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Subject: Re: [PATCH 6/9] x86, pkeys: add pkey set/get syscalls
To: Ingo Molnar <mingo@kernel.org>
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Cc: Andy Lutomirski <luto@amacapital.net>,
        linux-arch <linux-arch@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Al Viro <viro@zeniv.linux.org.uk>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Hugh Dickins <hughd@google.com>, "H. Peter Anvin" <hpa@zytor.com>,
        X86 ML <x86@kernel.org>, Dave Hansen <dave.hansen@linux.intel.com>
From: Dave Hansen <dave@sr71.net>
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Date: Tue, 12 Jul 2016 08:39:07 -0700
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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.