Re: [PATCH v9 29/51] mm/mprotect, powerpc/mm/pkeys, x86/mm/pkeys: Add sysfs interface

[Ram Pai <linuxram@us.ibm.com>]

On Mon, Dec 18, 2017 at 02:28:14PM -0800, Dave Hansen wrote:
> On 12/18/2017 02:18 PM, Ram Pai wrote:
> > b) minimum number of keys available to the application.
> > 	if libraries consumes a few, they could provide a library
> > 	interface to the application informing the number available to
> > 	the application.  The library interface can leverage (b) to
> > 	provide the information.
> 
> OK, let's see a real user of this including a few libraries.  Then we'll
> put it in the kernel.
> 
> > c) types of disable-rights supported by keys.
> > 	Helps the application to determine the types of disable-features
> > 	available. This is helpful, otherwise the app has to 
> > 	make pkey_alloc() call with the corresponding parameter set
> > 	and see if it suceeds or fails. Painful from an application
> > 	point of view, in my opinion.
> 
> Again, let's see a real-world use of this.  How does it look?  How does
> an app "fall back" if it can't set a restriction the way it wants to?
> 
> Are we *sure* that such an interface makes sense?  For instance, will it
> be possible for some keys to be execute-disable while other are only
> write-disable?

Can it be on x86?

its not possible on ppc. the keys can *all* be  the-same-attributes-disable all the
time.

However you are right. Its conceivable that some arch could provide a
feature where it can be x-attribute-disable for key 'a' and
y-attribute-disable for key 'b'.  But than its a bit of a headache
for an application to consume such a feature.

Ben: I recall you requesting this feature.  Thoughts?

> 
> > I think on x86 you look for some hardware registers to determine
> > which hardware features are enabled by the kernel.
> 
> No, we use CPUID.  It's a part of the ISA that's designed for
> enumerating CPU and (sometimes) OS support for CPU features.
> 
> > We do not have generic support for something like that on ppc.  The
> > kernel looks at the device tree to determine what hardware features
> > are available. But does not have mechanism to tell the hardware to
> > track which of its features are currently enabled/used by the
> > kernel; atleast not for the memory-key feature.
> 
> Bummer.  You're missing out.
> 
> But, you could still do this with a syscall.  "Hey, kernel, do you
> support this feature?"

or do powerpc specific sysfs interface.
or a debugfs interface.

RP