Subject: [PATCH,RFC] explicit connection confirmation

From: Lennert Buytenhek <[email-masked]>
Date: Thu, 14 Aug 2003 09:11:56 -0400

Hi,

Below is the original email I sent to netdev about nine months ago
announcing selective connection acceptance support for TCP sockets.
I have forward-ported the 2.4.18 patch to 2.6.0-test2, included below.
No functional changes have been made.

Could someone have a look at it?

cheers,
Lennert

On Thu, Nov 07, 2002 at 04:32:08AM -0500, buytenh wrote:

> (please CC on replies, I am not on this list)
> 
> Hi,
> 
> This patch gives userland the ability to decide whether to react
> with an incoming TCP SYN with a SYN-ACK or a RST.  It was hacked
> up after Linux Kongress 2001 and has been sitting on my patch
> pile since april this year or something.
> 
> The basic idea is this:
> - Put the listening TCP socket in TCP_CONFIRM_CONNECT mode.
> - Sockets returned from accept() on this socket after this will be
>   sockets in the SYN_RECV state instead of the ESTABLISHED state
>   (unless syncookies had to be used).  By writing to the socket,
>   you cause a SYN-ACK to be sent, and by immediately closing the
>   socket you cause a RST to be sent.
> 
> There are two issues left, AFAICS:
> - SYN_RECV sockets currently don't time out for some reason
> - it deadlocks instantly on SMP
> 
> It's against 2.4.18.  Could someone have a look at it please?  I
> unfortunately haven't had any time at all lately, so I would be
> really happy if someone else could take this over.  (Well, I can
> dream, can't I?)
> 
> 
> cheers,
> Lennert
> 

--- linux-2.6.0-test2/include/linux/tcp.h.orig	2003-08-14 14:19:20.886285797 +0200
+++ linux-2.6.0-test2/include/linux/tcp.h	2003-08-14 13:44:42.000000000 +0200
@@ -127,6 +127,7 @@
 #define TCP_WINDOW_CLAMP	10	/* Bound advertised window */
 #define TCP_INFO		11	/* Information about this connection. */
 #define TCP_QUICKACK		12	/* Block/reenable quick acks */
+#define TCP_CONFIRM_CONNECT	13	/* Let user control connection acceptance */
 
 #define TCPI_OPT_TIMESTAMPS	1
 #define TCPI_OPT_SACK		2
@@ -257,6 +258,7 @@
 	__u8	reordering;	/* Packet reordering metric.		*/
 	__u8	queue_shrunk;	/* Write queue has been shrunk recently.*/
 	__u8	defer_accept;	/* User waits for some data after accept() */
+	__u8	confirm_connect;/* User wants control over conn. acceptance */
 
 /* RTT measurement */
 	__u8	backoff;	/* backoff				*/
@@ -364,6 +366,11 @@
 	struct open_request	*accept_queue;
 	struct open_request	*accept_queue_tail;
 
+	/* Our corresponding open_request if this socket is unconfirmed
+	 * (i.e. if we haven't sent SYN-ACK or RST yet)
+	 */
+	struct open_request     *unconfirmed_openreq;
+
 	int			write_pending;	/* A write to socket waits to start. */
 
 	unsigned int		keepalive_time;	  /* time before keep alive takes place */
--- linux-2.6.0-test2/include/net/tcp.h.orig	2003-08-14 14:19:20.888285455 +0200
+++ linux-2.6.0-test2/include/net/tcp.h	2003-08-14 13:42:42.000000000 +0200
@@ -591,7 +591,8 @@
 		sack_ok : 1,
 		wscale_ok : 1,
 		ecn_ok : 1,
-		acked : 1;
+		acked : 1,
+		unconfirmed : 1;
 	/* The following two fields can be easily recomputed I think -AK */
 	__u32			window_clamp;	/* window clamp at creation time */
 	__u32			rcv_wnd;	/* rcv_wnd offered first time */
@@ -619,6 +620,17 @@
 	tcp_openreq_fastfree(req);
 }
 
