On Sat, Jun 03, 2017 at 12:50:58PM -0700, Darren Hart wrote: > 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? > > > > 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. > > > > Just a bump now that we're out of the merge window in case either Greg or Linus > care to follow up with the responses to this. > > To Greg's last point - any kernel state that is built up in conjunction with the > WMI interface could be invalidated by a userspace application. It may or may not > be recoverable, depending on the WMI implementation. This would be true for > multiple WMI userspace applications as well, and I suppose the question is, do > we defend the kernel drivers against this, or do we consider the kernel drivers > on equal footing with WMI applications, and say "don't do that then" when some > combination of apps and drivers don't play well together? In the end, this shouldn't really matter, as long as nothing breaks as far as a user notices. And that's the key here, apis can change, but if you do it in a way that breaks something, or anyone notices, then it's not ok. So I don't have a solid answer other than "good luck!" :) greg k-h
