linux-kernel - Re: [RFC][PATCH 0/8] x86, mpx: Support 32-bit binaries on 64-bit kernels

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Date:	Fri, 12 Dec 2014 13:41:08 -0800
From:	Dave Hansen <dave@...1.net>
To:	Andy Lutomirski <luto@...capital.net>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>
Subject: Re: [RFC][PATCH 0/8] x86, mpx: Support 32-bit binaries on 64-bit
 kernels

On 12/12/2014 12:48 PM, Andy Lutomirski wrote:
> On Fri, Dec 12, 2014 at 12:27 PM, Dave Hansen <dave@...1.net> wrote:
>> You want the same size structures with the same format for 32-bit and
>> 64-bit modes?
> 
> Yes.  Especially because programs can switch between 32-bit and 64-bit
> mode entirely in userspace.  I don't know whether any do in practice,
> but programs *can*.

So, you want a 2GB of the 32-bit address space dedicated to a bounds
directory, and half of the space for the bounds tables to be simply
zero-filled unused address bits?  That seems, um, a bit unreasonable.

> Or better yet: Intel could have skipped supporting it at all in 32-bit
> mode.  

So, we should not have this security feature for 32-bit apps... because
it costs us 50 lines of code in the kernel to support?  Did you look at
the diffstat?

> Isn't mpx somewhat of an address space hog anyway?

Yes, it will be troublesome for 32-bit apps that are already bumping up
against the virtual address space size to support it.  But, really, how
many of those *are* there these days?
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