[PATCH net-next v4 0/2] tcp: keep the route window metric when SO_RCVBUF is set before connect

[PATCH net-next v4 0/2] tcp: keep the route window metric when SO_RCVBUF is set before connect

[Shashank Mohan Jain]

Since commit b025461303d8 ("tcp: update window_clamp when SO_RCVBUF is
set"), SO_RCVBUF set before connect() or on a listener leaves a non-zero
window_clamp behind. The SYN, SYN-ACK and SYN cookie paths take that for
a TCP_WINDOW_CLAMP set by the user, so the route "window" metric
(RTAX_WINDOW) is skipped, and a real TCP_WINDOW_CLAMP set before
SO_RCVBUF is overwritten.

Patch 1 records the clamp the user set with TCP_WINDOW_CLAMP in a new
field and keys the SYN, SYN-ACK and SYN cookie paths off it. Patch 2
adds nine packetdrill tests (active open, passive open, SYN cookies over
IPv4 and IPv6, TCP_WINDOW_CLAMP reset, SO_RCVBUF raise on an established
socket). All nine fail on net-next without patch 1 and pass with it.

As Jakub asked, the commit message of patch 1 now says how the problem
was found and how it was tested.

Changes since v3, all from the AI review of v3 (each point was checked
against net-next before acting on it):

 - SYN cookies: cookie_v4_check() and cookie_v6_check() still used
   "window_clamp ?: RTAX_WINDOW". With SO_RCVBUF on the listener and a
   route window metric, v3 sent a SYN-ACK with wscale 0 while the cookie
   child used rcv_wscale 5 (reproduced on the wire with v3, and by the
   new cookie tests). The SYN-ACK path and both cookie paths now use
   one helper, tcp_req_window_clamp().

 - A single bit could leave window_clamp too low: v3's tcp_set_rcvbuf()
   returned early once TCP_WINDOW_CLAMP was set, so after
   tcp_init_buffer_space() lowered window_clamp, a larger SO_RCVBUF
   could not raise it again. v4 stores the requested clamp (u32
   window_clamp_user) and lets SO_RCVBUF move window_clamp up to it.

 - Lockless read of a bitfield: the listener side now reads a separate
   u32 with READ_ONCE(), written with WRITE_ONCE(), instead of a bit in
   a byte shared with nonagle and rate_app_limited.

 - window_clamp_user is listed in net_cachelines/tcp_sock.rst. It sits
   in the cold part of struct tcp_sock; pahole shows the size (2432
   bytes on x86_64, with and without MPTCP) and the fast path groups
   unchanged.

 - Selftests: symbolic TCP_FASTOPEN_CONNECT and TCP_FASTOPEN_NO_COOKIE
   (TCP_WINDOW_CLAMP stays numeric because packetdrill has no symbol for
   it, and the comment no longer refers to a local build); new listener
   tests for SO_RCVBUF alone and for TCP_WINDOW_CLAMP then SO_RCVBUF;
   SYN cookie tests for IPv4 and IPv6 that check tcpi_rcv_wscale of the
   child; a TCP_WINDOW_CLAMP=0 reset test; a test that SO_RCVBUF raises
   window_clamp up to the user's clamp and not above it.

 - Patch 1 changelog: how the bug was found and tested; the cookie paths
   are named.

 - Rebased on current net-next.

Link: https://lore.kernel.org/all/20261012132622.48550-1-jain.sm@gmail.com/ (v3)

Changes since v2:
 - track a user TCP_WINDOW_CLAMP instead of checking the socket state,
   so the TCP_SYN_SENT paths (TCP_FASTOPEN_CONNECT, sock_ops) are fixed
   too; corrected the changelog; two more selftests.

Changes since v1 (single [PATCH net]):
 - retargeted from net to net-next, added the packetdrill selftest, and
   rewrote the changelog.

