Re: [PATCH mptcp] mptcp: sockopt: make sync_socket_options propagate SOCK_KEEPOPEN

[Matthieu Baerts]

Hi Geliang,

On 07/09/2025 02:51, Geliang Tang wrote:
> Hi Matt,
> 
> On Sat, 2025-09-06 at 15:26 +0200, Matthieu Baerts wrote:
>> Hi Krister,
>>
>> On 06/09/2025 02:43, Krister Johansen wrote:
>>> Users reported a scenario where MPTCP connections that were
>>> configured
>>> with SO_KEEPALIVE prior to connect would fail to enable their
>>> keepalives
>>> if MTPCP fell back to TCP mode.
>>>
>>> After investigating, this affects keepalives for any connection
>>> where
>>> sync_socket_options is called on a socket that is in the closed or
>>> listening state.  Joins are handled properly. For connects,
>>> sync_socket_options is called when the socket is still in the
>>> closed
>>> state.  The tcp_set_keepalive() function does not act on sockets
>>> that
>>> are closed or listening, hence keepalive is not immediately
>>> enabled.
>>> Since the SO_KEEPOPEN flag is absent, it is not enabled later in
>>> the
>>> connect sequence via tcp_finish_connect.  Setting the keepalive via
>>> sockopt after connect does work, but would not address any
>>> subsequently
>>> created flows.
>>>
>>> Fortunately, the fix here is straight-forward: set SOCK_KEEPOPEN on
>>> the
>>> subflow when calling sync_socket_options.
>>>
>>> The fix was valdidated both by using tcpdump to observe keeplaive
>>> packets not being sent before the fix, and being sent after the
>>> fix.  It
>>> was also possible to observe via ss that the keepalive timer was
>>> not
>>> enabled on these sockets before the fix, but was enabled
>>> afterwards.
>>
>>
>> Thank you for the fix! Indeed, the SOCK_KEEPOPEN flag was missing!
>> This
>> patch looks good to me as well:
>>
>> Reviewed-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
>>
>>
>> @Netdev Maintainers: please apply this patch in 'net' directly. But I
>> can always re-send it later if preferred.
> 
> nit:
> 
> I just noticed his patch breaks 'Reverse X-Mas Tree' order in
> sync_socket_options(). If you think any changes are needed, please
> update this when you re-send it.

Sure, I can do the modification and send it with other fixes we have.

pw-bot: cr

Cheers,
Matt
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