linux-kernel - Re: [PATCH v8 08/10] x86, mpx: add prctl commands PR_MPX_REGISTER, PR_MPX

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Message-ID: <5417B9BE.1030209@intel.com>
Date:	Mon, 15 Sep 2014 21:17:02 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	"Ren, Qiaowei" <qiaowei.ren@...el.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 08/10] x86, mpx: add prctl commands PR_MPX_REGISTER,
 PR_MPX_UNREGISTER

On 09/15/2014 08:20 PM, Ren, Qiaowei wrote:
>> What are the semantics across execve() ?
>> 
> This will not impact on the semantics of execve(). One runtime
> library
> for MPX will be provided (or merged into Glibc), and when the
> application starts, this runtime will be called to initialize MPX
> runtime environment, including calling prctl() to notify the kernel to
> start managing the bounds directories. You can see the discussion
> about exec(): https://lkml.org/lkml/2014/1/26/199

I think he's asking what happens to the kernel value at execve() time.

The short answer is that it is zero'd along with the rest of a new mm.
It probably _shouldn't_ be, though.  It's actually valid to have a bound
directory at 0x0.  We probably need to initialize it to -1 instead, and
that means initializing to -1 at execve() time.
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