Shashank Mohan Jain (2):
  tcp: keep the route window metric when SO_RCVBUF is set before connect
  selftests/net/packetdrill: cover SO_RCVBUF vs the route window clamp

 .../networking/net_cachelines/tcp_sock.rst    |  1 +
 include/linux/tcp.h                           |  1 +
 include/net/tcp.h                             | 14 ++++++++++
 net/ipv4/syncookies.c                         |  3 +--
 net/ipv4/tcp.c                                | 14 +++++++++-
 net/ipv4/tcp_minisocks.c                      |  4 +--
 net/ipv4/tcp_output.c                         |  2 +-
 net/ipv6/syncookies.c                         |  2 +-
 .../tcp_fastopen_connect_rcvbuf_window.pkt    | 21 +++++++++++++++
 .../tcp_rcvbuf_before_connect_window.pkt      | 27 +++++++++++++++++++
 .../tcp_rcvbuf_listener_window.pkt            | 20 ++++++++++++++
 .../tcp_syncookies_rcvbuf_window_ip4.pkt      | 22 +++++++++++++++
 .../tcp_syncookies_rcvbuf_window_ip6.pkt      | 22 +++++++++++++++
 .../tcp_window_clamp_rcvbuf_raise.pkt         | 27 +++++++++++++++++++
 .../tcp_window_clamp_reset_rcvbuf.pkt         | 22 +++++++++++++++
 .../tcp_window_clamp_vs_rcvbuf.pkt            | 23 ++++++++++++++++
 .../tcp_window_clamp_vs_rcvbuf_listener.pkt   | 25 +++++++++++++++++
 17 files changed, 242 insertions(+), 8 deletions(-)
 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_rcvbuf_listener_window.pkt
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_syncookies_rcvbuf_window_ip4.pkt
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_syncookies_rcvbuf_window_ip6.pkt
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_window_clamp_rcvbuf_raise.pkt
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_window_clamp_reset_rcvbuf.pkt
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_window_clamp_vs_rcvbuf.pkt
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_window_clamp_vs_rcvbuf_listener.pkt


base-commit: 279d5c8afcd38bd841832a39bfe1aa4f8518d98e
--
2.43.0

[PATCH net-next v4 1/2] tcp: keep the route window metric when SO_RCVBUF is set before connect

[Shashank Mohan Jain]

Since commit b025461303d8 ("tcp: update window_clamp when SO_RCVBUF is
set"), setsockopt(SO_RCVBUF) writes tp->window_clamp in every socket
state through tcp_set_window_clamp().

tcp_connect_init(), tcp_openreq_init_rwin() and the SYN cookie paths
(cookie_v4_check(), cookie_v6_check()) treat any non-zero window_clamp
as a clamp the user asked for and, in that case, skip the route window
metric (RTAX_WINDOW, "ip route ... window N"). A clamp left behind by an
SO_RCVBUF call therefore has two observable effects:

 (1) on a route that carries an RTAX_WINDOW metric, SO_RCVBUF set before
     connect() (or on a listener) changes the window and window scale
     advertised in the SYN / SYN-ACK, because the route metric is no
     longer applied;

 (2) a window clamp the application set with setsockopt(TCP_WINDOW_CLAMP)
     is overwritten by a later SO_RCVBUF on the still unconnected socket
     or on a listener.

Without an RTAX_WINDOW metric and without TCP_WINDOW_CLAMP nothing
changes: before and after this patch the advertised window and scale
are derived from sk_rcvbuf (tcp_full_space()).

Keying the decision off the socket state is not enough, because the
socket is already in TCP_SYN_SENT before tcp_connect_init() runs.
SO_RCVBUF applied in that gap - through TCP_FASTOPEN_CONNECT before the
first write(), or through bpf_setsockopt() in a
BPF_SOCK_OPS_TCP_CONNECT_CB program - still lands before
tcp_connect_init().

Record instead the clamp the user set with TCP_WINDOW_CLAMP, in a new
tp->window_clamp_user field (0 when there is none; TCP_WINDOW_CLAMP=0
and tcp_disconnect() reset it), and use it as follows:

 - tcp_connect_init() applies the route metric whenever there is no
   user clamp, in any socket state.

 - tcp_openreq_init_rwin(), cookie_v4_check() and cookie_v6_check() all
   pick the request's clamp with the same new helper,
   tcp_req_window_clamp(): the listener's window_clamp if the user set
   one, else the route metric. The SYN-ACK and the SYN cookie path must
   agree, or the child of a cookie would use a different receive window
   scale than the one advertised in the SYN-ACK. The listener is not
   locked on these paths, so the field is written with WRITE_ONCE() and
   read with READ_ONCE().

