This adds support for SO_TIMESTAMP(NS).  Timestamps are passed to
userspace in the same way as for plain tcp sockets.
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Move the pre-check to the function that handles all SOL_SOCKET values.

At this point there is complete coverage for all values that were
accepted by the pre-check.

BUSYPOLL functions are accepted but will not have any functionality
yet until its clear how the expected mptcp behaviour should look like.
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This adds support for TIMESTAMP(NS) setsockopt.

This doesn't make things work yet, because the mptcp receive path
doesn't convert the skb timestamps to cmsgs for userspace consumption.

receive path cmsg support is added ina followup patch.
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

To avoid confusions, it seems better to parse this sysctl parameter as a
boolean. We use it as a boolean, no need to parse an integer and bring
confusions if we see a value different from 0 and 1, especially with
this parameter name: enabled.

It seems fine to do this modification because the default value is 1
(enabled). Then the only other interesting value to set is 0 (disabled).
All other values would not have changed the default behaviour.
Suggested-by: NFlorian Westphal <fw@strlen.de>
Acked-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

Since the introduction of the sysctl support in MPTCP with
commit 784325e9 ("mptcp: new sysctl to control the activation per NS"),
we don't check CONFIG_SYSCTL.

Until now, that was not an issue: the register and unregister functions
were replaced by NO-OP one if SYSCTL was not enabled in the config. The
only thing we could have avoid is not to reserve memory for the table
but that's for the moment only a small table per net-ns.

But the following commit is going to use SYSCTL_ZERO and SYSCTL_ONE
which are not be defined if SYSCTL is not enabled in the config. This
causes 'undefined reference' errors from the linker.
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

When add address with port, it is mean to create a listening socket,
and send an ADD_ADDR to remote, so it must have flag signal set,
add this check in mptcp_pm_parse_addr().

Fixes: a77e9179 ("mptcp: deal with MPTCP_PM_ADDR_ATTR_PORT in PM netlink")
Acked-by: NGeliang Tang <geliangtang@gmail.com>
Reviewed-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NJianguo Wu <wujianguo@chinatelecom.cn>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

Memory of struct pm_nl_pernet{} is allocated by kzalloc()
in setup_net()->ops_init(), so it's no need to reset counters
and zero bitmap in pm_nl_init_net().
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Reviewed-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NJianguo Wu <wujianguo@chinatelecom.cn>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

For outgoing subflow join, when recv SYNACK, in subflow_finish_connect(),
the mptcp_finish_join() may return false in some cases, and send a RESET
to remote, and no local hmac is required.
So generate subflow hmac after mptcp_finish_join().

Fixes: ec3edaa7 ("mptcp: Add handling of outgoing MP_JOIN requests")
Signed-off-by: NJianguo Wu <wujianguo@chinatelecom.cn>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

We have macro TOKEN_MAX_RETRIES for the number of token generate retries,
so using TOKEN_MAX_RETRIES in subflow_check_req().

And rename TOKEN_MAX_RETRIES to MPTCP_TOKEN_MAX_RETRIES as it is now
exposed.

Fixes: 535fb815 ("mptcp: token: move retry to caller")
Reviewed-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NJianguo Wu <wujianguo@chinatelecom.cn>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

After commit 2c5ebd00 ("mptcp: refactor token container"),
pr_debug() is called before mptcp_crypto_key_gen_sha() in
mptcp_token_new_connect(), so the output local_key, token and
idsn are 0, like:

  MPTCP: ssk=00000000f6b3c4a2, local_key=0, token=0, idsn=0

Move pr_debug() after mptcp_crypto_key_gen_sha().

Fixes: 2c5ebd00 ("mptcp: refactor token container")
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NJianguo Wu <wujianguo@chinatelecom.cn>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

When some mapping related errors occurs we close the main
MPC subflow with a RST. We should instead fallback gracefully
to TCP, and do the reset only for MPJ subflows.

Fixes: d22f4988 ("mptcp: process MP_CAPABLE data option")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/192Reported-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

In subflow_syn_recv_sock() we currently skip options parsing
for OoO packet, given that such packets may not carry the relevant
MPC option.

If the peer generates an MPC+data TSO packet and some of the early
segments are lost or get reorder, we server will ignore the peer key,
causing transient, unexpected fallback to TCP.

The solution is always parsing the incoming MPTCP options, and
do the fallback only for in-order packets. This actually cleans
the existing code a bit.

Fixes: d22f4988 ("mptcp: process MP_CAPABLE data option")
Reported-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

MPTCP sk_forward_memory handling is a bit special, as such field
is protected by the msk socket spin_lock, instead of the plain
socket lock.

Currently we have a code path updating such field without handling
the relevant lock:

__mptcp_retrans() -> __mptcp_clean_una_wakeup()

Several helpers in __mptcp_clean_una_wakeup() will update
sk_forward_alloc, possibly causing such field corruption, as reported
by Matthieu.

Address the issue providing and using a new variant of blamed function
which explicitly acquires the msk spin lock.

