[RFC v7 26/25] mm/mprotect, powerpc/mm/pkeys, x86/mm/pkeys: Add sysfs interface

[Ram Pai]

On Fri, Aug 11, 2017 at 02:34:43PM -0300, Thiago Jung Bauermann wrote:
> Expose useful information for programs using memory protection keys.
> Provide implementation for powerpc and x86.
> 
> On a powerpc system with pkeys support, here is what is shown:
> 
> $ head /sys/kernel/mm/protection_keys/*
> ==> /sys/kernel/mm/protection_keys/disable_execute_supported <==
> true

We should not just call out disable_execute_supported.
disable_access_supported and disable_write_supported should also 
be called out.

> 
> ==> /sys/kernel/mm/protection_keys/total_keys <==
> 32
> 

> ==> /sys/kernel/mm/protection_keys/usable_keys <==
> 30

This is little nebulous.  It depends on how we define
usable as.  Is it the number of keys that are available
to the app?  If that is the case that value is dynamic.
Sometime the OS steals one key for execute-only key.
And anything that is dynamic can be inherently racy.
So I think we should define 'usable' as guaranteed number
of keys available to the app and display a value that is
one less than what is available.

in the above example the value should be 29.

RP