[PATCH net v3 2/2] selftests/net: add packetdrill test for locked SO_RCVBUF SWS

[Ankit Jain <ankit-aj.jain@broadcom.com>]

Add a packetdrill test to verify that locked SO_RCVBUF sockets do not
suffer from scaling_ratio truesize penalties.

The test uses a standard 1460 MSS and sends medium-sized packets
(600, 700, 800 bytes) to trigger the recalculation logic. It checks
that the internal window clamp (tcpi_rcv_ssthresh) does not drop
unexpectedly.

Signed-off-by: Ankit Jain <ankit-aj.jain@broadcom.com>
---
 .../net/packetdrill/tcp_locked_rcvbuf_sws.pkt | 29 +++++++++++++++++++
 1 file changed, 29 insertions(+)
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_locked_rcvbuf_sws.pkt

diff --git a/tools/testing/selftests/net/packetdrill/tcp_locked_rcvbuf_sws.pkt b/tools/testing/selftests/net/packetdrill/tcp_locked_rcvbuf_sws.pkt
new file mode 100644
index 000000000000..43e1d00d5f26
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_locked_rcvbuf_sws.pkt
@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test that TCP does not reduce scaling_ratio for locked SO_RCVBUF.
+
+0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
++0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [32768], 4) = 0
++0 bind(3, ..., ...) = 0
++0 listen(3, 1) = 0
+
+// Establish connection with standard MSS.
++0 < S 0:0(0) win 65535 <mss 1460,nop,wscale 8>
++0 > S. 0:0(0) ack 1 <...>
++0 < . 1:1(0) ack 1 win 65535
++0 accept(3, ..., ...) = 4
+
+// Inject varying payload sizes to force scaling_ratio recalculation.
++0.1 < P. 1:601(600) ack 1 win 65535
++0 > . 1:1(0) ack 601 <...>
+
++0.1 < P. 601:1301(700) ack 1 win 65535
++0 > . 1:1(0) ack 1301 <...>
+
++0.1 < P. 1301:2101(800) ack 1 win 65535
++0 > . 1:1(0) ack 2101 <...>
+
+// Check that truesize penalty did not reduce the window clamp.
+// On unpatched kernels, rcv_ssthresh drops to ~22K.
++0.1 %{
+assert tcpi_rcv_ssthresh > 28000, f"rcv_ssthresh dropped unexpectedly: {tcpi_rcv_ssthresh}"
+}%
-- 
2.53.0