Re: WMI and Kernel:User interface

Darren Hart
2017-05-10T22:11:40Z

On Wed, May 10, 2017 at 10:02:46PM +0000, Mario.Limonciello@dell.com wrote:
> > -----Original Message-----
> > From: Darren Hart [mailto:dvhart@infradead.org]
> > Sent: Wednesday, May 10, 2017 1:12 AM
> > To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>; Limonciello, Mario
> > <Mario_Limonciello@Dell.com>; Pali Rohár <pali.rohar@gmail.com>; Andy
> > Shevchenko <andriy.shevchenko@linux.intel.com>; Rafael Wysocki
> > <rjw@rjwysocki.net>; Andy Lutomirski <luto@amacapital.net>; LKML <linux-
> > kernel@vger.kernel.org>; platform-driver-x86@vger.kernel.org
> > Subject: Re: WMI and Kernel:User interface
> >
> > On Wed, May 10, 2017 at 07:13:41AM +0200, Greg Kroah-Hartman wrote:
> > > On Tue, May 09, 2017 at 04:16:39PM -0700, Darren Hart wrote:
> > > > Linus and Greg,
> > > >
> > > > We are in the process of redesigning the Windows Management
> > Instrumentation
> > > > (WMI) [1] system in the kernel. WMI is the Microsoft implementation of Web-
> > Based
> > > > Enterprise Management (WBEM). We are looking to provide WMI access to
> > userspace,
> > > > while allowing the kernel to filter requests that conflict with its own usage.
> > > > We'd like your take on how this approach relates to our commitment to not
> > break
> > > > userspace.
> > > >
> > > > For this discussion, we are specifically referring to ACPI PNP0C14 WMI
> > > > devices, consisting of a GUID and a set of methods and events, as well as a
> > > > precompiled intermediate description of the methods and arguments (MOF).
> > Exposed
> > > > to userspace, these methods provide for BIOS interaction and are used for
> > system
> > > > management as well as LEDs, hot keys, radio switches, etc. There is vendor
> > > > interest in achieving feature parity with Windows by exposing WMI methods to
> > > > userspace for system management.
> > > >
> > > > While it appears WMI intended to be accessed from userspace, we have
> > > > made use of it in the kernel to support various laptop features by connecting
> > > > the WMI methods to other subsystems, notably input, leds, and rfkill [2]. The
> > > > challenge is continuing to use WMI for these platform features, while allowing
> > > > userspace to use it for system management tasks. Unfortunately, the WMI
> > methods
> > > > are not guaranteed to be split up along granular functional lines, and we will
> > > > certainly face situations where the same GUID::METHOD_ID will be needed for
> > a
> > > > kernel feature (say LED support) as well as a system management task.
> > > >
> > > > To address this, I have proposed [3] that exporting WMI be opt-in, only done at
> > > > the request of and in collaboration with a vendor, with the kernel platform
> > > > driver given the opportunity to filter requests. This filtering would need to be
> > > > at the method and argument inspection level, such as checking for specific bits
> > > > in the input buffer, and rejecting the request if they conflict with an in
> > > > kernel usage (that's worst case, in some cases just GUID or method ID could be
> > > > sufficient).
> > > >
> > > > Because the kernel and the platform drivers are under continual development,
> > and
> > > > new systems appear regularly, we will encounter necessary changes to the
> > > > platform driver WMI request filters. These changes could be considered a
> > change
> > > > to the kernel provided WMI interface to userspace. For example, we could
> > > > regularly accept a call to $GUID::$METHOD_ID with bit 4 of the buffer set, and
> > > > later deny the call when we determine it interferes with kernel usage.
> > > >
> > > > In your view, is it acceptable to provide a chardev interface, for example,
> > > > exposing WMI methods to userspace, with the understanding that the kernel
> > may
> > > > choose to filter certain requests which conflict with its own use? And that this
> > > > filtering may change as new features are added to the platform drivers?
> > >
> > > So, for example, if a new driver for a "brightness key" were added to
> > > the kernel, all of a sudden the "raw" access to the wmi data through the
> > > chardev would filtered away by the kernel and not seen by userspace?
> > >
> >
> > Pali provided some detail in the rather lengthy thread I linked to [3], but the
> > summary is that there is nothing to encourage vendors to separate out
> > functionality into separate method ids. Some do, the asus-wmi driver seems to
> > have a reasonable separation, others do a lot with a single method id.
> >
> > In the scenario you mention, it could be that the brightness key shows up in a
> > new laptop. Because we've never seen it before, the kernel driver doesn't detect
> > it and send the input event. The clever user realizes they can write a userspace
> > program to deal with this [a] by writing a my-laptop-brightness-key-daemon which
> > also makes a WMI method call to affect the change in brightness in response to
> > the keypress.
> >
> > Another user notices the dmesg is reporting "Unknown WMI Event" when they
> > press
> > that key and send a patch to make the corresponding wmi call to affect the
> > brightness change (which depends on driver data to track the brightness value).
> > To avoid conflicting with userspace, they also blacklist the WMI_BRIGHTNESS_CMD
> > from being called by userspace.
> >
> > Now the first user makes a call to set the brightness, and receives an error
> > code instead of a successful response. Their program no longer works - although
> > with a current kernel, their brightness key would.
> >
> > I glossed over a few things there, but the description isn't too far off from a
> > plausible scenario.
> >
> > > Why would you want to do that? What's wrong with providing "raw" access
> > > through a chardev, and the current in-kernel access as well at the same
> > > time?
> > >
> > > I don't really understand what would "break" over time here.
> >
> > Like the scenario above, if the behavior has state, if userspace and the kernel
> > attempt to control it, neither will have an accurate state representation.
> >
> > We could allow both and if things start not working, the user would have to
> > choose between the kernel driver and the userspace management application. This
> > was the scenario Pali was particularly keen to avoid.
> >
> > That said, to date I haven't come across anything that states there can only be
> > one userspace application accessing WMI at any given time. It should be
> > straightforward to serialize WMI calls such that they cannot "cross the
> > streams".
> >
> > >
> > > thanks,
> > >
> > > greg k-h
> > >
> >
> > a. This is perhaps a contrived scenario since hotkeys generate events, and as
> > far as I understand it, there is no interest in exporting events to userspace,
> > just the methods.
> >
> > --
> > Darren Hart
> > VMware Open Source Technology Center
>
> This above discussion is confusing because it's referring specifically to "WMI events".
> related to brightness keypresses that don't make sense to go to userspace.
>
> The more interesting (and potentially problematic) area is things that read and write
> data using ASL methods.
>
> So here's a "more" realistic scenario:
>
> OEM has support through a WMI function to control keyboard backlight timeouts
> and intensity. That same WMI function also can support turning on/off an individual
> USB port. Backlight timeouts are done by setting the first argument to "1" and USB
> port control is done by setting first argument to "2".
>
> Some userspace app is developed that can control both of these functions through
> the chardev. Later an enterprising young kernel developer realizes that backlight
> control should be done through a platform driver instead.
>
> They write a platform driver to do it, and add a filter to block "1" arguments from
> userspace. Now if the userspace app tries to call the chardev with the "1" argument
> some error code is returned indicating this request is not supported.
>
> The result is the userspace app broke, but it broke because the kernel is supporting
> the method in a much smarter and more scalable way.
>

Thank you Mario, that's a much better example.

--
Darren Hart
VMware Open Source Technology Center