Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752822AbbDPAru (ORCPT <rfc822;w@1wt.eu>);
	Wed, 15 Apr 2015 20:47:50 -0400
Received: from mail-la0-f53.google.com ([209.85.215.53]:35036 "EHLO
	mail-la0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752096AbbDPArk (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 15 Apr 2015 20:47:40 -0400
MIME-Version: 1.0
In-Reply-To: <20150415232716.GS889@ZenIV.linux.org.uk>
References: <20150415154551.GE6801@home.goodmis.org> <20150415163520.GA25105@kroah.com>
 <20150415130649.6f9ab20f@gandalf.local.home> <20150415173145.GA26146@kroah.com>
 <20150415225611.0c256ea6@lxorguk.ukuu.org.uk> <CALCETrWBvT7Vhwir+ntk7A31Ux2C_3CeuWFk7iQqA1fovh74yg@mail.gmail.com>
 <20150415221804.GP889@ZenIV.linux.org.uk> <CALCETrVSsyUUNujCi+oYTBcy+6ROahem4M0A2axCivRwrA4myw@mail.gmail.com>
 <20150415224854.GQ889@ZenIV.linux.org.uk> <CALCETrV=YKZcJtRQ7pB-pb4h6xYBvF42RUTuuYkvxx2MMMcSAg@mail.gmail.com>
 <20150415232716.GS889@ZenIV.linux.org.uk>
From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 15 Apr 2015 17:47:18 -0700
Message-ID: <CALCETrVJxSmL6P8eyb47TFZYYWiP_4dKAbgFVZ39zjqa4NtVDg@mail.gmail.com>
Subject: Re: [GIT PULL] kdbus for 4.1-rc1
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Richard Weinberger <richard.weinberger@gmail.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>, Tom Gundersen <teg@jklm.no>,
        Jiri Kosina <jkosina@suse.cz>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Daniel Mack <daniel@zonque.org>,
        David Herrmann <dh.herrmann@gmail.com>,
        Djalal Harouni <tixxdz@opendz.org>
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 2674
Lines: 57

On Wed, Apr 15, 2015 at 4:27 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Wed, Apr 15, 2015 at 03:54:10PM -0700, Andy Lutomirski wrote:
>> Huh, interesting.
>>
>> I was imagining that each of a server's peers (capability holders)
>> would have a fresh struct file, but maybe this wouldn't be needed at
>> all.  You'd still need a way to get replies to your request, but the
>> API could just as easily be:
>>
>> int send_to_capability(int dest, int source, const void *data, size_t len, ...);
>>
>> where dest would be the destination's fd and source would be whatever
>> receive queue I expect the response on.
>>
>> So maybe this is feasible.  It doesn't solve broadcasts, but dbus
>> unicast could easily layer over a facility like this and the context
>> switch problem would go away for unicast.
>>
>> Heck, I'd use it for my own proprietary stuff, too.  It would be way
>> easier than the absurd tangle of socketpairs I currently use.
>
> BTW, the main issue with AF_UNIX passing is that recepient isn't asleep
> awaiting for descriptors - they are thrown by sender at whoever's receiving
> and sit there until somebody gets around to picking them.
>
> _IF_ we had
> client: I want a desciptor <goes to sleep, interruptibly>
> kernel: assign it a sequence number
> server: sees request (including sequence number)
> server: give this fd to originator of request #N
> kernel: check if originator is still there, insert the damn thing into their
> descriptor table if they still are and return the obtained number
> or
> server: tell the originator of request #N to fuck off
> kernel: check if originator is still there and gleefully pass the "fuck off" if
> they still are
>
> we wouldn't have the in-flight state at all, and there goes the garbage
> collection shite.  With some elaboration, it could even carry the
> authentication traffic - "fuck off" might be "answer this challenge", with
> the next "I want a descriptor" carrying reply...

I wonder if we could get away with having the receiver pre-allocate
some placeholder fds and then have the kernel replace a placeholder
with a passed fd immediately when the fd is sent and enqueue *that* in
the cmsg data.  If you send an fd to someone who hasn't assigned any
placeholders to the receiving socket, then you get an error.

To keep the accounting sane, a placeholder would be a bona fide fd,
presumably a reference to a global placeholder anon_inode.

--Andy
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/