mptcp: fix ack generation for fallback msk

mptcp_cleanup_rbuf() needs to know the last most recent, mptcp-level
rcv_wnd sent, and such information is tracked into the msk->old_wspace
field, updated at ack transmission time by mptcp_write_options().

Fallback socket do not add any mptcp options, such helper is never
invoked, and msk->old_wspace value remain stale. That in turn makes
ack generation at recvmsg() time quite random.

Address the issue ensuring mptcp_write_options() is invoked even for
fallback sockets, and just update the needed info in such a case.

The issue went unnoticed for a long time, as mptcp currently overshots
the fallback socket receive buffer autotune significantly. It is going
to change in the near future.

Fixes: e385960 ("mptcp: better msk receive window updates")
Cc: stable@vger.kernel.org
Closes: multipath-tcp/mptcp_net-next#594
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Geliang Tang <geliang@kernel.org>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251118-net-mptcp-misc-fixes-6-18-rc6-v1-1-806d3781c95f@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Paolo Abeni

net/mptcp/options.c

@@ -838,8 +838,11 @@ bool mptcp_established_options(struct sock *sk, struct sk_buff *skb,
 
 	opts->suboptions = 0;
 
+	/* Force later mptcp_write_options(), but do not use any actual
+	 * option space.
+	 */
 	if (unlikely(__mptcp_check_fallback(msk) && !mptcp_check_infinite_map(skb)))
-		return false;
+		return true;
 
 	if (unlikely(skb && TCP_SKB_CB(skb)->tcp_flags & TCPHDR_RST)) {
 		if (mptcp_established_options_fastclose(sk, &opt_size, remaining, opts) ||
@@ -1319,6 +1322,20 @@ static void mptcp_set_rwin(struct tcp_sock *tp, struct tcphdr *th)
 	WRITE_ONCE(msk->old_wspace, tp->rcv_wnd);
 }
 
+static void mptcp_track_rwin(struct tcp_sock *tp)
+{
+	const struct sock *ssk = (const struct sock *)tp;
+	struct mptcp_subflow_context *subflow;
+	struct mptcp_sock *msk;
+
+	if (!ssk)
+		return;
+
+	subflow = mptcp_subflow_ctx(ssk);
+	msk = mptcp_sk(subflow->conn);
+	WRITE_ONCE(msk->old_wspace, tp->rcv_wnd);
+}
+
 __sum16 __mptcp_make_csum(u64 data_seq, u32 subflow_seq, u16 data_len, __wsum sum)
 {
 	struct csum_pseudo_header header;
@@ -1611,6 +1628,10 @@ void mptcp_write_options(struct tcphdr *th, __be32 *ptr, struct tcp_sock *tp,
 				      opts->reset_transient,
 				      opts->reset_reason);
 		return;
+	} else if (unlikely(!opts->suboptions)) {
+		/* Fallback to TCP */
+		mptcp_track_rwin(tp);
+		return;
 	}
 
 	if (OPTION_MPTCP_PRIO & opts->suboptions) {