 - tcp_set_rcvbuf() keeps updating window_clamp from the buffer size,
   but caps it at the user's clamp. SO_RCVBUF on an established socket
   thus still grows window_clamp, which is what the commit above was
   for, also after tcp_init_buffer_space() lowered it below the user's
   clamp, but no longer above a clamp set with TCP_WINDOW_CLAMP.

A value is stored, not a flag, so that SO_RCVBUF can raise window_clamp
back up to the user's clamp. The new u32 sits in the cold part of
struct tcp_sock; on x86_64 the size of struct tcp_sock (2432 bytes, with
and without MPTCP) and the fast path cacheline groups do not change.

Example: two netns joined by a veth pair, "ip route ... window 4096" on
the routes between them, SO_RCVBUF set to 1 MiB:

  before:  SYN win 65535, wscale 5      (SO_RCVBUF before connect())
           SYN-ACK win 65535, wscale 5  (SO_RCVBUF on the listener)
  after:   SYN win 2920, wscale 0
           SYN-ACK win 2896, wscale 0

(the window scale option is still sent, with a shift of 0.)

The problem was found by code inspection, assisted by an LLM (Claude
Code, Claude Opus 5.5), while reviewing the commit above. It was
confirmed with a reproducer in a qemu guest (two netns joined by a veth
pair, "ip route ... window 4096", tcpdump of the SYN and SYN-ACK, and
TCP_INFO of both ends, including a SYN cookie case), and with the
packetdrill tests added by the next patch, which fail without this
patch and pass with it.

Fixes: b025461303d8 ("tcp: update window_clamp when SO_RCVBUF is set")
Assisted-by: LLM
Signed-off-by: Shashank Mohan Jain <jain.sm@gmail.com>
---
v4:
 - also convert cookie_v4_check() and cookie_v6_check(); the SYN-ACK and
   cookie paths now share tcp_req_window_clamp()
 - store the user's clamp (u32 window_clamp_user, READ_ONCE/WRITE_ONCE)
   instead of a bit, and let SO_RCVBUF raise window_clamp up to it
 - add the field to net_cachelines/tcp_sock.rst
 - say in the changelog how the bug was found and tested

 .../networking/net_cachelines/tcp_sock.rst         |  1 +
 include/linux/tcp.h                                |  1 +
 include/net/tcp.h                                  | 14 ++++++++++++++
 net/ipv4/syncookies.c                              |  3 +--
 net/ipv4/tcp.c                                     | 14 +++++++++++++-
 net/ipv4/tcp_minisocks.c                           |  4 +---
 net/ipv4/tcp_output.c                              |  2 +-
 net/ipv6/syncookies.c                              |  2 +-
 8 files changed, 33 insertions(+), 8 deletions(-)

diff --git a/Documentation/networking/net_cachelines/tcp_sock.rst b/Documentation/networking/net_cachelines/tcp_sock.rst
index 103328dc409..f317c7154d2 100644
--- a/Documentation/networking/net_cachelines/tcp_sock.rst
+++ b/Documentation/networking/net_cachelines/tcp_sock.rst
@@ -152,6 +152,7 @@ u32                           urg_seq
 unsigned_int                  keepalive_time
 unsigned_int                  keepalive_intvl
 int                           linger2
+u32                           window_clamp_user
 u8                            bpf_sock_ops_cb_flags
 u8:1                          bpf_chg_cc_inprogress
 u16                           timeout_rehash
diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index e4d1720f49f..4f28fa6c4f4 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -474,6 +474,7 @@ struct tcp_sock {
 	unsigned int		keepalive_intvl;  /* time interval between keep alive probes */
 
 	int			linger2;
+	u32	window_clamp_user; /* TCP_WINDOW_CLAMP set by the user, 0 if none */
 
 
 /* Sock_ops bpf program related variables */
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 0265c27a5fd..35b64fffd74 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1879,6 +1879,20 @@ extern void tcp_openreq_init_rwin(struct request_sock *req,
 				  const struct sock *sk_listener,
 				  const struct dst_entry *dst);
 
