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

[Geliang Tang <geliang@kernel.org>]

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@kernel.org>
> 
> 
> @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.

Thanks,
-Geliang

> 
> Cheers,
> Matt