> - Where else does Linux use user mode drivers?
> * Frame buffer access for performance: DRI, DRM.
> * Printers, mostly (nowadays) using a serial byte-stream
> model to less performance-sensitive rendering systems.
> * Scanners (SANE) using serial byte-stream models
> * Various serial devices (most digital cameras :) that
> use serial byte-stream models.
> * ... surely a few others, but notice that pattern.
> - Where do other UNIXes use them?
> * Same as Linux
> * ... any notable differences?
> - How about non-POSIXey systems like Win32, Mac, BeOS?
> * ... not just ones that keep apps from touching hardware ...
There is graphics, which is a long standing sore point.
Other than that character devices without direct access to the CPU bus.
Applicable only to some USB devices.
> One complication for USB -- not shared with any of the more
> traditional device models! -- is hotplugging. Devices can be,
> and are, added/removed at any time. They can move around.
> The primary policy hook for security is to name a device. But
> hotplugging means those names can change...
>
> There needs to be some step that says "here's a new device,
> how should it be exposed for general use?" And the answer
> to that question needs to include permissions. I suppose we
> should assume those still get associated with device names,
> so as not to rock the boat too much.
That is reasonable.
But the actual setting of permissions needs additional information. In all
likelihood it is based at least in large part on the type or class of the
attached device.
> Of course they don't _always_ move around, and in cases
> where the device doesn't move (tree of USB devices that's
> set up and not changed, maybe) it'd be a good idea to save
> those answers and expect them to work next time around.
How do you tell them apart ?
Falsely applied generic permissions are preferrable to falsely applied
specific permissions.
> That relies on user space tools having stable names for
> USB (and other hotpluggable) devices; we don't have those
> today, due to "usbdevfs" limitations. (The Java USB APIs
> layer such names on top, but with some difficulty.)
Those are preferrable but not necessary. I agree.
> In fact you can look at that "device config is stable" setup
> as the traditional hardware norm, the one for which today's
> major/minor and /dev/* (even "devfs") naming scheme was
> designed.
Partially. Devfs overcomes part of the limitation.
> That's just some musings on the problem, which I'm hoping
> may help trigger useful insights from other folk. No answers
> yet, beyond saying that the unstable naming in "usbdevfs"
> is part of the problem.
Definitely.
Regards
Oliver
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