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

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