selftests/net: packetdrill: add tcp_rcv_neg_window.pkt

The test ensures we correctly apply the maximum advertised window limit
when rcv_nxt advances past rcv_mwnd_seq, so that the "usable window"
is properly clamped to zero rather than becoming negative.

Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260309-tcp_rfc7323_retract_wnd_rfc-v3-6-4c7f96b1ec69@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/tools/testing/selftests/net/packetdrill/tcp_rcv_neg_window.pkt b/tools/testing/selftests/net/packetdrill/tcp_rcv_neg_window.pkt
new file mode 100644 (file)
index 0000000..15a9b49
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_rcv_neg_window.pkt
@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0
+
+--mss=1000
+
+`./defaults.sh`
+
+// Establish a connection.
+   +0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [20000], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+   +0 < S 0:0(0) win 32792 <mss 1000,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 win 18980 <mss 1460,nop,wscale 0>
+  +.1 < . 1:1(0) ack 1 win 257
+
+   +0 accept(3, ..., ...) = 4
+
+// A too big packet is accepted if the receive queue is empty
+   +0 < P. 1:20001(20000) ack 1 win 257
+// Send a RST immediately so that there is no rcv_wup/rcv_mwnd_seq update yet
+   +0 < R. 20001:20001(0) ack 1 win 257
+
+  +.1 %{ assert tcpi_state == TCP_CLOSE, tcpi_state }%
+