Fixes: 64b9cea7 ("mptcp: fix spurious retransmissions")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/172Reported-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Tested-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

when Linux receives an echo-ed ADD_ADDR, it checks the IP address against
the list of "announced" addresses. In case of a positive match, the timer
that handles retransmissions is stopped regardless of the 'Address Id' in
the received packet: this behaviour does not comply with RFC8684 3.4.1.

Fix it by validating the 'Address Id' in received echo-ed ADD_ADDRs.
Tested using packetdrill, with the following captured output:

 unpatched kernel:

 Out <...> Flags [.], ack 1, win 256, options [mptcp add-addr v1 id 1 198.51.100.2 hmac 0xfd2e62517888fe29,mptcp dss ack 3007449509], length 0
 In  <...> Flags [.], ack 1, win 257, options [mptcp add-addr v1-echo id 1 1.2.3.4,mptcp dss ack 3013740213], length 0
 Out <...> Flags [.], ack 1, win 256, options [mptcp add-addr v1 id 1 198.51.100.2 hmac 0xfd2e62517888fe29,mptcp dss ack 3007449509], length 0
 In  <...> Flags [.], ack 1, win 257, options [mptcp add-addr v1-echo id 90 198.51.100.2,mptcp dss ack 3013740213], length 0
        ^^^ retransmission is stopped here, but 'Address Id' is 90

 patched kernel:

 Out <...> Flags [.], ack 1, win 256, options [mptcp add-addr v1 id 1 198.51.100.2 hmac 0x1cf372d59e05f4b8,mptcp dss ack 3007449509], length 0
 In  <...> Flags [.], ack 1, win 257, options [mptcp add-addr v1-echo id 1 1.2.3.4,mptcp dss ack 1672384568], length 0
 Out <...> Flags [.], ack 1, win 256, options [mptcp add-addr v1 id 1 198.51.100.2 hmac 0x1cf372d59e05f4b8,mptcp dss ack 3007449509], length 0
 In  <...> Flags [.], ack 1, win 257, options [mptcp add-addr v1-echo id 90 198.51.100.2,mptcp dss ack 1672384568], length 0
 Out <...> Flags [.], ack 1, win 256, options [mptcp add-addr v1 id 1 198.51.100.2 hmac 0x1cf372d59e05f4b8,mptcp dss ack 3007449509], length 0
 In  <...> Flags [.], ack 1, win 257, options [mptcp add-addr v1-echo id 1 198.51.100.2,mptcp dss ack 1672384568], length 0
        ^^^ retransmission is stopped here, only when both 'Address Id' and 'IP Address' match

Fixes: 00cfd77b ("mptcp: retransmit ADD_ADDR when timeout")
Signed-off-by: NDavide Caratti <dcaratti@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Another left-over. Avoid flooding dmesg with useless text,
we already have a MIB for that event.

Fixes: 648ef4b8 ("mptcp: Implement MPTCP receive path")
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This is a left-over of early day. A malicious peer can flood
the kernel logs with useless messages, just drop it.

Fixes: f296234c ("mptcp: Add handling of incoming MP_JOIN requests")
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

We can't use tcp_set_congestion_control() on an mptcp socket, as
such function can end-up accessing a tcp-specific field -
prior_ssthresh - causing an OOB access.

To allow propagating the correct ca algo on subflow, cache the ca
name at initialization time.

Additionally avoid overriding the user-selected CA (if any) at
clone time.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/182
Fixes: aa1fbd94 ("mptcp: sockopt: add TCP_CONGESTION and TCP_INFO")
Acked-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Maxim reported several issues when forcing a TCP transparent proxy
to use the MPTCP protocol for the inbound connections. He also
provided a clean reproducer.

The problem boils down to 'mptcp_frag_can_collapse_to()' assuming
that only MPTCP will use the given page_frag.

If others - e.g. the plain TCP protocol - allocate page fragments,
we can end-up re-using already allocated memory for mptcp_data_frag.

Fix the issue ensuring that the to-be-expanded data fragment is
located at the current page frag end.

v1 -> v2:
 - added missing fixes tag (Mat)

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/178Reported-and-tested-by: NMaxim Galaganov <max@internet.ru>
Fixes: 18b683bf ("mptcp: queue data for mptcp level retransmission")
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

If userspace exits before calling accept() on a listener that had at least
one new connection ready, we get:

   Attempt to release TCP socket in state 8

This happens because the mptcp socket gets cloned when the TCP connection
is ready, but the socket is never exposed to userspace.

The client additionally sends a DATA_FIN, which brings connection into
CLOSE_WAIT state.  This in turn prevents the orphan+state reset fixup
in mptcp_sock_destruct() from doing its job.

Fixes: 3721b9b6 ("mptcp: Track received DATA_FIN sequence number and add related helpers")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/185Tested-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Link: https://lore.kernel.org/r/20210507001638.225468-1-mathew.j.martineau@linux.intel.comSigned-off-by: NJakub Kicinski <kuba@kernel.org>

This patch adds support for MSG_PEEK flag. Packets are not removed
from the receive_queue if MSG_PEEK set in recv() system call.
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NYonglong Li <liyonglong@chinatelecom.cn>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Currently mptcp_sendmsg() fails with EOPNOTSUPP if the
user-space provides some unsupported flag. That is unexpected
and may foul existing applications migrated to MPTCP, which
expect a different behavior.