+static inline int tcp_is_unconfirmed(struct tcp_opt *tp)
+{
+	struct open_request *req;
+
+	req = tp->unconfirmed_openreq;
+	if (req != NULL && req->unconfirmed)
+		return 1;
+
+	return 0;
+}
+
 #if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
 #define TCP_INET_FAMILY(fam) ((fam) == AF_INET)
 #else
@@ -1762,6 +1774,7 @@
 	req->acked = 0;
 	req->ecn_ok = 0;
 	req->rmt_port = skb->h.th->source;
+	req->unconfirmed = 0;
 }
 
 #define TCP_MEM_QUANTUM	((int)PAGE_SIZE)
--- linux-2.6.0-test2/net/ipv4/af_inet.c.orig	2003-08-14 14:19:20.890285113 +0200
+++ linux-2.6.0-test2/net/ipv4/af_inet.c	2003-08-14 13:47:14.000000000 +0200
@@ -685,8 +685,8 @@
 
 	lock_sock(sk2);
 
-	BUG_TRAP((1 << sk2->sk_state) &
-		 (TCPF_ESTABLISHED | TCPF_CLOSE_WAIT | TCPF_CLOSE));
+	BUG_TRAP((1 << sk2->sk_state) & (TCPF_SYN_RECV | TCPF_ESTABLISHED |
+		TCPF_CLOSE_WAIT | TCPF_CLOSE));
 
 	sock_graft(sk2, newsock);
 
--- linux-2.6.0-test2/net/ipv4/tcp.c.orig	2003-08-14 14:19:20.891284941 +0200
+++ linux-2.6.0-test2/net/ipv4/tcp.c	2003-08-14 14:16:08.697201584 +0200
@@ -206,6 +206,7 @@
  *					lingertime == 0 (RFC 793 ABORT Call)
  *	Hirokazu Takahashi	:	Use copy_from_user() instead of
  *					csum_and_copy_from_user() if possible.
+ *	Lennert Buytenhek	:	Explicit connection confirmation
  *
  *		This program is free software; you can redistribute it and/or
  *		modify it under the terms of the GNU General Public License
@@ -374,6 +375,15 @@
 	return tcp_sk(sk)->accept_queue ? (POLLIN | POLLRDNORM) : 0;
 }
 
+static void tcp_confirm(struct sock *sk)
+{
+	struct tcp_opt *tp = tcp_sk(sk);
+	struct open_request *req = tp->unconfirmed_openreq;
+
+	req->unconfirmed = 0;
+	req->class->rtx_syn_ack(sk, req, NULL);
+}
+
 /*
  *	Wait for a TCP event.
  *
@@ -662,6 +672,9 @@
 	struct task_struct *tsk = current;
 	DEFINE_WAIT(wait);
 
+	if (tcp_is_unconfirmed(tp))
+		tcp_confirm(sk);
+
 	while ((1 << sk->sk_state) & ~(TCPF_ESTABLISHED | TCPF_CLOSE_WAIT)) {
 		if (sk->sk_err)
 			return sock_error(sk);
@@ -1939,7 +1952,7 @@
 void tcp_close(struct sock *sk, long timeout)
 {
 	struct sk_buff *skb;
-	int data_was_unread = 0;
+	int should_send_rst = 0;
 
 	lock_sock(sk);
 	sk->sk_shutdown = SHUTDOWN_MASK;
@@ -1960,12 +1973,19 @@
 	while ((skb = __skb_dequeue(&sk->sk_receive_queue)) != NULL) {
 		u32 len = TCP_SKB_CB(skb)->end_seq - TCP_SKB_CB(skb)->seq -
 			  skb->h.th->fin;
-		data_was_unread += len;
+		should_send_rst += len;
 		__kfree_skb(skb);
 	}
 
 	tcp_mem_reclaim(sk);
 
