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>

3 files changed, 53 insertions, 4 deletions

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index f637b659b35a..3ca5ed02de6d 100644
--- a/include/linux/tcp.h
+++ b/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 */
diff --git a/include/net/tcp_ecn.h b/include/net/tcp_ecn.h
index 133fb6b79500..f13e5cd2b1ac 100644
--- a/include/net/tcp_ecn.h
+++ b/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,
-				      u8 ip_dsfield)
+static inline void tcp_ecn_rcv_synack(struct sock *sk, const struct sk_buff *skb,
+				      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;
-		tp->accecn_opt_demand = 2;
+		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)) {
diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
index 53e0e85b52be..dce3113787a7 100644
--- a/include/uapi/linux/tcp.h
+++ b/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 */