Re: [PATCH 6/9] x86, pkeys: add pkey set/get syscalls

[Dave Hansen]

On 07/12/2016 12:13 AM, Ingo Molnar wrote:
>> > Remember, PKRU is just a *bitmap*.  The only place keys are stored is in the 
>> > page tables.
> A pkey is an index *and* a protection mask. So by representing it as a bitmask we 
> lose per thread information. This is what I meant by 'incomplete shadowing' - for 
> example the debug code couldn't work: if we cleared a pkey in a task we wouldn't 
> know what to restore it to with the current data structures, right?

Right.  I actually have some code to do the shadowing that I wrote to
explore how to do different PKRU values in signal handlers.  The code
only shadowed the keys that were currently allocated, and used the
(mm-wide) allocation map to figure that out.  It did not have a separate
per-thread concept of which parts of PKRU need to be shadowed.

It essentially populated the shadow value on all pkru_set() calls.