACK);
 		break;
 	}
-	if ((tp->t_state == TCPS_LAST_ACK) &&
-	    (tp->t_flags & TF_SENTFIN)) {
-		/*
-		 * If we have reached LAST_ACK, and
-		 * we sent a FIN (e.g. via MSG_EOR), then
-		 * we really should move to either FIN_WAIT_1
-		 * or FIN_WAIT_2 depending on snd_max/snd_una.
-		 */
-		if (tp->snd_una == tp->snd_max) {
-			/* The FIN is acked */
-			tcp_state_change(tp, TCPS_FIN_WAIT_2);
-		} else {
-			/* The FIN is still outstanding */
-			tcp_state_change(tp, TCPS_FIN_WAIT_1);
-		}
-	}
 	if (tp->t_state >= TCPS_FIN_WAIT_2) {
 		soisdisconnected(tp->t_inpcb->inp_socket);
 		/* Prevent the connection hanging in FIN_WAIT_2 forever. */