[PATCH net-next v3 2/2] selftests/net/packetdrill: cover SO_RCVBUF vs the route window clamp
From: Shashank Mohan Jain
Date: Mon Oct 12 2026 - 09:38:08 EST
Add packetdrill tests for the interaction between SO_RCVBUF, a user
TCP_WINDOW_CLAMP and the route "window" metric (RTAX_WINDOW) on the SYN:
- tcp_rcvbuf_before_connect_window.pkt: SO_RCVBUF set before connect()
must not defeat the route metric; the SYN must advertise the
route-clamped window and window scale 0.
- tcp_window_clamp_vs_rcvbuf.pkt: a clamp set with TCP_WINDOW_CLAMP must
be honoured on the SYN and not be overwritten by a later SO_RCVBUF on
the still unconnected socket.
- tcp_fastopen_connect_rcvbuf_window.pkt: with TCP_FASTOPEN_CONNECT the
SYN is deferred to the first write() while the socket sits in
TCP_SYN_SENT; SO_RCVBUF applied in that gap must not defeat the route
metric on the deferred SYN either.
Without the first patch of this series the SYN carried win 65535,
wscale 5 in the first and third cases (the route metric was skipped) and
the SO_RCVBUF-derived window in the second (the user clamp was lost).
The tests are picked up automatically by the packetdrill selftest
Makefile (TEST_PROGS := $(wildcard *.pkt)).
Assisted-by: LLM
Signed-off-by: Shashank Mohan Jain <jain.sm@xxxxxxxxx>
---
.../tcp_fastopen_connect_rcvbuf_window.pkt | 22 +++++++++++++++
.../tcp_rcvbuf_before_connect_window.pkt | 27 +++++++++++++++++++
.../tcp_window_clamp_vs_rcvbuf.pkt | 24 +++++++++++++++++
3 files changed, 73 insertions(+)
create mode 100644 tools/testing/selftests/net/packetdrill/tcp_fastopen_connect_rcvbuf_window.pkt
create mode 100644 tools/testing/selftests/net/packetdrill/tcp_rcvbuf_before_connect_window.pkt
create mode 100644 tools/testing/selftests/net/packetdrill/tcp_window_clamp_vs_rcvbuf.pkt
diff --git a/tools/testing/selftests/net/packetdrill/tcp_fastopen_connect_rcvbuf_window.pkt b/tools/testing/selftests/net/packetdrill/tcp_fastopen_connect_rcvbuf_window.pkt
new file mode 100644
index 000000000..7892ac566
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_fastopen_connect_rcvbuf_window.pkt
@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-2.0
+// TCP_FASTOPEN_CONNECT defers the SYN until the first write(); between
+// connect() and that write the socket is in TCP_SYN_SENT. SO_RCVBUF applied
+// in that gap must not defeat the route "window" metric on the deferred SYN.
+// optname 30 = TCP_FASTOPEN_CONNECT, 34 = TCP_FASTOPEN_NO_COOKIE on SOL_TCP.
+--ip_version=ipv4
+
+`./defaults.sh
+sysctl -q net.ipv4.tcp_fastopen=5
+ip route replace 192.0.2.1 via 192.168.0.1 dev tun0 window 4096`
+
+ 0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+ +0 setsockopt(3, SOL_TCP, 30, [1], 4) = 0
+ +0 setsockopt(3, SOL_TCP, 34, [1], 4) = 0
+ +0 connect(3, ..., ...) = 0
+ +0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [1048576], 4) = 0
+ +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
+ +0 write(3, ..., 1) = 1
+
+// The deferred SYN (carrying 1 byte of data) must advertise the route
+// clamped window and window scale 0 (not win 65535, wscale 5).
+ +0 > S 0:1(1) win 2920 <mss 1460,sackOK,TS val 0 ecr 0,nop,wscale 0>
diff --git a/tools/testing/selftests/net/packetdrill/tcp_rcvbuf_before_connect_window.pkt b/tools/testing/selftests/net/packetdrill/tcp_rcvbuf_before_connect_window.pkt
new file mode 100644
index 000000000..7ec7f14d8
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_rcvbuf_before_connect_window.pkt
@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0
+// Setting SO_RCVBUF before connect() must not defeat the route "window"
+// metric on the SYN.
+//
+// The route to the peer carries "window 4096" (RTAX_WINDOW). With SO_RCVBUF
+// also set before connect(), the SYN the kernel sends must still advertise
+// the small, route-clamped receive window and a window scale of 0 (the
+// window scale option is sent, with a shift of 0).
+//
+// Since commit b025461303d8 ("tcp: update window_clamp when SO_RCVBUF is
+// set"), setsockopt(SO_RCVBUF) wrote tp->window_clamp while the socket was
+// still in TCP_CLOSE. tcp_connect_init() then treated that as a user clamp
+// and skipped RTAX_WINDOW, so the SYN carried win 65535, wscale 5 instead.
+--ip_version=ipv4
+
+`./defaults.sh
+ip route replace 192.0.2.1 via 192.168.0.1 dev tun0 window 4096`
+
+ 0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+ +0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [1048576], 4) = 0
+ +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
+ +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
+
+// The SYN must advertise the route-clamped window and window scale 0.
+ +0 > S 0:0(0) win 2920 <mss 1460,sackOK,TS val 0 ecr 0,nop,wscale 0>
+ +.1 < S. 0:0(0) ack 1 win 65535 <mss 1460,nop,wscale 7>
+ +0 > . 1:1(0) ack 1
diff --git a/tools/testing/selftests/net/packetdrill/tcp_window_clamp_vs_rcvbuf.pkt b/tools/testing/selftests/net/packetdrill/tcp_window_clamp_vs_rcvbuf.pkt
new file mode 100644
index 000000000..3d73178df
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_window_clamp_vs_rcvbuf.pkt
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0
+// A window clamp the user set with setsockopt(TCP_WINDOW_CLAMP) must be
+// honoured on the SYN even when SO_RCVBUF is set afterwards on the still
+// unconnected socket: SO_RCVBUF must not overwrite the user's clamp, and
+// the route "window" metric must not replace it either.
+//
+// TCP_WINDOW_CLAMP is optname 10 on SOL_TCP (this packetdrill build has no
+// symbol for it).
+--ip_version=ipv4
+
+`./defaults.sh
+ip route replace 192.0.2.1 via 192.168.0.1 dev tun0 window 4096`
+
+ 0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+ +0 setsockopt(3, SOL_TCP, 10, [16384], 4) = 0
+ +0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [1048576], 4) = 0
+ +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
+ +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
+
+// The SYN advertises the user's clamp (win 16060), not the route window
+// (2920) and not the SO_RCVBUF-derived full window.
+ +0 > S 0:0(0) win 16060 <mss 1460,sackOK,TS val 0 ecr 0,nop,wscale 0>
+ +.1 < S. 0:0(0) ack 1 win 65535 <mss 1460,nop,wscale 7>
+ +0 > . 1:1(0) ack 1
--
2.43.0