[PATCH net-next 2/2] selftests/net/packetdrill: test SO_RCVBUF before connect keeps route window
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COOLING6d
From: Shashank Mohan Jain <hidden>
Date: 2026-10-11 11:16:23
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lkml
Subsystem:
kernel selftest framework, networking [general], the rest · Maintainers:
Shuah Khan, Shuah Khan, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds
Add a packetdrill test for the SYN window and window scale when SO_RCVBUF is set before connect() and the route to the peer carries a "window" metric (RTAX_WINDOW). The test puts "window 4096" on the route to the peer, sets SO_RCVBUF, connects, and checks that the SYN advertises the route-clamped window and a window scale of 0. Without the first patch of this series the SYN carried win 65535, wscale 5, because setsockopt(SO_RCVBUF) had written tp->window_clamp while the socket was still in TCP_CLOSE and tcp_connect_init() then skipped the route metric. The test is picked up automatically by the packetdrill selftest Makefile (TEST_PROGS := $(wildcard *.pkt)). Assisted-by: LLM Signed-off-by: Shashank Mohan Jain <redacted> --- .../tcp_rcvbuf_before_connect_window.pkt | 27 +++++++++++++++++++ 1 file changed, 27 insertions(+) create mode 100644 tools/testing/selftests/net/packetdrill/tcp_rcvbuf_before_connect_window.pkt
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..a3557ce80
--- /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 change 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
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2.43.0