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From: Andy Lutomirski <luto@amacapital.net>
Date: Mon, 23 Jun 2014 15:00:46 -0700
Message-ID: <CALCETrX7B8yxrNgojCR8YNH+6zHX8vq+GMFn0RjdWXdLmhA-Fg@mail.gmail.com>
Subject: Re: [PATCH v6 08/10] x86, mpx: add prctl commands PR_MPX_REGISTER, PR_MPX_UNREGISTER
To: Dave Hansen <dave.hansen@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Qiaowei Ren <qiaowei.ren@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>
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Sender: linux-kernel-owner@vger.kernel.org

On Jun 23, 2014 1:09 PM, "Dave Hansen" <dave.hansen@intel.com> wrote:
>
> On 06/23/2014 01:00 PM, Andy Lutomirski wrote:
> > On 06/18/2014 02:44 AM, Qiaowei Ren wrote:
> >> > This patch adds the PR_MPX_REGISTER and PR_MPX_UNREGISTER prctl()
> >> > commands. These commands can be used to register and unregister MPX
> >> > related resource on the x86 platform.
> >> >
> >> > The base of the bounds directory is set into mm_struct during
> >> > PR_MPX_REGISTER command execution. This member can be used to
> >> > check whether one application is mpx enabled.
> > The register and unregister operations seem to be almost the same thing.
> >  How about just PR_SYNC_MPX?
>
> That wouldn't support a usage model where userspace wanted to keep using
> MPX, but wanted the kernel to butt out and not try to free unused bounds
> tables.  That's not super-important, but it does lose some level of
> flexibility.
>

Hmm.  How about PR_SET/GET_MPX_BOUNDS_TABLE, to update the kernel's
copy.  No fpu magic needed.

This has an added benefit: CRIU will need updating for MPX, and
they'll appreciate having the required interface already exist.
(They'll want a way to allocate "MPX" memory, too, but that's probably
somewhat less important, and it won't result in duplicated
functionality.)

> FWIW, I think it would also be handy to support a PR_MPX_DISABLE prctl
> too.  That way, a wrapper program could set a flag that any children
> could notice if they try a PR_MPX_REGISTER.  That way we could
> software-disable MPX in cases in a process tree where it was not wanted.

Seccomp can do this :)

--Andy
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