selftests/net: packetdrill: add tcp_rcv_toobig.pkt

Check that TCP receiver behavior after "tcp: stronger sk_rcvbuf checks"

Too fat packet is dropped unless receive queue is empty.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250711114006.480026-9-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/packetdrill/tcp_rcv_toobig.pkt

@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: GPL-2.0
+
+--mss=1000
+
+`./defaults.sh`
+
+    0 `nstat -n`
+
+// 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
+
+   +0 < P. 1:20001(20000) ack 1 win 257
+ +.04 > .  1:1(0) ack 20001 win 18000
+
+   +0 setsockopt(4, SOL_SOCKET, SO_RCVBUF, [12000], 4) = 0
+   +0 < P. 20001:80001(60000) ack 1 win 257
+   +0 > .  1:1(0) ack 20001 win 18000
+
+   +0 read(4, ..., 20000) = 20000
+// A too big packet is accepted if the receive queue is empty
+   +0 < P. 20001:80001(60000) ack 1 win 257
+   +0 > .  1:1(0) ack 80001 win 0
+