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

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Message-ID: <20160714080701.GA14613@gmail.com>
Date:	Thu, 14 Jul 2016 10:07:01 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Dave Hansen <dave.hansen@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Al Viro <viro@...iv.linux.org.uk>, X86 ML <x86@...nel.org>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux API <linux-api@...r.kernel.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Arnd Bergmann <arnd@...db.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 6/9] x86, pkeys: add pkey set/get syscalls


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

> On Wed, Jul 13, 2016 at 12:56 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >
> > * Andy Lutomirski <luto@...capital.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