tcp: accecn: AccECN option failure handling

AccECN option may fail in various way, handle these:
- Attempt to negotiate the use of AccECN on the 1st retransmitted SYN
	- From the 2nd retransmitted SYN, stop AccECN negotiation
- Remove option from SYN/ACK rexmits to handle blackholes
- If no option arrives in SYN/ACK, assume Option is not usable
        - If an option arrives later, re-enabled
- If option is zeroed, disable AccECN option processing

This patch use existing padding bits in tcp_request_sock and
holes in tcp_sock without increasing the size.

Signed-off-by: Ilpo Järvinen <ij@kernel.org>
Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250916082434.100722-9-chia-yu.chang@nokia-bell-labs.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/linux/tcp.h

@@ -173,6 +173,7 @@ struct tcp_request_sock {
 	u8				syn_ect_snt: 2,
 					syn_ect_rcv: 2,
 					accecn_fail_mode:4;
+	u8				saw_accecn_opt  :2;
 #ifdef CONFIG_TCP_AO
 	u8				ao_keyid;
 	u8				ao_rcv_next;
@@ -407,7 +408,8 @@ struct tcp_sock {
 		syn_fastopen_child:1; /* created TFO passive child socket */
 
 	u8	keepalive_probes; /* num of allowed keep alive probes	*/
-	u8	accecn_fail_mode:4;	/* AccECN failure handling */
+	u8	accecn_fail_mode:4,	/* AccECN failure handling */
+		saw_accecn_opt:2;	/* An AccECN option was seen */
 	u32	tcp_tx_delay;	/* delay (in usec) added to TX packets */
 
 /* RTT measurement */

include/net/tcp_ecn.h

@@ -91,6 +91,11 @@ static inline void tcp_accecn_fail_mode_set(struct tcp_sock *tp, u8 mode)
 	tp->accecn_fail_mode |= mode;
 }
 
+#define TCP_ACCECN_OPT_NOT_SEEN		0x0
+#define TCP_ACCECN_OPT_EMPTY_SEEN	0x1
+#define TCP_ACCECN_OPT_COUNTER_SEEN	0x2
+#define TCP_ACCECN_OPT_FAIL_SEEN	0x3
+
 static inline u8 tcp_accecn_ace(const struct tcphdr *th)
 {
 	return (th->ae << 2) | (th->cwr << 1) | th->ece;
@@ -146,6 +151,14 @@ static inline bool tcp_accecn_validate_syn_feedback(struct sock *sk, u8 ace,
 	return true;
 }
 
+static inline void tcp_accecn_saw_opt_fail_recv(struct tcp_sock *tp,
+						u8 saw_opt)
+{
+	tp->saw_accecn_opt = saw_opt;
+	if (tp->saw_accecn_opt == TCP_ACCECN_OPT_FAIL_SEEN)
+		tcp_accecn_fail_mode_set(tp, TCP_ACCECN_OPT_FAIL_RECV);
+}
+
 /* Validate the 3rd ACK based on the ACE field, see Table 4 of AccECN spec */
 static inline void tcp_accecn_third_ack(struct sock *sk,
 					const struct sk_buff *skb, u8 sent_ect)
@@ -428,9 +441,35 @@ static inline void tcp_accecn_set_ace(struct tcp_sock *tp, struct sk_buff *skb,
 	}
 }
 
+static inline u8 tcp_accecn_option_init(const struct sk_buff *skb,
+					u8 opt_offset)
+{
+	u8 *ptr = skb_transport_header(skb) + opt_offset;
+	unsigned int optlen = ptr[1] - 2;
+
+	if (WARN_ON_ONCE(ptr[0] != TCPOPT_ACCECN0 && ptr[0] != TCPOPT_ACCECN1))
+		return TCP_ACCECN_OPT_FAIL_SEEN;
+	ptr += 2;
+
+	/* Detect option zeroing: an AccECN connection "MAY check that the
+	 * initial value of the EE0B field or the EE1B field is non-zero"
+	 */
+	if (optlen < TCPOLEN_ACCECN_PERFIELD)
+		return TCP_ACCECN_OPT_EMPTY_SEEN;
+	if (get_unaligned_be24(ptr) == 0)
+		return TCP_ACCECN_OPT_FAIL_SEEN;
+	if (optlen < TCPOLEN_ACCECN_PERFIELD * 3)
+		return TCP_ACCECN_OPT_COUNTER_SEEN;
+	ptr += TCPOLEN_ACCECN_PERFIELD * 2;
+	if (get_unaligned_be24(ptr) == 0)
+		return TCP_ACCECN_OPT_FAIL_SEEN;
+
+	return TCP_ACCECN_OPT_COUNTER_SEEN;
+}
+
 /* See Table 2 of the AccECN draft */
-static inline void tcp_ecn_rcv_synack(struct sock *sk, const struct tcphdr *th,
+static inline void tcp_ecn_rcv_synack(struct sock *sk, const struct sk_buff *skb,
-				      u8 ip_dsfield)
+				      const struct tcphdr *th, u8 ip_dsfield)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	u8 ace = tcp_accecn_ace(th);
@@ -469,7 +508,13 @@ static inline void tcp_ecn_rcv_synack(struct sock *sk, const struct tcphdr *th,
 	default:
 		tcp_ecn_mode_set(tp, TCP_ECN_MODE_ACCECN);
 		tp->syn_ect_rcv = ip_dsfield & INET_ECN_MASK;
+		if (tp->rx_opt.accecn &&
+		    tp->saw_accecn_opt < TCP_ACCECN_OPT_COUNTER_SEEN) {
+			u8 saw_opt = tcp_accecn_option_init(skb, tp->rx_opt.accecn);
+
+			tcp_accecn_saw_opt_fail_recv(tp, saw_opt);
 			tp->accecn_opt_demand = 2;
+		}
 		if (INET_ECN_is_ce(ip_dsfield) &&
 		    tcp_accecn_validate_syn_feedback(sk, ace,
 						     tp->syn_ect_snt)) {

