mptcp: be sure to send ack when mptcp-level window re-opens

mptcp_cleanup_rbuf() is responsible to send acks when the user-space
reads enough data to update the receive windows significantly.

It tries hard to avoid acquiring the subflow sockets locks by checking
conditions similar to the ones implemented at the TCP level.

To avoid too much code duplication - the MPTCP protocol can't reuse the
TCP helpers as part of the relevant status is maintained into the msk
socket - and multiple costly window size computation, mptcp_cleanup_rbuf
uses a rough estimate for the most recently advertised window size:
the MPTCP receive free space, as recorded as at last-ack time.

Unfortunately the above does not allow mptcp_cleanup_rbuf() to detect
a zero to non-zero win change in some corner cases, skipping the
tcp_cleanup_rbuf call and leaving the peer stuck.

After commit ea66758 ("tcp: allow MPTCP to update the announced
window"), MPTCP has actually cheap access to the announced window value.
Use it in mptcp_cleanup_rbuf() for a more accurate ack generation.

Fixes: e385960 ("mptcp: better msk receive window updates")
Cc: [email protected]
Reported-by: Jakub Kicinski <[email protected]>
Closes: https://lore.kernel.org/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>
Reviewed-by: Matthieu Baerts (NGI0) <[email protected]>
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>
Link: https://patch.msgid.link/20250113-net-mptcp-connect-st-flakes-v1-1-0d986ee7b1b6@kernel.org
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Paolo Abeni

net/mptcp/options.c

@@ -607,7 +607,6 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
 	}
 	opts->ext_copy.use_ack = 1;
 	opts->suboptions = OPTION_MPTCP_DSS;
-	WRITE_ONCE(msk->old_wspace, __mptcp_space((struct sock *)msk));
 
 	/* Add kind/length/subtype/flag overhead if mapping is not populated */
 	if (dss_size == 0)
@@ -1288,7 +1287,7 @@ static void mptcp_set_rwin(struct tcp_sock *tp, struct tcphdr *th)
 			}
 			MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_RCVWNDCONFLICT);
 		}
-		return;
+		goto update_wspace;
 	}
 
 	if (rcv_wnd_new != rcv_wnd_old) {
@@ -1313,6 +1312,9 @@ static void mptcp_set_rwin(struct tcp_sock *tp, struct tcphdr *th)
 		th->window = htons(new_win);
 		MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_RCVWNDSHARED);
 	}
+
+update_wspace:
+	WRITE_ONCE(msk->old_wspace, tp->rcv_wnd);
 }
 
 __sum16 __mptcp_make_csum(u64 data_seq, u32 subflow_seq, u16 data_len, __wsum sum)