linux-kernel - Re: [PATCH 0/4] Add 32 bit VDSO time function support

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Message-ID: <52EA93E4.6010504@zytor.com>
Date:	Thu, 30 Jan 2014 10:03:16 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Stefani Seibold <stefani@...bold.net>
CC:	Greg KH <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	andriy.shevchenko@...ux.intel.com, Martin.Runge@...de-schwarz.com,
	Andreas.Brief@...de-schwarz.com
Subject: Re: [PATCH 0/4] Add 32 bit VDSO time function support

On 01/30/2014 09:57 AM, Andy Lutomirski wrote:
> 
> By definition there aren't any broken users of the new functions,
> because there aren't any users at all.  So... should we start
> randomizing this thing from the beginning?
> 

The vdso already exists.  It isn't new.  Randomizing it might be a good
idea, though; it already is randomized on 64 bits.

> Also, since the VVAR page has a real vma, should something be done to
> prevent mprotect or ptrace from COWing it?  Users will be rather
> surprised if it suddenly stops updating.

What happens currently on 64 bits?  I think we just take the attitude
that "don't do that, then", and it hasn't seemed to be a problem.

> Finally, this might be the time to kill off the userspace mapping of
> the HPET.  I suspect that there are few if any machines for which the
> HPET is fast enough that avoiding a syscall matters at all.  (On my
> box at work, reading the HPET takes ~500 nanoseconds.  I can do a lot
> of syscalls in that amount of time.)

I think this can be independent of extending the current 64-bit
functionality to 32 bits.  It is a valid question, though.

	-hpa

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