Re: [PATCH net] tcp: reject completely old segments during sequence validation

On Tue, 18 Aug 2026 23:52:30 +0300 Michael Cohen wrote:

tcp_sequence() rejects an incoming segment when end_seq is before
rcv_wup. Since end_seq is one past the last sequence number consumed by
the segment, this misses the boundary case where end_seq is equal to
rcv_wup.

A segment that consumes sequence space and has end_seq equal to rcv_wup
is therefore allowed to reach later processing, including ACK handling,
even though it should be rejected as a completely old segment.

Reject this boundary case while retaining the existing behavior for
segments that consume no sequence space.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Michael Cohen <redacted>
Reported-by: Tamir Shahar <redacted>
Reported-by: Amit Klein <redacted>
Signed-off-by: Michael Cohen <redacted>

CI says the AccECN cases need to be adjusted.
Example failure output (tcp_accecn_synack_rexmit.pkt):

  tcp_accecn_synack_rexmit.pkt:15: error handling packet: live packet field
    tcp_ece: expected: 0 (0x0) vs actual: 1 (0x1)
  script packet:  0.104 .W  1:1(0) ack 1 <ECN e1b 1 ceb 0 e0b 1,nop>
  actual packet:  0.102 .EA 1:1(0) ack 1 win 1050 <ECN e1b 1 ceb 0 e0b 1,nop>

The AccECN tests expect the kernel to observe and react to the retransmitted
SYN-ACK, but the new boundary rejection prevents that.