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

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Message-Id: <1447957669.2293924.444570833.0E151DD6@webmail.messagingengine.com>
Date:	Thu, 19 Nov 2015 19:27:49 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	David Miller <davem@...emloft.net>, tom@...bertland.com
Cc:	eric.dumazet@...il.com, zenczykowski@...il.com, lorenzo@...gle.com,
	stephen@...workplumber.org, netdev@...r.kernel.org,
	edumazet@...gle.com, ek@...gle.com, dtor@...gle.com
Subject: Re: Add a SOCK_DESTROY operation to close sockets from userspace

On Thu, Nov 19, 2015, at 19:09, David Miller wrote:
> From: Tom Herbert <tom@...bertland.com>
> Date: Thu, 19 Nov 2015 09:38:37 -0800
> 
> > 1) We need transparency. If a third party kills a TCP connection then
> > the application should be informed of specifically that. This seems
> > easy enough to just pick an appropriate error number as I suggested.
> 
> Agreed.
> 
> > 2) We need constraints. This feature seems to be specific to a very
> > narrow use case. It is not at all clear to me if there are any
> > legitimate uses cases beyond Android, enabling this by default in the
> > stack creates a non-zero amount of risk and liability for abuse. It
> > seems like this should be an opt-in sort of feature, with a kernel
> > CONFIG or maybe opt-in per socket.
> 
> And this will probably be the point of contention.

For me this looks like the revoke(const char *) syscall for TCP. I would
be willing to enable this by default if we can guarantee that for
TIMEWAIT_LEN we don't have reuse of that quadruple. I don't see anything
bad happening by tearing down any connection besides too quick reuse of
that identifier, so why not enable it by default for everyone? I
certainly am often in the situation that I would like to kill
connections on my laptop which are hanging and I know the reason,
actually. A nice user interface would be certainly a place. Also we
should of course wake up any poll_wait_time wchans waiting on the socket
so user space can move forward.

Probably I would really like to see that we send a reset, even though it
won't probably reach its destination in case the source address is
removed from the system.

I will research the semantics behind tcpdrop:
<http://www.freebsd.org/cgi/man.cgi?query=tcpdrop&apropos=0&sektion=0&manpath=FreeBSD+10.2-RELEASE&arch=default&format=html>

Bye,
Hannes
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