[PATCH net v2 2/2] selftests: net: check timestamp echo after an old ACK

[Jeff Jo <jeffjo@openai.com>]

Add a regression test for a gap-filling packet whose acknowledgment has
become old. Linux first sends data to the peer. Deliver two peer data
packets out of order: the later packet acknowledges Linux's data, while
the delayed packet still carries the earlier acknowledgment.

Require the ACK that closes the receive gap to echo the delayed packet's
timestamp, 301000. Without the fix, Linux accepts the data but still echoes
the previously saved timestamp, 1000. Also check that the application can
read all 34 bytes.

Use a large jump in peer timestamps to represent the idle interval, with
no real wait, loss or retransmission. The test directly checks the outgoing
timestamp echo. It requires packetdrill's merged TSecr verification fix
(linked below); older tools incorrectly pass on an unfixed kernel.

Use the existing packetdrill selftest runner for IPv4, IPv6 and
IPv4-mapped IPv6.

Link: https://github.com/google/packetdrill/commit/83f72d3f9085d0e26eb4d206fe4d7cfab5b6d872
Assisted-by: LLM
Signed-off-by: Jeff Jo <jeffjo@openai.com>
---
 .../net/packetdrill/tcp_old_ack_ts.pkt        | 22 +++++++++++++++++++
 1 file changed, 22 insertions(+)
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_old_ack_ts.pkt

diff --git a/tools/testing/selftests/net/packetdrill/tcp_old_ack_ts.pkt b/tools/testing/selftests/net/packetdrill/tcp_old_ack_ts.pkt
new file mode 100644
index 000000000000..f072e5e4c449
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_old_ack_ts.pkt
@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-2.0
+// An out-of-order segment advances SND.UNA. The gap filler still carries
+// its original, older ACK. The reply must echo the gap filler's timestamp.
+--tcp_ts_tick_usecs=1000
+
+`./defaults.sh`
+
+0.000 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
+0.100 < S 0:0(0) win 20000 <mss 1000,sackOK,TS val 900 ecr 0>
++0 > S. 0:0(0) ack 1 <mss 1460,sackOK,TS val 100 ecr 900>
+0.200 < . 1:1(0) ack 1 win 20000 <nop,nop,TS val 1000 ecr 100>
++0 accept(3, ..., ...) = 4
+0.300 write(4, ..., 17) = 17
++0 > P. 1:18(17) ack 1 <nop,nop,TS val 300 ecr 1000>
+0.310 < P. 18:35(17) ack 18 win 20000 <nop,nop,TS val 301001 ecr 300>
++0 > . 18:18(0) ack 1 <nop,nop,TS val 310 ecr 1000,nop,nop,sack 18:35>
+0.320 < P. 1:18(17) ack 1 win 20000 <nop,nop,TS val 301000 ecr 100>
++0 > . 18:18(0) ack 35 <nop,nop,TS val 320 ecr 301000>
++0 read(4, ..., 34) = 34
-- 
2.55.0