tcp: refine tcp_sequence() for the FIN exception

Commit 0e24d17bd966 ("tcp: implement RFC 7323 window retraction
receiver requirements") removed the special FIN case that
was added in commit 1e3bb184e941 ("tcp: re-enable acceptance of
FIN packets when RWIN is 0").

If a peer sends a segment containing data and a FIN flag before
it learns about our window retraction and has a buggy TCP stack,
it might place the FIN one byte beyond what it thinks is the
right edge of the window (i.e., max_window_edge + 1).

The data portion (end_seq - th->fin) will end exactly at max_window_edge.
In this case, we will drop the packet if our receive queue is not empty,
even though the data was sent within the window we previously allowed.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Simon Baatz <gmbnomis@gmail.com>
Link: https://patch.msgid.link/20260608151452.706822-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions