linux-kernel - Re: kdbus: add documentation

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Date:	Tue, 20 Jan 2015 13:54:00 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Daniel Mack <daniel@...que.org>, Florian Weimer <fw@...eb.enyo.de>,
	David Herrmann <dh.herrmann@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	mtk.manpages@...il.com, Arnd Bergmann <arnd@...db.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Tom Gundersen <teg@...m.no>, Jiri Kosina <jkosina@...e.cz>,
	Andy Lutomirski <luto@...capital.net>,
	Linux API <linux-api@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Djalal Harouni <tixxdz@...ndz.org>
Subject: Re: kdbus: add documentation

On 01/20/2015 09:25 AM, Daniel Mack wrote:
> Hi Michael,
> 
> On 01/20/2015 09:09 AM, Michael Kerrisk (man-pages) wrote:
>> On 11/30/2014 06:23 PM, Florian Weimer wrote:
>>> * David Herrmann:
>>>
>>>> On Sun, Nov 30, 2014 at 10:02 AM, Florian Weimer <fw@...eb.enyo.de> wrote:
>>>>> * Greg Kroah-Hartman:
>>>>>
>>>>>> +7.4 Receiving messages
>>>
>>>>> What happens if this is not possible because the file descriptor limit
>>>>> of the processes would be exceeded?  EMFILE, and the message will not
>>>>> be received?
>>>>
>>>> The message is returned without installing the FDs. This is signaled
>>>> by EMFILE, but a valid pool offset.
>>>
>>> Oh.  This is really surprising, so it needs documentation.  But it's
>>> probably better than the alternative (return EMFILE and leave the
>>> message stuck, so that you receive it immediately again—this behavior
>>> makes non-blocking accept rather difficult to use correctly).
>>
>> So, was this point in the end explicitly documented? I not
>> obvious that it is documented in the revised kdbus.txt that
>> Greg K-H sent out 4 days ago.
> 
> No, we've revisited this point and changed the kernel behavior again in
> v3. We're no longer returning -EMFILE in this case, but rather set
> KDBUS_RECV_RETURN_INCOMPLETE_FDS in a new field in the receive ioctl
> struct called 'return_flags'. We believe that's a nicer way of signaling
> specific errors. The message will carry -1 for all FDs that failed to
> get installed, so the user can actually see which one is missing.
> 
> That's also documented in kdbus.txt, but we missed putting it into the
> Changelog - sorry for that.

Thanks for the info, Daniel.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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