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

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Message-ID: <CALCETrWPM_jrF6saHPxoUL4vkjhhGBfizATXrFOW=+z5xycToQ@mail.gmail.com>
Date:	Sun, 17 Jul 2016 21:43:31 -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 Thu, Jul 14, 2016 at 1:07 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * 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.

But how does the vdso code find it?  FS and GS are both spoken for by
existing userspace.

--Andy