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

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Date:	Thu, 19 Nov 2015 14:33:01 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <tom@...bertland.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, 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 : 

Please do not even bother trying to send additional data, it is not
worth wasting precious resource.

There is currently no system call telling the stack : 

Please remove all outstanding data for this socket.

And even if it was present, how many applications need to be changed,
because all they do is probably a close(), after a shutdown() if they
really were cautious...



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