netdev - Re: Add a SOCK_DESTROY operation to close sockets from userspace

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Date:	Wed, 18 Nov 2015 14:31:55 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Lorenzo Colitti <lorenzo@...gle.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
	Erik Kline <ek@...gle.com>,
	Maciej Żenczykowski <maze@...gle.com>,
	Dmitry Torokhov <dtor@...gle.com>
Subject: Re: Add a SOCK_DESTROY operation to close sockets from userspace

Hello,

On Wed, Nov 18, 2015, at 14:04, Lorenzo Colitti wrote:
> On Wed, Nov 18, 2015 at 8:19 PM, Hannes Frederic Sowa
> <hannes@...essinduktion.org> wrote:
> > I was wondering why you didn't use tcp_close function, because still we
> > could have the address and we would like to do a proper shutdown of the
> > connection. While this patchset wants to tear down sockets for addresses
> > no longer alive, it still can be used with full sockets.
> 
> From the perspective of the TCP state machine, there's not much
> difference. In most TCP states, tcp_close takes the socket straight to
> TCP_CLOSE (not into TCP_TIME_WAIT).

Active close will end up in time wait in the end anyway (with some
exceptions).

> There is a difference in that tcp_close() sends a RST by calling
> tcp_send_active_reset. We could make tcp_diag_destroy do that too. Not
> sure it's worth it because in most of the the cases where you'd want
> to use SOCK_DESTROY (e.g., you've lost a network connection, a VPN
> connected, etc.), tcp_send_active_reset is either not going to send a
> RST at all or it's going send on the wrong network. Even if we're
> still connected to the same network (e.g., in the case where you're
> running "ss --kill" to close a socket instead of the bad old days
> where you had to load your process in gdb and call close() from there
> :-)), not sending a RST is not the end of the world, because as soon
> as the peer sends us a packet we'll send a RST anyway.

Ack. I don't think it makes sense to provide a FIN/RST less way of
closing a socket, just invoke a shutdown() from an interface might be
okayish IMHO.

> In any case calling tcp_close itself won't work - that's intended for
> userspace closes. It calls sock_orphan, which nulls out the
> backpointer to the userspace socket structure, and assumes that there
> are no userspace references to the protocol socket. If we make
> SOCK_DESTROY call tcp_close without releasing the userspace
> components, things blow up as soon as the app calls close().

I was not saying using tcp_close literally, sorry for not making that
clear, but just model the state transitions after tcp_close. At least it
seems like a normal close to me.

Thanks,
Hannes
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