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Date: Mon, 15 Sep 2014 21:17:02 -0700
From: Dave Hansen <dave.hansen@intel.com>
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To: "Ren, Qiaowei" <qiaowei.ren@intel.com>,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
CC: "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, "x86@kernel.org" <x86@kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v8 08/10] x86, mpx: add prctl commands PR_MPX_REGISTER,
 PR_MPX_UNREGISTER
References: <1410425210-24789-1-git-send-email-qiaowei.ren@intel.com>	<1410425210-24789-9-git-send-email-qiaowei.ren@intel.com> <20140915010025.5940c946@alan.etchedpixels.co.uk> <9E0BE1322F2F2246BD820DA9FC397ADE017AE183@shsmsx102.ccr.corp.intel.com>
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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.