linux-kernel - Re: [PATCH v6 08/10] x86, mpx: add prctl commands PR_MPX_REGISTER, PR_MPX

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Message-ID: <53A88966.5080400@intel.com>
Date:	Mon, 23 Jun 2014 13:09:10 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Qiaowei Ren <qiaowei.ren@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
CC:	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 08/10] x86, mpx: add prctl commands PR_MPX_REGISTER,
 PR_MPX_UNREGISTER

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.

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.
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