[PATCH net-next v2 3/3] selftests: packetdrill: cover scaled rwnd quantization slack

[Wesley Atwell <atwellwea@gmail.com>]

Add a packetdrill reproducer for the free_space-limited scaled
no-shrink quantization case.

Grow rcv_ssthresh with in-order data, then queue tiny OOO skbs so
receive memory drives raw free_space just below rcv_ssthresh without
advancing rcv_nxt. The final ACK reaches the case where raw free_space
sits just above 84 scaled units while rcv_ssthresh stays slightly
larger.

Old code rounds that final free_space value up and advertises 85. With
the fix, the ACK stays at 84.

This gives fail-before/pass-after coverage for the actual raw
free_space bug rather than the separate rcv_ssthresh-limited ALIGN-up
behavior.

Signed-off-by: Wesley Atwell <atwellwea@gmail.com>
---
v2:
- replace the old rcv_ssthresh-limited sequence with an OOO-memory
  reproducer for the raw free_space case
- drop the follow-on ACK transition and keep the final free_space
  failure case that proves the bug
- drop the IPv4-only restriction after verifying the test passes for
  ipv4, ipv6, and ipv4-mapped-ipv6
- reword the in-file comments to describe the observed behavior without
  referring to a removed path as "buggy"

 .../packetdrill/tcp_rcv_quantization_credit.pkt    | 62 ++++++++++++++++++++++
 1 file changed, 62 insertions(+)
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_rcv_quantization_credit.pkt

diff --git a/tools/testing/selftests/net/packetdrill/tcp_rcv_quantization_credit.pkt b/tools/testing/selftests/net/packetdrill/tcp_rcv_quantization_credit.pkt
new file mode 100644
index 000000000000..bb4f63772326
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_rcv_quantization_credit.pkt
@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: GPL-2.0
+
+--mss=1000
+
+`./defaults.sh
+sysctl -q net.ipv4.tcp_moderate_rcvbuf=0
+sysctl -q net.ipv4.tcp_shrink_window=0
+sysctl -q net.ipv4.tcp_rmem="4096 131072 $((32*1024*1024))"`
+
+// Exercise the scaled no-shrink path when raw backed free_space, not
+// rcv_ssthresh, limits the post-ACK window in __tcp_select_window().
+//
+// Grow rcv_ssthresh with in-order data, then queue tiny OOO skbs so receive
+// memory drops raw free_space to just over 84 scaled units while the current
+// rcv_ssthresh stays slightly larger. The final OOO ACK should keep the
+// sender-visible window at 84 rather than reopening it to 85.
+   +0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+   +0 < S 0:0(0) win 32792 <mss 1000,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 <mss 1460,nop,wscale 10>
+   +0 < . 1:1(0) ack 1 win 257
+
+   +0 accept(3, ..., ...) = 4
+
+   +0 < P. 1:10001(10000) ack 1 win 257
+   * > .  1:1(0) ack 10001
+
+   +0 < P. 10001:11024(1023) ack 1 win 257
+   * > .  1:1(0) ack 11024
+
+   +0 < P. 12024:12025(1) ack 1 win 257
+   * > .  1:1(0) ack 11024
+
+   +0 < P. 13024:13025(1) ack 1 win 257
+   * > .  1:1(0) ack 11024
+
+   +0 < P. 14024:14025(1) ack 1 win 257
+   * > .  1:1(0) ack 11024
+
+   +0 < P. 15024:15025(1) ack 1 win 257
+   * > .  1:1(0) ack 11024
+
+   +0 < P. 16024:16025(1) ack 1 win 257
+   * > .  1:1(0) ack 11024
+
+   +0 < P. 17024:17025(1) ack 1 win 257
+   * > .  1:1(0) ack 11024
+
+   +0 < P. 18024:18025(1) ack 1 win 257
+   * > .  1:1(0) ack 11024
+
+   +0 < P. 19024:19025(1) ack 1 win 257
+   * > .  1:1(0) ack 11024
+
+// The last tiny OOO skb pushes raw free_space just below rcv_ssthresh
+// without crossing the next lower scaled unit.
+   +0 < P. 20024:20025(1) ack 1 win 257
+   * > .  1:1(0) ack 11024 win 84
+   +0 %{ assert (tcpi_rcv_wnd >> 10) == 84, tcpi_rcv_wnd }%
-- 
2.43.0