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Date: Thu, 14 Jul 2016 10:07:01 +0200
From: Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 6/9] x86, pkeys: add pkey set/get syscalls
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To: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@intel.com>, Thomas Gleixner <tglx@linutronix.de>, Dave Hansen <dave.hansen@linux.intel.com>, Al Viro <viro@zeniv.linux.org.uk>, X86 ML <x86@kernel.org>, Hugh Dickins <hughd@google.com>, Andrew Morton <akpm@linux-foundation.org>, Linux API <linux-api@vger.kernel.org>, Mel Gorman <mgorman@techsingularity.net>, Linus Torvalds <torvalds@linux-foundation.org>, linux-arch <linux-arch@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, Arnd Bergmann <arnd@arndb.de>, Peter Zijlstra <a.p.zijlstra@chello.nl>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "H. Peter Anvin" <hpa@zytor.com>


* Andy Lutomirski <luto@amacapital.net> wrote:

> On Wed, Jul 13, 2016 at 12:56 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Andy Lutomirski <luto@amacapital.net> wrote:
> >
> >> > If we push a PKRU value into a thread between the rdpkru() and wrpkru(), we'll
> >> > lose the content of that "push".  I'm not sure there's any way to guarantee
> >> > this with a user-controlled register.
> >>
> >> We could try to insist that user code uses some vsyscall helper that tracks
> >> which bits are as-yet-unassigned.  That's quite messy, though.
> >
> > Actually, if we turned the vDSO into something more like a minimal user-space 
> > library with the ability to run at process startup as well to prepare stuff 
> > then it's painful to get right only *once*, and there will be tons of other 
> > areas where a proper per thread data storage on the user-space side would be 
> > immensely useful!
> 
> Doing this could be tricky: how exactly is the vDSO supposed to find per-thread 
> data without breaking existing glibc?

So I think the way this could be done is by allocating it itself. The vDSO vma 
itself is 'external' to glibc as well to begin with - this would be a small 
extension to that concept.

Thanks,

	Ingo

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