+/* Window clamp for a request socket of listener @tp: the listener's clamp
+ * if the user set one with TCP_WINDOW_CLAMP, else the route window metric.
+ * The SYN-ACK and the SYN cookie paths must make the same choice, or the
+ * child's window scale would not match the one sent in the SYN-ACK.
+ * The listener is not locked here.
+ */
+static inline u32 tcp_req_window_clamp(const struct tcp_sock *tp,
+				       const struct dst_entry *dst)
+{
+	if (READ_ONCE(tp->window_clamp_user))
+		return READ_ONCE(tp->window_clamp);
+	return dst_metric(dst, RTAX_WINDOW);
+}
+
 void tcp_enter_memory_pressure(struct sock *sk);
 void tcp_leave_memory_pressure(struct sock *sk);
 
diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
index 73e12976818..6e78efbdc31 100644
--- a/net/ipv4/syncookies.c
+++ b/net/ipv4/syncookies.c
@@ -472,8 +472,7 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
 	}
 
 	/* Try to redo what tcp_v4_send_synack did. */
-	req->rsk_window_clamp = READ_ONCE(tp->window_clamp) ? :
-				dst_metric(&rt->dst, RTAX_WINDOW);
+	req->rsk_window_clamp = tcp_req_window_clamp(tp, &rt->dst);
 	/* limit the window selection if the user enforce a smaller rx buffer */
 	full_space = tcp_full_space(sk);
 	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 99d7237a623..64465bc257d 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1859,7 +1859,15 @@ int tcp_set_rcvlowat(struct sock *sk, int val)
 
 void tcp_set_rcvbuf(struct sock *sk, int val)
 {
-	tcp_set_window_clamp(sk, tcp_win_from_space(sk, val));
+	u32 user_clamp = tcp_sk(sk)->window_clamp_user;
+	int win = tcp_win_from_space(sk, val);
+
+	/* window_clamp follows the receive buffer, but never goes above a
+	 * clamp the user set with TCP_WINDOW_CLAMP.
+	 */
+	if (user_clamp)
+		win = min_t(u32, win, user_clamp);
+	tcp_set_window_clamp(sk, win);
 }
 
 #ifdef CONFIG_MMU
@@ -3429,6 +3437,7 @@ int tcp_disconnect(struct sock *sk, int flags)
 	tp->is_cwnd_limited = 0;
 	tp->max_packets_out = 0;
 	tp->window_clamp = 0;
+	WRITE_ONCE(tp->window_clamp_user, 0);
 	tp->delivered = 0;
 	tp->delivered_ce = 0;
 	tp->accecn_fail_mode = 0;
@@ -4058,6 +4067,9 @@ int do_tcp_setsockopt(struct sock *sk, int level, int optname,
 
 	case TCP_WINDOW_CLAMP:
 		err = tcp_set_window_clamp(sk, val);
+		if (!err)
+			WRITE_ONCE(tp->window_clamp_user,
+				   val ? tp->window_clamp : 0);
 		break;
 
 	case TCP_QUICKACK:
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index 0ddfd5af6e5..c17dc5c4648 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -435,15 +435,13 @@ void tcp_openreq_init_rwin(struct request_sock *req,
 	struct inet_request_sock *ireq = inet_rsk(req);
 	const struct tcp_sock *tp = tcp_sk(sk_listener);
 	int full_space = tcp_full_space(sk_listener);
-	u32 window_clamp;
 	__u8 rcv_wscale;
 	u32 rcv_wnd;
 	int mss;
 
 	mss = tcp_mss_clamp(tp, tcp_dst_advmss(dst));
-	window_clamp = READ_ONCE(tp->window_clamp);
 	/* Set this up on the first call only */
-	req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);
+	req->rsk_window_clamp = tcp_req_window_clamp(tp, dst);
 
 	/* limit the window selection if the user enforce a smaller rx buffer */
 	if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK &&
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index c8865205cdc..21b4ae303dd 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -4137,7 +4137,7 @@ static void tcp_connect_init(struct sock *sk)
 
 	tcp_ca_dst_init(sk, dst);
 
-	if (!tp->window_clamp)
+	if (!tp->window_clamp_user)
 		WRITE_ONCE(tp->window_clamp, dst_metric(dst, RTAX_WINDOW));
 	tp->advmss = tcp_mss_clamp(tp, tcp_dst_advmss(dst));
 
diff --git a/net/ipv6/syncookies.c b/net/ipv6/syncookies.c
index b581cb1ee2e..d7d61f8d25e 100644
--- a/net/ipv6/syncookies.c
+++ b/net/ipv6/syncookies.c
@@ -254,7 +254,7 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 		}
 	}
 
