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

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Date:	Thu, 19 Nov 2015 16:04:18 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	zenczykowski <zenczykowski@...il.com>,
	Lorenzo Colitti <lorenzo@...gle.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>, Erik Kline <ek@...gle.com>,
	Dmitry Torokhov <dtor@...gle.com>
Subject: Re: Add a SOCK_DESTROY operation to close sockets from userspace

On Thu, Nov 19, 2015 at 2:33 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2015-11-19 at 14:14 -0800, Tom Herbert wrote:
>> On Thu, Nov 19, 2015 at 2:07 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Thu, 2015-11-19 at 13:53 -0800, Tom Herbert wrote:
>> >
>> >> That covers the case where the local address is removed, but the not
>> >> the case where the network manager is informed of an error in the path
>> >> and wants to signal the application. My understanding was that
>> >> SIOCKILLADDR would work for the first case, but this patch was need to
>> >> cover the second case.
>> >>
>> >> btw, instead of closing the TCP socket can we just report an error and
>> >> wake up the application without affecting the connection? That is this
>> >> just becomes an error on the socket. The response by the application
>> >> will be the same in any case, porbablly just close the socket and try
>> >> to reestablish the connection.
>> >
>> > I thought this was the patch intent ?
>> >
>> > Application gets a EPOLLIN|EPOLLOUT|POLLERR notification (if it is
>> > willing to receive it, or blocked in a socket syscall) and closes the
>> > socket.
>> >
>> >          sk->sk_err = ETIMEDOUT;
>> >          sk->sk_error_report(sk);
>> >          tcp_done(sk);
>> >
>> The tcp_done is not needed here. This is the difference between the
>> application having the connection closed underneath them, versus the
>> application performing a close to terminate the connection. The latter
>> behavior preserves the semantics that only the stack or the
>> application owning the socket can initiate a state change on the
>> connection.
>
>
> The code behaves like we received a formal RST :
>
But we didn't get a RST, closing the connection is not being done
under the auspices of the protocol. The most comparable event to be a
timeout on the socket read operation.

> Please do not even bother trying to send additional data, it is not
> worth wasting precious resource.
>
It is reasonable to think of this mechanism as way to indicate loss of
reachability. But if this is being viewed as a way to do resource
management that is a little worrisome. What is to stop someone from
using this mechanism from implementing to implement a security layer,
or imposing a global time limit to how long someone can be connected,
or to start killing connections based on some arbitrary policy when
under memory pressure. I am not necessarily saying this stuff is
necessarily bad or an abuse of the mechanism, but the flexibility and
power of the mechanism opens the door for these use cases.

Tom
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