linux-kernel - Re: [dm-devel] clone() with CLONE_NEWNET breaks kobject_uevent

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Date:	Fri, 19 Aug 2011 04:43:39 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Milan Broz <mbroz@...hat.com>
Cc:	device-mapper development <dm-devel@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kay Sievers <kay.sievers@...y.org>,
	"David S. Miller" <davem@...emloft.net>, containers@...ts.osdl.org
Subject: Re: [dm-devel] clone() with CLONE_NEWNET breaks kobject_uevent_env()

Milan Broz <mbroz@...hat.com> writes:

> On 08/19/2011 11:13 AM, Eric W. Biederman wrote:
>> Milan Broz <mbroz@...hat.com> writes:
>> 
>> I think the proper fix is to remove the error return from
>> kobject_uevent_env and kobject_uevent, and make it harder to get calling
>> of this function wrong.  Possibly in conjunction with that tag all of
>> the memory allocations of kobject_uevent_env with GFP_NOFAIL or
>> something so the memory allocator knows that this path is totally
>> not able to deal with failure.
>> 
>> Is kobject_uevent_env anything except an asynchronous best effort
>> notification to user-space that a device has come or gone?
>
> Unfortunately it is for device-mapper. libdevmapper
> depends on information that uevent was sent because udev rules uses
> semaphore to inform that some action was taken.
> So if dm-ioctl returns flag that uevent was not sent, it fallback
> to different error path (otherwise it waits for completion forever).
> (TBH I am more and more convinced this was not quite clever concept.)

If I understand your description and the code right the guarantee that
you need is that kobject_uevent will return success only if it has
queued a packet in every listening netlink socket.

We already ignore ENOBUFS so the guarantee you appear to need in
libdevmapper does not appear to be present in kobject_uevent.

Does the libdevmapper code work despite getting a spurious failure?

If libdevmapper does not work despite a spurious failure I don't see how
we could possibly fix kobject_uevent when there is more than one netlink
listener.

Eric
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