+	if (tcp_sk(sk)->unconfirmed_openreq != NULL) {
+		if (tcp_is_unconfirmed(tcp_sk(sk)))
+			should_send_rst = 1;
+		tcp_openreq_free(tcp_sk(sk)->unconfirmed_openreq);
+		tcp_sk(sk)->unconfirmed_openreq = NULL;
+	}
+
 	/* As outlined in draft-ietf-tcpimpl-prob-03.txt, section
 	 * 3.10, we send a RST here because data was lost.  To
 	 * witness the awful effects of the old behavior of always
@@ -1975,7 +1995,7 @@
 	 * the FTP client, wheee...  Note: timeout is always zero
 	 * in such a case.
 	 */
-	if (data_was_unread) {
+	if (should_send_rst) {
 		/* Unread data was tossed, zap the connection. */
 		NET_INC_STATS_USER(TCPAbortOnClose);
 		tcp_set_state(sk, TCP_CLOSE);
@@ -2145,6 +2165,11 @@
 	if (!(sk->sk_userlocks & SOCK_BINDADDR_LOCK))
 		inet_reset_saddr(sk);
 
+	if (tp->unconfirmed_openreq) {
+		tcp_openreq_free(tp->unconfirmed_openreq);
+		tp->unconfirmed_openreq = NULL;
+	}
+
 	sk->sk_shutdown = 0;
 	sock_reset_flag(sk, SOCK_DONE);
 	tp->srtt = 0;
@@ -2258,8 +2283,10 @@
 
  	newsk = req->sk;
 	tcp_acceptq_removed(sk);
-	tcp_openreq_fastfree(req);
-	BUG_TRAP(newsk->sk_state != TCP_SYN_RECV);
+	if (tcp_sk(newsk)->unconfirmed_openreq == NULL)
+		tcp_openreq_fastfree(req);
+	BUG_TRAP(tcp_sk(newsk)->unconfirmed_openreq ||
+		 newsk->sk_state != TCP_SYN_RECV);
 	release_sock(sk);
 	return newsk;
 
@@ -2428,6 +2455,10 @@
 		}
 		break;
 
+	case TCP_CONFIRM_CONNECT:
+		tp->confirm_connect = !!val;
+		break;
+
 	default:
 		err = -ENOPROTOOPT;
 		break;
@@ -2553,6 +2584,9 @@
 	case TCP_QUICKACK:
 		val = !tp->ack.pingpong;
 		break;
+	case TCP_CONFIRM_CONNECT:
+		val = tp->confirm_connect || tcp_is_unconfirmed(tp);
+		break;
 	default:
 		return -ENOPROTOOPT;
 	};
--- linux-2.6.0-test2/net/ipv4/tcp_input.c.orig	2003-08-14 14:19:20.894284428 +0200
+++ linux-2.6.0-test2/net/ipv4/tcp_input.c	2003-08-14 13:42:42.000000000 +0200
@@ -3938,6 +3938,11 @@
 		switch(sk->sk_state) {
 		case TCP_SYN_RECV:
 			if (acceptable) {
+				if (tp->unconfirmed_openreq != NULL) {
+					tcp_openreq_free(tp->unconfirmed_openreq);
+					tp->unconfirmed_openreq = NULL;
+				}
+
 				tp->copied_seq = tp->rcv_nxt;
 				mb();
 				tcp_set_state(sk, TCP_ESTABLISHED);
--- linux-2.6.0-test2/net/ipv4/tcp_ipv4.c.orig	2003-08-14 14:19:20.895284256 +0200
+++ linux-2.6.0-test2/net/ipv4/tcp_ipv4.c	2003-08-14 14:34:31.383363445 +0200
@@ -1403,12 +1403,14 @@
 
 int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
 {
+	struct tcp_opt *master_tp = tcp_sk(sk);
 	struct tcp_opt tp;
 	struct open_request *req;
 	__u32 saddr = skb->nh.iph->saddr;
 	__u32 daddr = skb->nh.iph->daddr;
 	__u32 isn = TCP_SKB_CB(skb)->when;
 	struct dst_entry *dst = NULL;
+	int dont_confirm = 0;
 #ifdef CONFIG_SYN_COOKIES
 	int want_cookie = 0;
 #else
@@ -1445,6 +1447,9 @@
 	if (!req)
 		goto drop;
 
