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

JIRA: https://issues.redhat.com/browse/RHEL-84571
Upstream Status: net.git commit 2ca06a2

commit 2ca06a2
Author: Paolo Abeni <pabeni@redhat.com>
Date:   Mon Jan 13 16:44:56 2025 +0100

    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: stable@vger.kernel.org
    Reported-by: Jakub Kicinski <kuba@kernel.org>
    Closes: https://lore.kernel.org/20250107131845.5e5de3c5@kernel.org
    Signed-off-by: Paolo Abeni <pabeni@redhat.com>
    Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
    Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
    Link: https://patch.msgid.link/20250113-net-mptcp-connect-st-flakes-v1-1-0d986ee7b1b6@kernel.org
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Davide Caratti <dcaratti@redhat.com>

Author: dcaratti

net/mptcp/options.c

@@ -606,7 +606,6 @@ 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)
@@ -1286,7 +1285,7 @@ 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) {
@@ -1311,6 +1310,9 @@ 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)