linux-kernel - Re: [GIT pull] x86 mpx support for 3.19

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Date:	Fri, 12 Dec 2014 09:31:34 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Dave Hansen <dave.hansen@...ux.intel.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [GIT pull] x86 mpx support for 3.19


* Dave Hansen <dave.hansen@...ux.intel.com> wrote:

> +config X86_INTEL_MPX
> +	prompt "Intel MPX (Memory Protection Extensions)"
> +	def_bool y
> +	depends on CPU_SUP_INTEL
> +	---help---
> +	  MPX provides hardware features that can be used in
> +	  conjunction with compiler-instrumented code to check
> +	  memory references.  It is designed to detect buffer
> +	  overflow or underflow bugs.
> +
> +	  This option enables running applications which are
> +	  instrumented or otherwise use MPX.  It does not use MPX
> +	  itself inside the kernel or to protect the kernel
> +	  against bad memory references.
> +
> +	  Enabling this option will make the kernel larger:
> +	  ~8k of kernel text and 36 bytes of data on a 64-bit
> +	  defconfig.  It adds a long to the 'mm_struct' which
> +	  will increase the kernel memory overhead of each
> +	  process and adds some branches to paths used during
> +	  exec() and munmap().
> +
> +	  If unsure, say Y.

That description looks pretty good to me.

Linus, what's your take on the default-y? To me it seems there's 
arguments both ways: usually we allow unprivileged ABIs to be 
disabled only under EXPERT and make them default-y (to help 
adaption and to define what the default ABI of 'Linux' is - we 
did a default-y for SECCOMP for example) - OTOH this is a new 
ABI, the processors aren't even out yet, and there's costs to the 
MM code even in the non-MPX case, so it's not as clear-cut as in 
the SECCOMP case which had essentially zero runtime cost.

Thanks,

	Ingo
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