-	req->rsk_window_clamp = READ_ONCE(tp->window_clamp) ? :dst_metric(dst, RTAX_WINDOW);
+	req->rsk_window_clamp = tcp_req_window_clamp(tp, dst);
 	/* limit the window selection if the user enforce a smaller rx buffer */
 	full_space = tcp_full_space(sk);
 	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
-- 
2.43.0

[PATCH net-next v4 2/2] selftests/net/packetdrill: cover SO_RCVBUF vs the route window clamp

[Shashank Mohan Jain]

Add packetdrill tests for the interaction between SO_RCVBUF, a user
TCP_WINDOW_CLAMP and the route "window" metric (RTAX_WINDOW).

Active open, checked on the SYN:

 - tcp_rcvbuf_before_connect_window.pkt: SO_RCVBUF set before connect()
   must not defeat the route metric.

 - tcp_window_clamp_vs_rcvbuf.pkt: a clamp set with TCP_WINDOW_CLAMP must
   not be overwritten by a later SO_RCVBUF on the unconnected socket.

 - tcp_window_clamp_reset_rcvbuf.pkt: TCP_WINDOW_CLAMP=0 removes the
   user's clamp, so a later SO_RCVBUF must again not defeat the route
   metric.

 - 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 either.

Passive open, checked on the SYN-ACK and on the child:

 - tcp_rcvbuf_listener_window.pkt: SO_RCVBUF on the listener must not
   defeat the route metric.

 - tcp_window_clamp_vs_rcvbuf_listener.pkt: TCP_WINDOW_CLAMP and then
   SO_RCVBUF on the listener; the SYN-ACK must use the user's clamp.

 - tcp_syncookies_rcvbuf_window_ip4.pkt and its _ip6 twin: with
   net.ipv4.tcp_syncookies=2, the child created from the cookie must use
   the window scale the SYN-ACK advertised (tcpi_rcv_wscale).

Established socket:

 - tcp_window_clamp_rcvbuf_raise.pkt: after tcp_init_buffer_space()
   lowered window_clamp, a larger SO_RCVBUF must raise it again, up to
   the value set with TCP_WINDOW_CLAMP and not beyond.

All nine tests fail without the previous patch and pass with it. They
are picked up by the packetdrill selftest Makefile
(TEST_PROGS := $(wildcard *.pkt)).

Assisted-by: LLM
Signed-off-by: Shashank Mohan Jain <jain.sm@gmail.com>
---
v4:
 - symbolic TCP_FASTOPEN_CONNECT / TCP_FASTOPEN_NO_COOKIE
 - new: listener (SYN-ACK) tests, SYN cookie tests for IPv4 and IPv6,
   TCP_WINDOW_CLAMP=0 reset test, SO_RCVBUF raise test

