Date: Thu, 14 Jan 2016 20:54:52 -0800
From: Isaac Dunham <ibid.ag@gmail.com>
To: Laurent Bercot <ska-devel@skarnet.org>
Cc: alpine-devel@lists.alpinelinux.org
Subject: Re: [alpine-devel] udev replacement on Alpine Linux
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On Thu, Jan 14, 2016 at 12:36:02PM +0100, Laurent Bercot wrote:
> On 14/01/2016 06:55, Jude Nelson wrote:
> >I think you're close.  The jist of it is that vdev needs to supply a
> >lot more information than the kernel gives it.  In particular, its
> >helper programs go on to query the properties and status of each
> >device (this often requires root privileges, i.e. via privileged
> >ioctl()s), and vdev gathers the information into a (much larger)
> >event packet and stores it in a directory tree under /dev for
> >subsequent query by less-privileged programs.
> 
>  I see.
>  I think this is exactly what could be made modular. I've heard
> people say they were reluctant to using vdev because it's not KISS, and
> I suspect the ioctl machinery and data gathering is a large part of
> the complexity. If that part could be pluggable, i.e. if admins could
> choose a "data gatherer" just complex enough for their needs, I believe
> it could encourage adoption. In other words, I'm looking at a 3-part
> program:
>  - the netlink listener
>  - the data gatherer
>  - the event publisher
> 
>  Of course, for libudev to work, you would need the full data gatherer;
> but if people aren't using libudev programs, they can use a simpler one,
> closer to what mdev is doing.
>  It's all from a very high point-of-view, and I don't know the details of
> the code so I have no idea whether it's envisionable for vdev, but that's
> what I'm thinking off the top of my head.

I haven't really looked at the vdevd code in a while, but from what
I recollect...
The vdev/libudev-compat "solution" is split up as follows:
-the netlink listener, vdevd
-several data gatherers, which are invoked according to a set of rules
for vdevd (analogous to mdev.conf)
-helper scripts to create extra links and so on, run via the same rules
-I don't recall exactly how the event publishing is done, but IIRC,
there's a daemon watching the directory where the helpers write out the
data, and then distributing it as described
-libudev-compat is libudev, patched to read events that are published
as described
-IIRC, there's some helper that will (also? instead?) maintain a list of
devices more like the way udev does, so you don't *need* libudev-compat.


Jude went to a bit of effort to design vdevd and libudev-compat so that
it would be possible to use part alongside mdev.

The discussion is in the Devuan archives; I don't recall how far back,
though.

HTH,
Isaac Dunham


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