linux-kernel - Re: [PATCH 0/6][RFC] Rework vsyscall to avoid truncation/rounding issue in timekeeping core

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Message-ID: <CALCETrVc0Lk62_jgB69h62g5pTPt8RFesQf6nOg7xLsb3GqWxw@mail.gmail.com>
Date:	Wed, 19 Sep 2012 14:15:46 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	John Stultz <john.stultz@...aro.org>,
	Richard Cochran <richardcochran@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Paul Turner <pjt@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/6][RFC] Rework vsyscall to avoid truncation/rounding
 issue in timekeeping core

On Wed, Sep 19, 2012 at 1:50 PM, Luck, Tony <tony.luck@...el.com> wrote:
>> Does anything except the vDSO actually use the vDSO data page?  It's
>> mapped as part of the vDSO image (i.e. at a non-constant address), and
>> it's not immediate obvious how userspace would locate that page.
>
> Just for reference - on ia64 the address of the entry point for the magic
> fast system call page is passed to each applications via the "auxv" structure
> that exec(2) drops at the top of stack after args/env in the AT_SYSINFO
> entry. Apps look for it to find out where to jump for fast system call entry
> (if it isn't there, they fall back to regular slow syscall path).
>
> Same method could be used to provide the address of a magic read-only
> for users page that kernel fills with stuff for simple system calls.

Erk.  I'd much rather *not* pass this information to user apps -- if
they only ever access this stuff via the vDSO, then there's no ABI
issue the next time the data structure changes.  (e.g. on x86-64 it's
changed at least twice this year, and nothing noticed.)

--Andy
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