linux-kernel - Re: [GIT pull] x86 mpx support for 3.19

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Date:	Wed, 10 Dec 2014 18:30:00 -0800
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org
Subject: Re: [GIT pull] x86 mpx support for 3.19

On 12/10/2014 06:14 PM, Eric W. Biederman wrote:
> Thomas Gleixner <tglx@...utronix.de> writes:
>> please pull the latest x86-mpx-for-linus git tree from:
>>
>>    git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86-mpx-for-linus
>>
>> This enables support for x86 MPX:
>>
>>   MPX is a new debug feature for bound checking in user space. It
>>   requires kernel support to handle the bound tables and decode the
>>   bound violating instruction in the trap handler.
> 
> I some really dumb questions.
> 
> Given that mpx is both architecture and cpu specific why use prctl?
> I would think arch_prctl would be a much more appropriate place to
> expose this logic.

I actually never considered arch_prctl().  It doesn't seem a bad fit for
any reason, just that I never thought of it and no one suggested it up
to this point.

Is there any *real* advantage to arch_prctl()?  We have some gcc code
that's going to be using these prctls and if we need to change the
interface, we've got to get that code changed too... fast.

> Why don't you have a call to let an application query to see if mpx
> management is enabled?  I am trying to imagine how checkpoint/restart is
> going to be supported for mpx applications.  If I can't even query if
> mpx is enabled I don't have a clue how we could save this state and
> duplicate it in another process on another machine.

You probably need to actually save off the contents of mm->bd_addr
itself.  I guess you can do it along with all the other internals of the
mm that get saved off currently.
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