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>

1 files changed, 3 insertions, 0 deletions

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 090f9ac43d4c..5b5c655ded1d 100644
--- a/net/ipv4/tcp.c
+++ b/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];