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

From: Dave Hansen
Date: Tue Jul 12 2016 - 11:39:15 EST

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.