MPTCP initialize the receive buffer stamp in mptcp_rcv_space_init(),
using the provided subflow stamp. Such helper is invoked in several
places; for passive sockets, space init happened at clone time.

In such scenario, MPTCP ends-up accesses the subflow stamp before
its initialization, leading to quite randomic timing for the first
receive buffer auto-tune event, as the timestamp for newly created
subflow is not refreshed there.

Fix the issue moving the stamp initialization out of the mentioned helper,
at the data transfer start, and always using a fresh timestamp.

Fixes: 013e3179dbd2 ("mptcp: fix rcv space initialization")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
v5 -> v6
  - really remove the stamp init from mptcp_rcv_space_init()

v1 -> v2:
  - factor out only the tstamp change for better reviewability
---
 net/mptcp/protocol.c | 9 ++++-----
 net/mptcp/protocol.h | 5 +++++
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 64f592a7897c..d6d2d9d1849a 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -2089,8 +2089,8 @@ static void mptcp_rcv_space_adjust(struct mptcp_sock *msk, int copied)
 
 	msk->rcvq_space.copied += copied;
 
-	mstamp = div_u64(tcp_clock_ns(), NSEC_PER_USEC);
-	time = tcp_stamp_us_delta(mstamp, msk->rcvq_space.time);
+	mstamp = mptcp_stamp();
+	time = tcp_stamp_us_delta(mstamp, READ_ONCE(msk->rcvq_space.time));
 
 	rtt_us = msk->rcvq_space.rtt_us;
 	if (rtt_us && time < (rtt_us >> 3))
@@ -3530,7 +3530,7 @@ struct sock *mptcp_sk_clone_init(const struct sock *sk,
 	mptcp_copy_inaddrs(nsk, ssk);
 	__mptcp_propagate_sndbuf(nsk, ssk);
 
-	mptcp_rcv_space_init(msk, ssk);
+	msk->rcvq_space.time = mptcp_stamp();
 
 	if (mp_opt->suboptions & OPTION_MPTCP_MPC_ACK)
 		__mptcp_subflow_fully_established(msk, subflow, mp_opt);
@@ -3548,8 +3548,6 @@ void mptcp_rcv_space_init(struct mptcp_sock *msk, const struct sock *ssk)
 	msk->rcvq_space.copied = 0;
 	msk->rcvq_space.rtt_us = 0;
 
-	msk->rcvq_space.time = tp->tcp_mstamp;
-
 	/* initial rcv_space offering made to peer */
 	msk->rcvq_space.space = min_t(u32, tp->rcv_wnd,
 				      TCP_INIT_CWND * tp->advmss);
@@ -3745,6 +3743,7 @@ void mptcp_finish_connect(struct sock *ssk)
 	 * accessing the field below
 	 */
 	WRITE_ONCE(msk->local_key, subflow->local_key);
+	WRITE_ONCE(msk->rcvq_space.time, mptcp_stamp());
 
 	mptcp_pm_new_connection(msk, ssk, 0);
 }
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 199f28f3dd5e..95c62f2ac705 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -917,6 +917,11 @@ static inline bool mptcp_is_fully_established(struct sock *sk)
 	       READ_ONCE(mptcp_sk(sk)->fully_established);
 }
 
+static inline u64 mptcp_stamp(void)
+{
+	return div_u64(tcp_clock_ns(), NSEC_PER_USEC);
+}
+
 void mptcp_rcv_space_init(struct mptcp_sock *msk, const struct sock *ssk);
 void mptcp_data_ready(struct sock *sk, struct sock *ssk);
 bool mptcp_finish_join(struct sock *sk);
-- 
2.51.1