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

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Date:	Tue, 12 Jul 2016 08:39:07 -0700
From:	Dave Hansen <dave@...1.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	linux-arch <linux-arch@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux API <linux-api@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Hugh Dickins <hughd@...gle.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 6/9] x86, pkeys: add pkey set/get syscalls

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.