[PATCH net v2 2/2] selftests/net: add packetdrill test for locked SO_RCVBUF SWS

Ankit Jain
2026-05-04T14:53:51Z

Add a packetdrill test to verify that TCP does not aggressively penalize
the `scaling_ratio` when an application explicitly locks the receive
buffer via SO_RCVBUF.

This test establishes a connection with a very small MSS, then injects
small payloads. On buggy kernels, the payload-to-truesize memory penalty
drops the `scaling_ratio` to 1, which shrinks the dynamically calculated
window and causes Silly Window Syndrome (SWS).

The test asserts that `tcpi_rcv_ssthresh` (the exposed window clamp)
remains protected and stays at a healthy level.

Signed-off-by: Ankit Jain <ankit-aj.jain@broadcom.com>
---
.../net/packetdrill/tcp_locked_rcvbuf_sws.pkt | 34 +++++++++++++++++++
1 file changed, 34 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..96d3a0813548
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_locked_rcvbuf_sws.pkt
@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test that TCP does not apply aggressive truesize penalties
+// to the scaling_ratio when the application has explicitly locked
+// the receive buffer, preventing Silly Window Syndrome.
+
+// 1. Create a socket and lock SO_RCVBUF to 32K
+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
+
+// 2. Establish connection with a TINY MSS (48 bytes)
+// This forces the kernel's rcv_mss expectation to initialize at rock bottom.
++0 < S 0:0(0) win 65535 <mss 48,nop,wscale 8>
++0 > S. 0:0(0) ack 1 <...>
++0 < . 1:1(0) ack 1 win 65535
++0 accept(3, ..., ...) = 4
+
+// 3. Inject 100 bytes.
+// 100 is > 48, so this forces tcp_measure_rcv_mss() to recalculate the scaling ratio.
+// Because the payload (100B) is small compared to the sk_buff truesize,
+// kernels without the fix will incorrectly drop the scaling_ratio.
++0.1 < P. 1:101(100) ack 1 win 65535
++0 > . 1:1(0) ack 101 <...>
+
+// 4. Inject 110 bytes to bypass the length jitter optimization and force
+// a second recalculation, driving the scaling_ratio to 1 and crushing the clamp.
++0.1 < P. 101:211(110) ack 1 win 65535
++0 > . 1:1(0) ack 211 <...>
+
+// 5. Assert the window clamp (tcpi_rcv_ssthresh) is protected from the penalty.
++0.1 %{
+assert tcpi_rcv_ssthresh > 15000, f"BUG DETECTED: scaling_ratio crushed! rcv_ssthresh={tcpi_rcv_ssthresh}"
+}%
--
2.53.0