With fastopen in place the first subflow socket is created
before the MPC handshake completes, and we need to properly initialize
the sequence numbers at MPC ACK reception.

Signed-off-by: Dmytro Shytyi <dmytro@shytyi.net>
---
 net/mptcp/Makefile   |  2 +-
 net/mptcp/fastopen.c | 19 +++++++++++++++++++
 net/mptcp/options.c  |  5 +++++
 net/mptcp/protocol.h |  6 ++++++
 4 files changed, 31 insertions(+), 1 deletion(-)
 create mode 100644 net/mptcp/fastopen.c

diff --git a/net/mptcp/Makefile b/net/mptcp/Makefile
index 8a7f68efa35f..c42ad8609876 100644
--- a/net/mptcp/Makefile
+++ b/net/mptcp/Makefile
@@ -2,7 +2,7 @@
 obj-$(CONFIG_MPTCP) += mptcp.o
 
 mptcp-y := protocol.o subflow.o options.o token.o crypto.o ctrl.o pm.o diag.o \
-	   mib.o pm_netlink.o sockopt.o pm_userspace.o sched.o
+	   mib.o pm_netlink.o sockopt.o pm_userspace.o sched.o fastopen.o
 
 obj-$(CONFIG_SYN_COOKIES) += syncookies.o
 obj-$(CONFIG_INET_MPTCP_DIAG) += mptcp_diag.o
diff --git a/net/mptcp/fastopen.c b/net/mptcp/fastopen.c
new file mode 100644
index 000000000000..d6fb45e6be4f
--- /dev/null
+++ b/net/mptcp/fastopen.c
@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0
+ * MPTCP Fast Open Mechanism. Copyright (c) 2021-2022, Dmytro SHYTYI
+ */
+
+#include "protocol.h"
+
+void mptcp_gen_msk_ackseq_fastopen(struct mptcp_sock *msk, struct mptcp_subflow_context *subflow,
+				   struct mptcp_options_received mp_opt)
+{
+	u64 ack_seq;
+
+	WRITE_ONCE(msk->can_ack, true);
+	WRITE_ONCE(msk->remote_key, mp_opt.sndr_key);
+	mptcp_crypto_key_sha(msk->remote_key, NULL, &ack_seq);
+	ack_seq++;
+	WRITE_ONCE(msk->ack_seq, ack_seq);
+	pr_debug("ack_seq=%llu sndr_key=%llu", msk->ack_seq, mp_opt.sndr_key);
+	atomic64_set(&msk->rcv_wnd_sent, ack_seq);
+}
diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 30d289044e71..0b6c4535750c 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -1208,6 +1208,11 @@ bool mptcp_incoming_options(struct sock *sk, struct sk_buff *skb)
 			mpext->dsn64 = 1;
 			mpext->mpc_map = 1;
 			mpext->data_fin = 0;
+
+			if (msk->is_mptfo) {
+				mptcp_gen_msk_ackseq_fastopen(msk, subflow, mp_opt);
+				mpext->data_seq = READ_ONCE(msk->ack_seq);
+			}
 		} else {
 			mpext->data_seq = mp_opt.data_seq;
 			mpext->subflow_seq = mp_opt.subflow_seq;
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 2358a4083eb3..f4b6f817ec46 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -282,6 +282,7 @@ struct mptcp_sock {
 	bool		use_64bit_ack; /* Set when we received a 64-bit DSN */
 	bool		csum_enabled;
 	bool		allow_infinite_fallback;
+	bool		is_mptfo;
 	u8		mpc_endpoint_id;
 	u8		recvmsg_inq:1,
 			cork:1,
@@ -841,6 +842,11 @@ void mptcp_event_addr_announced(const struct sock *ssk, const struct mptcp_addr_
 void mptcp_event_addr_removed(const struct mptcp_sock *msk, u8 id);
 bool mptcp_userspace_pm_active(const struct mptcp_sock *msk);
 
+// Fast Open Mechanism functions begin
+void mptcp_gen_msk_ackseq_fastopen(struct mptcp_sock *msk, struct mptcp_subflow_context *subflow,
+				   struct mptcp_options_received mp_opt);
+// Fast Open Mechanism functions end
+
 static inline bool mptcp_pm_should_add_signal(struct mptcp_sock *msk)
 {
 	return READ_ONCE(msk->pm.addr_signal) &
-- 
2.34.1