 .../tcp_fastopen_connect_rcvbuf_window.pkt    | 21 +++++++++++++++
 .../tcp_rcvbuf_before_connect_window.pkt      | 27 +++++++++++++++++++
 .../tcp_rcvbuf_listener_window.pkt            | 20 ++++++++++++++
 .../tcp_syncookies_rcvbuf_window_ip4.pkt      | 22 +++++++++++++++
 .../tcp_syncookies_rcvbuf_window_ip6.pkt      | 22 +++++++++++++++
 .../tcp_window_clamp_rcvbuf_raise.pkt         | 27 +++++++++++++++++++
 .../tcp_window_clamp_reset_rcvbuf.pkt         | 22 +++++++++++++++
 .../tcp_window_clamp_vs_rcvbuf.pkt            | 23 ++++++++++++++++
 .../tcp_window_clamp_vs_rcvbuf_listener.pkt   | 25 +++++++++++++++++
 9 files changed, 209 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_rcvbuf_listener_window.pkt
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_syncookies_rcvbuf_window_ip4.pkt
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_syncookies_rcvbuf_window_ip6.pkt
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_window_clamp_rcvbuf_raise.pkt
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_window_clamp_reset_rcvbuf.pkt
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_window_clamp_vs_rcvbuf.pkt
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_window_clamp_vs_rcvbuf_listener.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 00000000000..b297294ed16
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_fastopen_connect_rcvbuf_window.pkt
@@ -0,0 +1,21 @@
+// 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.
+--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, TCP_FASTOPEN_CONNECT, [1], 4) = 0
+   +0 setsockopt(3, SOL_TCP, TCP_FASTOPEN_NO_COOKIE, [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 00000000000..7ec7f14d8e8
--- /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_rcvbuf_listener_window.pkt b/tools/testing/selftests/net/packetdrill/tcp_rcvbuf_listener_window.pkt
new file mode 100644
index 00000000000..ed20a75fa9a
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_rcvbuf_listener_window.pkt
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0
+// Passive side: SO_RCVBUF set on a listener must not defeat the route
+// "window" metric on the SYN-ACK, and the child must use the window scale
+// that the SYN-ACK advertised.
+--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_REUSEADDR, [1], 4) = 0
+   +0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [1048576], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+   +0 < S 0:0(0) win 65535 <mss 1460,sackOK,TS val 100 ecr 0,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 win 2896 <mss 1460,sackOK,TS val 100 ecr 100,nop,wscale 0>
+ +.01 < . 1:1(0) ack 1 win 514 <nop,nop,TS val 110 ecr 100>
+   +0 accept(3, ..., ...) = 4
+   +0 %{ assert tcpi_rcv_wscale == 0, tcpi_rcv_wscale }%
diff --git a/tools/testing/selftests/net/packetdrill/tcp_syncookies_rcvbuf_window_ip4.pkt b/tools/testing/selftests/net/packetdrill/tcp_syncookies_rcvbuf_window_ip4.pkt
new file mode 100644
index 00000000000..e26f78704ab
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_syncookies_rcvbuf_window_ip4.pkt
@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-2.0
+// SYN cookies: the child built by cookie_v4_check() must use the same
+// window clamp rule as the SYN-ACK. With SO_RCVBUF set on the listener and
+// a route "window" metric, the SYN-ACK advertises window scale 0, so the
+// child must use a receive window scale of 0 too.
+--ip_version=ipv4
+
+`./defaults.sh
+sysctl -q net.ipv4.tcp_syncookies=2
+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_REUSEADDR, [1], 4) = 0
+   +0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [1048576], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 10) = 0
+
+   +0 < S 0:0(0) win 65535 <mss 1460,sackOK,TS val 100 ecr 0,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 win 2896 <mss 1460,sackOK,TS val 4000 ecr 100,nop,wscale 0>
+ +.01 < . 1:1(0) ack 1 win 514 <nop,nop,TS val 110 ecr 4000>
+   +0 accept(3, ..., ...) = 4
+   +0 %{ assert tcpi_rcv_wscale == 0, tcpi_rcv_wscale }%
diff --git a/tools/testing/selftests/net/packetdrill/tcp_syncookies_rcvbuf_window_ip6.pkt b/tools/testing/selftests/net/packetdrill/tcp_syncookies_rcvbuf_window_ip6.pkt
new file mode 100644
index 00000000000..8b5dd71d765
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_syncookies_rcvbuf_window_ip6.pkt
@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-2.0
+// SYN cookies: the child built by cookie_v6_check() must use the same
+// window clamp rule as the SYN-ACK. With SO_RCVBUF set on the listener and
+// a route "window" metric, the SYN-ACK advertises window scale 0, so the
+// child must use a receive window scale of 0 too.
+--ip_version=ipv6
+
+`./defaults.sh