include/uapi/linux/tcp.h

@@ -323,6 +323,8 @@ struct tcp_info {
 	__u32	tcpi_received_e1_bytes;
 	__u32	tcpi_received_e0_bytes;
 	__u32	tcpi_received_ce_bytes;
+	__u16	tcpi_accecn_fail_mode;
+	__u16	tcpi_accecn_opt_seen;
 };
 
 /* netlink attributes types for SCM_TIMESTAMPING_OPT_STATS */

net/ipv4/tcp.c

@@ -3409,6 +3409,7 @@ int tcp_disconnect(struct sock *sk, int flags)
 	tp->delivered = 0;
 	tp->delivered_ce = 0;
 	tp->accecn_fail_mode = 0;
+	tp->saw_accecn_opt = TCP_ACCECN_OPT_NOT_SEEN;
 	tcp_accecn_init_counters(tp);
 	tp->prev_ecnfield = 0;
 	tp->accecn_opt_tstamp = 0;
@@ -4287,6 +4288,8 @@ void tcp_get_info(struct sock *sk, struct tcp_info *info)
 	if (tp->rto_stamp)
 		info->tcpi_total_rto_time += tcp_clock_ms() - tp->rto_stamp;
 
+	info->tcpi_accecn_fail_mode = tp->accecn_fail_mode;
+	info->tcpi_accecn_opt_seen = tp->saw_accecn_opt;
 	info->tcpi_received_ce = tp->received_ce;
 	info->tcpi_delivered_e1_bytes = tp->delivered_ecn_bytes[ect1_idx];
 	info->tcpi_delivered_e0_bytes = tp->delivered_ecn_bytes[ect0_idx];

net/ipv4/tcp_input.c

@@ -398,7 +398,22 @@ static bool tcp_accecn_process_option(struct tcp_sock *tp,
 	unsigned int i;
 	u8 *ptr;
 
+	if (tcp_accecn_opt_fail_recv(tp))
+		return false;
+
 	if (!(flag & FLAG_SLOWPATH) || !tp->rx_opt.accecn) {
+		if (!tp->saw_accecn_opt) {
+			/* Too late to enable after this point due to
+			 * potential counter wraps
+			 */
+			if (tp->bytes_sent >= (1 << 23) - 1) {
+				u8 saw_opt = TCP_ACCECN_OPT_FAIL_SEEN;
+
+				tcp_accecn_saw_opt_fail_recv(tp, saw_opt);
+			}
+			return false;
+		}
+
 		if (estimate_ecnfield) {
 			u8 ecnfield = estimate_ecnfield - 1;
 
@@ -415,6 +430,13 @@ static bool tcp_accecn_process_option(struct tcp_sock *tp,
 	order1 = (ptr[0] == TCPOPT_ACCECN1);
 	ptr += 2;
 
+	if (tp->saw_accecn_opt < TCP_ACCECN_OPT_COUNTER_SEEN) {
+		tp->saw_accecn_opt = tcp_accecn_option_init(skb,
+							    tp->rx_opt.accecn);
+		if (tp->saw_accecn_opt == TCP_ACCECN_OPT_FAIL_SEEN)
+			tcp_accecn_fail_mode_set(tp, TCP_ACCECN_OPT_FAIL_RECV);
+	}
+
 	res = !!estimate_ecnfield;
 	for (i = 0; i < 3; i++) {
 		u32 init_offset;
@@ -6123,8 +6145,14 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 	if (th->syn) {
 		if (tcp_ecn_mode_accecn(tp)) {
 			accecn_reflector = true;
+			if (tp->rx_opt.accecn &&
+			    tp->saw_accecn_opt < TCP_ACCECN_OPT_COUNTER_SEEN) {
+				u8 saw_opt = tcp_accecn_option_init(skb, tp->rx_opt.accecn);
+
+				tcp_accecn_saw_opt_fail_recv(tp, saw_opt);
 				tcp_accecn_opt_demand_min(sk, 1);
 			}
+		}
 		if (sk->sk_state == TCP_SYN_RECV && sk->sk_socket && th->ack &&
 		    TCP_SKB_CB(skb)->seq + 1 == TCP_SKB_CB(skb)->end_seq &&
 		    TCP_SKB_CB(skb)->seq + 1 == tp->rcv_nxt &&
@@ -6606,7 +6634,8 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
 		 */
 