Change the mentioned function to silently ignore the unsupported
flags except MSG_FASTOPEN. This is the only flags currently not
supported by MPTCP with user-space visible side-effects.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/162Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

The mentioned flag is currently silenlty ignored. This
change implements the TCP-like behaviour, dropping the
pending data up to the specified length.
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Sigend-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

mptcp_recvmsg() currently silently ignores MSG_ERRQUEUE, returning
input data instead of error cmsg.

This change provides a dummy implementation for MSG_ERRQUEUE - always
returns no data. That is consistent with the current lack of a suitable
IP_RECVERR setsockopt() support.
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

With this change, the MPTCP-level retransmission timer is used to resend
DATA_FIN. The retranmit timer is not stopped while waiting for a
MPTCP-level ACK of DATA_FIN, and retransmitted DATA_FINs are sent on all
subflows. The retry interval starts at TCP_RTO_MIN and then doubles on
each attempt, up to TCP_RTO_MAX.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/146
Fixes: 43b54c6e ("mptcp: Use full MPTCP-level disconnect state machine")
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This patch used the macro helper mptcp_for_each_subflow() instead of
list_for_each_entry() in mptcp_close.
Signed-off-by: NGeliang Tang <geliangtang@gmail.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This patch added a tracepoint in subflow_check_data_avail() to show the
mapping status.
Suggested-by: NPaolo Abeni <pabeni@redhat.com>
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NGeliang Tang <geliangtang@gmail.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This patch added a tracepoint in ack_update_msk() to track the
incoming data_ack and window/snd_una updates.
Suggested-by: NPaolo Abeni <pabeni@redhat.com>
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NGeliang Tang <geliangtang@gmail.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This patch added a tracepoint in the mapping status function
get_mapping_status() to dump every mpext field.
Suggested-by: NPaolo Abeni <pabeni@redhat.com>
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NGeliang Tang <geliangtang@gmail.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This patch added a tracepoint in the packet scheduler function
mptcp_subflow_get_send().
Suggested-by: NPaolo Abeni <pabeni@redhat.com>
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NGeliang Tang <geliangtang@gmail.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This patch moved the static function mptcp_subflow_active to protocol.h
as an inline one.
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NGeliang Tang <geliangtang@gmail.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Some of the sequence numbers are printed as the negative ones in the debug
log:

[   46.250932] MPTCP: DSS
[   46.250940] MPTCP: data_fin=0 dsn64=0 use_map=0 ack64=1 use_ack=1
[   46.250948] MPTCP: data_ack=2344892449471675613
[   46.251012] MPTCP: msk=000000006e157e3f status=10
[   46.251023] MPTCP: msk=000000006e157e3f snd_data_fin_enable=0 pending=0 snd_nxt=2344892449471700189 write_seq=2344892449471700189
[   46.251343] MPTCP: msk=00000000ec44a129 ssk=00000000f7abd481 sending dfrag at seq=-1658937016627538668 len=100 already sent=0
[   46.251360] MPTCP: data_seq=16787807057082012948 subflow_seq=1 data_len=100 dsn64=1

This patch used the format specifier %u instead of %d for the unsigned int
values to fix it.

Fixes: d9ca1de8 ("mptcp: move page frag allocation in mptcp_sendmsg()")
Reviewed-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NGeliang Tang <geliangtang@gmail.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Drop 'S' from end of CONFIG_MPTCP_KUNIT_TESTS in order to adhere to the
KUNIT *_KUNIT_TEST config name format.

Fixes: a00a5822 (mptcp: move crypto test to KUNIT)
Reviewed-by: NDavid Gow <davidgow@google.com>
Reviewed-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NNico Pache <npache@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

TCP_CONGESTION is set for all subflows.
The mptcp socket gains icsk_ca_ops too so it can be used to keep the
authoritative state that should be set on new/future subflows.

TCP_INFO will return first subflow only.
The out-of-tree kernel has a MPTCP_INFO getsockopt, this could be added
later on.
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Handle SO_DEBUG and set it on all subflows.
Ignore those values not implemented on TCP sockets.
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Replicate to all subflows.
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Value is synced to all subflows.
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Similar to PRIORITY/KEEPALIVE: needs to be mirrored to all subflows.
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Similar to previous patch: needs to be mirrored to all subflows.

Device bind is simpler: it is only done on the initial (listener) sk.
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

start with something simple: both take an integer value, both
need to be mirrored to all subflows.
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Paolo Abeni suggested to avoid re-syncing new subflows because
they inherit options from listener. In case options were set on
listener but are not set on mptcp-socket there is no need to
do any synchronisation for new subflows.

This change sets sockopt_seq of new mptcp sockets to the seq of
the mptcp listener sock.

Subflow sequence is set to the embedded tcp listener sk.
Add a comment explaing why sk_state is involved in sockopt_seq
generation.
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>