+sysctl -q net.ipv4.tcp_syncookies=2
+ip -6 route replace fd3d:fa7b:d17d::1 via fd3d:0a0b:17d6:8888::1 dev tun0 window 4096`
+
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [1048576], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 10) = 0
+
+   +0 < S 0:0(0) win 65535 <mss 1460,sackOK,TS val 100 ecr 0,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 win 2896 <mss 1460,sackOK,TS val 4000 ecr 100,nop,wscale 0>
+ +.01 < . 1:1(0) ack 1 win 514 <nop,nop,TS val 110 ecr 4000>
+   +0 accept(3, ..., ...) = 4
+   +0 %{ assert tcpi_rcv_wscale == 0, tcpi_rcv_wscale }%
diff --git a/tools/testing/selftests/net/packetdrill/tcp_window_clamp_rcvbuf_raise.pkt b/tools/testing/selftests/net/packetdrill/tcp_window_clamp_rcvbuf_raise.pkt
new file mode 100644
index 00000000000..08ef005bf99
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_window_clamp_rcvbuf_raise.pkt
@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0
+// After the handshake, tcp_init_buffer_space() lowers window_clamp to the
+// receive space. A later, larger SO_RCVBUF must raise window_clamp again,
+// but only up to the clamp the user set with TCP_WINDOW_CLAMP.
+//
+// packetdrill has no symbol for TCP_WINDOW_CLAMP, so optname 10 is used.
+--ip_version=ipv4
+
+`./defaults.sh`
+
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_TCP, 10, [40000], 4) = 0
+   +0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [8192], 4) = 0
+   +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
+   +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
+   +0 > S 0:0(0) <...>
+  +.1 < S. 0:0(0) ack 1 win 65535 <mss 1460,nop,wscale 7>
+   +0 > . 1:1(0) ack 1
+
+// The small receive buffer keeps window_clamp below the user's clamp:
+// 8192 (tcp_full_space()) minus the one segment (advmss 1460) that
+// tcp_init_buffer_space() reserves.
+   +0 getsockopt(3, SOL_TCP, 10, [6732], [4]) = 0
+
+// A larger SO_RCVBUF raises it again, up to the user's clamp.
+   +0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [212992], 4) = 0
+   +0 getsockopt(3, SOL_TCP, 10, [40000], [4]) = 0
diff --git a/tools/testing/selftests/net/packetdrill/tcp_window_clamp_reset_rcvbuf.pkt b/tools/testing/selftests/net/packetdrill/tcp_window_clamp_reset_rcvbuf.pkt
new file mode 100644
index 00000000000..458f2f313c7
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_window_clamp_reset_rcvbuf.pkt
@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-2.0
+// TCP_WINDOW_CLAMP set back to 0 on an unconnected socket removes the
+// user's clamp: a later SO_RCVBUF must then not stop the route "window"
+// metric from being applied on the SYN.
+//
+// packetdrill has no symbol for TCP_WINDOW_CLAMP, so optname 10 is used.
+--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_TCP, 10, [0], 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)
+
+// No user clamp is left, so the SYN advertises the route window.
+   +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 00000000000..69a05959e42
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_window_clamp_vs_rcvbuf.pkt
@@ -0,0 +1,23 @@
+// 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 raise window_clamp above the
+// user's clamp, and the route "window" metric must not replace it either.
+//
+// packetdrill has no symbol for TCP_WINDOW_CLAMP, so optname 10 is used.
+--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
diff --git a/tools/testing/selftests/net/packetdrill/tcp_window_clamp_vs_rcvbuf_listener.pkt b/tools/testing/selftests/net/packetdrill/tcp_window_clamp_vs_rcvbuf_listener.pkt
new file mode 100644
index 00000000000..f7e7ddb5000
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_window_clamp_vs_rcvbuf_listener.pkt
@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0
+// Passive side: a TCP_WINDOW_CLAMP set on a listener must be honoured on
+// the SYN-ACK even when SO_RCVBUF is set afterwards on the listener, and
+// the route "window" metric must not replace it.
+//
+// packetdrill has no symbol for TCP_WINDOW_CLAMP, so optname 10 is used.
+--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_REUSEADDR, [1], 4) = 0
+   +0 setsockopt(3, SOL_TCP, 10, [16384], 4) = 0
+   +0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [1048576], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+// The SYN-ACK advertises the user's clamp, not the route window and not
+// the SO_RCVBUF-derived full window.
+   +0 < S 0:0(0) win 65535 <mss 1460,sackOK,TS val 100 ecr 0,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 win 15928 <mss 1460,sackOK,TS val 100 ecr 100,nop,wscale 0>
+ +.01 < . 1:1(0) ack 1 win 514 <nop,nop,TS val 110 ecr 100>
+   +0 accept(3, ..., ...) = 4
+   +0 %{ assert tcpi_rcv_wscale == 0, tcpi_rcv_wscale }%
-- 
2.43.0