selftests: drv-net: psp: add test for auto-adjusting TCP MSS

Test TCP MSS getting auto-adjusted. PSP adds an encapsulation overhead
of 40B per packet, when used in transport mode without any
virtualization cookie or other optional PSP header fields. The kernel
should adjust the MSS for a connection after PSP tx state is reached.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Link: https://patch.msgid.link/20250927225420.1443468-8-kuba@kernel.org
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/drivers/net/psp.py

@@ -431,6 +431,45 @@ def data_send_disconnect(cfg):
         s.close()
 
 
+def _data_mss_adjust(cfg, ipver):
+    _init_psp_dev(cfg)
+
+    # First figure out what the MSS would be without any adjustments
+    s = _make_clr_conn(cfg, ipver)
+    s.send(b"0123456789abcdef" * 1024)
+    _check_data_rx(cfg, 16 * 1024)
+    mss = s.getsockopt(socket.IPPROTO_TCP, socket.TCP_MAXSEG)
+    _close_conn(cfg, s)
+
+    s = _make_psp_conn(cfg, 0, ipver)
+    try:
+        rx_assoc = cfg.pspnl.rx_assoc({"version": 0,
+                                     "dev-id": cfg.psp_dev_id,
+                                     "sock-fd": s.fileno()})
+        rx = rx_assoc['rx-key']
+        tx = _spi_xchg(s, rx)
+
+        rxmss = s.getsockopt(socket.IPPROTO_TCP, socket.TCP_MAXSEG)
+        ksft_eq(mss, rxmss)
+
+        cfg.pspnl.tx_assoc({"dev-id": cfg.psp_dev_id,
+                          "version": 0,
+                          "tx-key": tx,
+                          "sock-fd": s.fileno()})
+
+        txmss = s.getsockopt(socket.IPPROTO_TCP, socket.TCP_MAXSEG)
+        ksft_eq(mss, txmss + 40)
+
+        data_len = _send_careful(cfg, s, 100)
+        _check_data_rx(cfg, data_len)
+        _check_data_outq(s, 0)
+
+        txmss = s.getsockopt(socket.IPPROTO_TCP, socket.TCP_MAXSEG)
+        ksft_eq(mss, txmss + 40)
+    finally:
+        _close_psp_conn(cfg, s)
+
+
 def data_stale_key(cfg):
     """ Test send on a double-rotated key """
     _init_psp_dev(cfg)
@@ -470,6 +509,15 @@ def psp_ip_ver_test_builder(name, test_func, psp_ver, ipver):
     return test_case
 
 
+def ipver_test_builder(name, test_func, ipver):
+    """Build test cases for each IP version"""
+    def test_case(cfg):
+        cfg.require_ipver(ipver)
+        test_case.__name__ = f"{name}_ip{ipver}"
+        test_func(cfg, ipver)
+    return test_case
+
+
 def main() -> None:
     """ Ksft boiler plate main """
 
@@ -497,6 +545,10 @@ def main() -> None:
                     for version in range(0, 4)
                     for ipver in ("4", "6")
                 ]
+                cases += [
+                    ipver_test_builder("data_mss_adjust", _data_mss_adjust, ipver)
+                    for ipver in ("4", "6")
+                ]
 
                 ksft_run(cases=cases, globs=globals(),
                          case_pfx={"dev_", "data_", "assoc_"},