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

[Krister Johansen]

On Mon, Sep 08, 2025 at 07:13:12PM +0200, Matthieu Baerts wrote:
> 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.

Thanks for the reviews, Geliang and Matt.  If you'd like me to fix the
formatting up and send a v2, I'm happy to do that as well.  Just let me
know.

-K