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

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Date:	Wed, 13 Jul 2016 11:43:24 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
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

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?