 		if (tcp_ecn_mode_any(tp))
-			tcp_ecn_rcv_synack(sk, th, TCP_SKB_CB(skb)->ip_dsfield);
+			tcp_ecn_rcv_synack(sk, skb, th,
+					   TCP_SKB_CB(skb)->ip_dsfield);
 
 		tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
 		tcp_try_undo_spurious_syn(sk);
@@ -7177,6 +7206,8 @@ static void tcp_openreq_init(struct request_sock *req,
 	tcp_rsk(req)->snt_tsval_first = 0;
 	tcp_rsk(req)->last_oow_ack_time = 0;
 	tcp_rsk(req)->accecn_ok = 0;
+	tcp_rsk(req)->saw_accecn_opt = TCP_ACCECN_OPT_NOT_SEEN;
+	tcp_rsk(req)->accecn_fail_mode = 0;
 	tcp_rsk(req)->syn_ect_rcv = 0;
 	tcp_rsk(req)->syn_ect_snt = 0;
 	req->mss = rx_opt->mss_clamp;

net/ipv4/tcp_minisocks.c

@@ -463,6 +463,7 @@ static void tcp_ecn_openreq_child(struct sock *sk,
 		tcp_ecn_mode_set(tp, TCP_ECN_MODE_ACCECN);
 		tp->syn_ect_snt = treq->syn_ect_snt;
 		tcp_accecn_third_ack(sk, skb, treq->syn_ect_snt);
+		tp->saw_accecn_opt = treq->saw_accecn_opt;
 		tp->prev_ecnfield = treq->syn_ect_rcv;
 		tp->accecn_opt_demand = 1;
 		tcp_ecn_received_counters_payload(sk, skb);
@@ -678,6 +679,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 	bool own_req;
 
 	tmp_opt.saw_tstamp = 0;
+	tmp_opt.accecn = 0;
 	if (th->doff > (sizeof(struct tcphdr)>>2)) {
 		tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL);
 
@@ -855,6 +857,18 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 	if (!(flg & TCP_FLAG_ACK))
 		return NULL;
 
+	if (tcp_rsk(req)->accecn_ok && tmp_opt.accecn &&
+	    tcp_rsk(req)->saw_accecn_opt < TCP_ACCECN_OPT_COUNTER_SEEN) {
+		u8 saw_opt = tcp_accecn_option_init(skb, tmp_opt.accecn);
+
+		tcp_rsk(req)->saw_accecn_opt = saw_opt;
+		if (tcp_rsk(req)->saw_accecn_opt == TCP_ACCECN_OPT_FAIL_SEEN) {
+			u8 fail_mode = TCP_ACCECN_OPT_FAIL_RECV;
+
+			tcp_rsk(req)->accecn_fail_mode |= fail_mode;
+		}
+	}
+
 	/* For Fast Open no more processing is needed (sk is the
 	 * child socket).
 	 */

net/ipv4/tcp_output.c

@@ -985,9 +985,14 @@ static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
 		}
 	}
 
-	/* Simultaneous open SYN/ACK needs AccECN option but not SYN */
+	/* Simultaneous open SYN/ACK needs AccECN option but not SYN.
+	 * It is attempted to negotiate the use of AccECN also on the first
+	 * retransmitted SYN, as mentioned in "3.1.4.1. Retransmitted SYNs"
+	 * of AccECN draft.
+	 */
 	if (unlikely((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_ACK) &&
 		     tcp_ecn_mode_accecn(tp) &&
+		     inet_csk(sk)->icsk_retransmits < 2 &&
 		     READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_option) &&
 		     remaining >= TCPOLEN_ACCECN_BASE)) {
 		opts->use_synack_ecn_bytes = 1;
@@ -1076,7 +1081,7 @@ static unsigned int tcp_synack_options(const struct sock *sk,
 
 	if (treq->accecn_ok &&
 	    READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_option) &&
-	    remaining >= TCPOLEN_ACCECN_BASE) {
+	    req->num_timeout < 1 && remaining >= TCPOLEN_ACCECN_BASE) {
 		opts->use_synack_ecn_bytes = 1;
 		remaining -= tcp_options_fit_accecn(opts, 0, remaining);
 	}
@@ -1156,7 +1161,7 @@ static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
 	if (tcp_ecn_mode_accecn(tp)) {
 		int ecn_opt = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_option);
 
-		if (ecn_opt &&
+		if (ecn_opt && tp->saw_accecn_opt && !tcp_accecn_opt_fail_send(tp) &&
 		    (ecn_opt >= TCP_ACCECN_OPTION_FULL || tp->accecn_opt_demand ||
 		     tcp_accecn_option_beacon_check(sk))) {
 			opts->use_synack_ecn_bytes = 0;