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

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Date:	Wed, 18 Nov 2015 22:04:20 +0900
From:	Lorenzo Colitti <lorenzo@...gle.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	"netdev@...r.kernel.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

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).

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.

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().
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