[PATCH 6.18.y] mptcp: update window_clamp on subflows when SO_RCVBUF is set

[Sasha Levin <sashal@kernel.org>]

From: Gang Yan <yangang@kylinos.cn>

[ Upstream commit 3a543ae0e2092d5c2085d5f21f7a7dbafdffea3c ]

Add __mptcp_subflow_set_rcvbuf() helper to write the subflow sk_rcvbuf,
but also to call the recently added tcp_set_rcvbuf() helper to update
window_clamp. This is needed because the window clap is updated when
scaling_ratio changes, in tcp_measure_rcv_mss(). Until scaling_ratio
changes, the subflow is stuck with the old window clamp which may be
based on a small initial buffer.

Use this new helper in both mptcp_sol_socket_sync_intval() (setsockopt
path) and sync_socket_options() (new subflow creation path).

Note that this patch depends on commit b025461303d8 ("tcp: update
window_clamp when SO_RCVBUF is set"): it fixes the issue on TCP side,
but the same fix is needed on MPTCP side as well.

Fixes: a2cbb1603943 ("tcp: Update window clamping condition")
Cc: stable@vger.kernel.org
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/619
Signed-off-by: Gang Yan <yangang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260515-net-mptcp-misc-fixes-7-1-rc4-v2-5-701e96419f2f@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
[ inlined missing `tcp_set_rcvbuf()` to `tcp_set_window_clamp(ssk, tcp_win_from_space(ssk, val))` ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/mptcp/sockopt.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index de12b3c548edd..566ab5e6c8621 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -67,6 +67,12 @@ static int mptcp_get_int_option(struct mptcp_sock *msk, sockptr_t optval,
 	return 0;
 }
 
+static void __mptcp_subflow_set_rcvbuf(struct sock *ssk, int val)
+{
+	WRITE_ONCE(ssk->sk_rcvbuf, val);
+	tcp_set_window_clamp(ssk, tcp_win_from_space(ssk, val));
+}
+
 static void mptcp_sol_socket_sync_intval(struct mptcp_sock *msk, int optname, int val)
 {
 	struct mptcp_subflow_context *subflow;
@@ -100,7 +106,7 @@ static void mptcp_sol_socket_sync_intval(struct mptcp_sock *msk, int optname, in
 		case SO_RCVBUF:
 		case SO_RCVBUFFORCE:
 			ssk->sk_userlocks |= SOCK_RCVBUF_LOCK;
-			WRITE_ONCE(ssk->sk_rcvbuf, sk->sk_rcvbuf);
+			__mptcp_subflow_set_rcvbuf(ssk, sk->sk_rcvbuf);
 			break;
 		case SO_MARK:
 			if (READ_ONCE(ssk->sk_mark) != sk->sk_mark) {
@@ -1558,7 +1564,7 @@ static void sync_socket_options(struct mptcp_sock *msk, struct sock *ssk)
 			mptcp_subflow_ctx(ssk)->cached_sndbuf = sk->sk_sndbuf;
 		}
 		if (sk->sk_userlocks & SOCK_RCVBUF_LOCK)
-			WRITE_ONCE(ssk->sk_rcvbuf, sk->sk_rcvbuf);
+			__mptcp_subflow_set_rcvbuf(ssk, sk->sk_rcvbuf);
 	}
 
 	if (sock_flag(sk, SOCK_LINGER)) {
-- 
2.53.0