RE: [PATCH v3 4/8] platform/x86: wmi: create character devices when requested by drivers

[Mario.Limonciello]

> -----Original Message-----
> From: Greg Kroah-Hartman [mailto:gregkh@xxxxxxxxxxxxxxxxxxx]
> Sent: Saturday, September 30, 2017 3:12 AM
> To: Darren Hart <dvhart@xxxxxxxxxxxxx>
> Cc: Limonciello, Mario <Mario_Limonciello@xxxxxxxx>; Andy Shevchenko
> <andy.shevchenko@xxxxxxxxx>; LKML <linux-kernel@xxxxxxxxxxxxxxx>;
> platform-driver-x86@xxxxxxxxxxxxxxx; Andy Lutomirski <luto@xxxxxxxxxx>;
> quasisec@xxxxxxxxxx; pali.rohar@xxxxxxxxx; Rafael Wysocki
> <rjw@xxxxxxxxxxxxx>; Matthew Garrett <mjg59@xxxxxxxxxx>; Christoph Hellwig
> <hch@xxxxxx>
> Subject: Re: [PATCH v3 4/8] platform/x86: wmi: create character devices when
> requested by drivers
> 
> On Fri, Sep 29, 2017 at 06:52:28PM -0700, Darren Hart wrote:
> >
> > On Wed, Sep 27, 2017 at 11:02:16PM -0500, Mario Limonciello wrote:
> > > For WMI operations that are only Set or Query read or write sysfs
> > > attributes created by WMI vendor drivers make sense.
> > >
> > > For other WMI operations that are run on Method, there needs to be a
> > > way to guarantee to userspace that the results from the method call
> > > belong to the data request to the method call.  Sysfs attributes don't
> > > work well in this scenario because two userspace processes may be
> > > competing at reading/writing an attribute and step on each other's
> > > data.
> > >
> > > When a WMI vendor driver declares a set of functions in a
> > > file_operations object the WMI bus driver will create a character
> > > device that maps to those file operations.
> > >
> > > That character device will correspond to this path:
> > > /dev/wmi/$driver
> > >
> > > This policy is selected as one driver may map and use multiple
> > > GUIDs and it would be better to only expose a single character
> > > device.
> > >
> > > The WMI vendor drivers will be responsible for managing access to
> > > this character device and proper locking on it.
> > >
> > > When a WMI vendor driver is unloaded the WMI bus driver will clean
> > > up the character device.
> > >
> > > Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxxx>
> > > ---
> > >  drivers/platform/x86/wmi.c | 98
> +++++++++++++++++++++++++++++++++++++++++++---
> > >  include/linux/wmi.h        |  1 +
> > >  2 files changed, 94 insertions(+), 5 deletions(-)
> >
> > +Greg, Rafael, Matthew, and Christoph
> >
> > You each provided feedback regarding the method of exposing WMI methods
> > to userspace. This and subsequent patches from Mario lay some of the
> > core groundwork.
> >
> > They implement an implicit whitelist as only drivers requesting the char
> > dev will see it created.
> >
> > https://lkml.org/lkml/2017/9/28/8
> 
> If you want patchs reviewed, it's best to actually cc: us on the patch
> itself :(

Greg,

I'll make sure to CC you on V4 after I address the other concerns that have
been recently raised.

I think what's Darren's most interested in is the that conceptually this is
an approach you can agree with.

"A WMI vendor driver binds to the WMI bus and requests the WMI bus to create
a character device.  The WMI bus creates a character device /dev/wmi/$driver
which the WMI vendor driver will process all various file operations."

Thanks,