From: Chia-Yu Chang <redacted>

According to Section 3.2.2.1 of AccECN spec (RFC9768), if the Server
is in AccECN mode and in SYN-RCVD state, and if it receives a value of
zero on a pure ACK with SYN=0 and no SACK blocks, for the rest of the
connection the Server MUST NOT set ECT on outgoing packets and MUST
NOT respond to AccECN feedback. Nonetheless, as a Data Receiver it
MUST NOT disable AccECN feedback.

Signed-off-by: Chia-Yu Chang <redacted>
---
 include/net/tcp_ecn.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/net/tcp_ecn.h b/include/net/tcp_ecn.h
index 2256d2efa5ec..8317c3f279c9 100644
--- a/include/net/tcp_ecn.h
+++ b/include/net/tcp_ecn.h

@@ -169,7 +169,10 @@ static inline void tcp_accecn_third_ack(struct sock *sk,
 	switch (ace) {
 	case 0x0:
 		/* Invalid value */
-		tcp_accecn_fail_mode_set(tp, TCP_ACCECN_ACE_FAIL_RECV);
+		if (!TCP_SKB_CB(skb)->sacked) {
+			tcp_accecn_fail_mode_set(tp, TCP_ACCECN_ACE_FAIL_RECV |
+						     TCP_ACCECN_OPT_FAIL_RECV);
+		}
 		break;
 	case 0x7:
 	case 0x5:

-- 
2.34.1