[PATCH net-next 2/2] selftests/net/packetdrill: test SO_RCVBUF before connect keeps route window

[Shashank Mohan Jain <jain.sm@gmail.com>]

Add a packetdrill test for the SYN window and window scale when
SO_RCVBUF is set before connect() and the route to the peer carries a
"window" metric (RTAX_WINDOW).

The test puts "window 4096" on the route to the peer, sets SO_RCVBUF,
connects, and checks that the SYN advertises the route-clamped window
and a window scale of 0. Without the first patch of this series the SYN
carried win 65535, wscale 5, because setsockopt(SO_RCVBUF) had written
tp->window_clamp while the socket was still in TCP_CLOSE and
tcp_connect_init() then skipped the route metric.

The test is picked up automatically by the packetdrill selftest Makefile
(TEST_PROGS := $(wildcard *.pkt)).

Assisted-by: LLM
Signed-off-by: Shashank Mohan Jain <jain.sm@gmail.com>
---
 .../tcp_rcvbuf_before_connect_window.pkt      | 27 +++++++++++++++++++
 1 file changed, 27 insertions(+)
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_rcvbuf_before_connect_window.pkt

diff --git a/tools/testing/selftests/net/packetdrill/tcp_rcvbuf_before_connect_window.pkt b/tools/testing/selftests/net/packetdrill/tcp_rcvbuf_before_connect_window.pkt
new file mode 100644
index 000000000..a3557ce80
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_rcvbuf_before_connect_window.pkt
@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0
+// Setting SO_RCVBUF before connect() must not defeat the route "window"
+// metric on the SYN.
+//
+// The route to the peer carries "window 4096" (RTAX_WINDOW). With SO_RCVBUF
+// also set before connect(), the SYN the kernel sends must still advertise
+// the small, route-clamped receive window and a window scale of 0 (the
+// window scale option is sent, with a shift of 0).
+//
+// Since commit b025461303d8 ("tcp: update window_clamp when SO_RCVBUF is
+// set"), setsockopt(SO_RCVBUF) wrote tp->window_clamp while the socket was
+// still in TCP_CLOSE. tcp_connect_init() then treated that as a user clamp
+// and skipped RTAX_WINDOW, so the SYN carried win 65535, wscale 5 instead.
+--ip_version=ipv4
+
+`./defaults.sh
+ip route change 192.0.2.1 via 192.168.0.1 dev tun0 window 4096`
+
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [1048576], 4) = 0
+   +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
+   +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
+
+// The SYN must advertise the route-clamped window and window scale 0.
+   +0 > S 0:0(0) win 2920 <mss 1460,sackOK,TS val 0 ecr 0,nop,wscale 0>
+  +.1 < S. 0:0(0) ack 1 win 65535 <mss 1460,nop,wscale 7>
+   +0 > . 1:1(0) ack 1
-- 
2.43.0