+	if (!want_cookie && master_tp->confirm_connect)
+		dont_confirm = 1;
+
 	tcp_clear_options(&tp);
 	tp.mss_clamp = 536;
 	tp.user_mss  = tcp_sk(sk)->user_mss;
@@ -1533,11 +1538,31 @@
 	}
 	req->snt_isn = isn;
 
-	if (tcp_v4_send_synack(sk, req, dst))
+	if (!dont_confirm && tcp_v4_send_synack(sk, req, dst))
 		goto drop_and_free;
 
 	if (want_cookie) {
 	   	tcp_openreq_free(req);
+	} else if (dont_confirm) {
+		struct sock *child;
+		__u8 rcv_wscale;
+
+		req->window_clamp = dst ? dst_metric(dst, RTAX_WINDOW) : 0;
+		tcp_select_initial_window(tcp_full_space(sk), req->mss,
+				&req->rcv_wnd, &req->window_clamp,
+				0, &rcv_wscale);
+		req->rcv_wscale = rcv_wscale;
+
+		child = tcp_v4_syn_recv_sock(sk, skb, req, NULL);
+		if (child != NULL) {
+			req->unconfirmed = 1;
+			tcp_sk(child)->unconfirmed_openreq = req;
+			tcp_acceptq_queue(sk, req, child);
+			sk->sk_data_ready(sk, 0);
+			sock_put(child);
+		} else {
+			tcp_openreq_free(req);
+		}
 	} else {
 		tcp_v4_synq_add(sk, req);
 	}
--- linux-2.6.0-test2/net/ipv4/tcp_minisocks.c.orig	2003-08-14 14:19:20.897283914 +0200
+++ linux-2.6.0-test2/net/ipv4/tcp_minisocks.c	2003-08-14 13:42:42.000000000 +0200
@@ -732,6 +732,7 @@
 		tcp_init_wl(newtp, req->snt_isn, req->rcv_isn);
 
 		newtp->retransmits = 0;
+		newtp->confirm_connect = 0;
 		newtp->backoff = 0;
 		newtp->srtt = 0;
 		newtp->mdev = TCP_TIMEOUT_INIT;
@@ -884,7 +885,8 @@
 		 * Enforce "SYN-ACK" according to figure 8, figure 6
 		 * of RFC793, fixed by RFC1122.
 		 */
-		req->class->rtx_syn_ack(sk, req, NULL);
+		if (!req->unconfirmed)
+			req->class->rtx_syn_ack(sk, req, NULL);
 		return NULL;
 	}
 
@@ -955,7 +957,7 @@
 	if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
 					  req->rcv_isn+1, req->rcv_isn+1+req->rcv_wnd)) {
 		/* Out of window: send ACK and drop. */
-		if (!(flg & TCP_FLAG_RST))
+		if (!req->unconfirmed && !(flg & TCP_FLAG_RST))
 			req->class->send_ack(skb, req);
 		if (paws_reject)
 			NET_INC_STATS_BH(PAWSEstabRejected);
@@ -991,6 +993,12 @@
 		return NULL;
 	}
 
+	/* @@@ If we are in SYN_RECV and haven't confirmed/rejected
+	 * the connection yet, this ACK is acking a never-sent packet.
+	 */
+	if (tcp_is_unconfirmed(tp))
+		return NULL;
+
 	/* OK, ACK is valid, create big socket and
 	 * feed this segment to it. It will repeat all
 	 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
--- linux-2.6.0-test2/net/ipv4/tcp_timer.c.orig	2003-08-14 14:19:20.899283572 +0200
+++ linux-2.6.0-test2/net/ipv4/tcp_timer.c	2003-08-14 13:42:42.000000000 +0200
@@ -519,7 +519,8 @@
 			if (time_after_eq(now, req->expires)) {
 				if ((req->retrans < thresh ||
 				     (req->acked && req->retrans < max_retries))
-				    && !req->class->rtx_syn_ack(sk, req, NULL)) {
+				    && (req->unconfirmed ||
+					!req->class->rtx_syn_ack(sk, req, NULL))) {
 					unsigned long timeo;
 
 					if (req->retrans++ == 0)