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 <[email protected]> --- .../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
