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Date: Fri, 22 Sep 2017 11:57:09 -0700
From: Kees Cook <keescook@...omium.org>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Ingo Molnar <mingo@...nel.org>, Thomas Garnier <thgarnie@...gle.com>, 
	Herbert Xu <herbert@...dor.apana.org.au>, "David S . Miller" <davem@...emloft.net>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...hat.com>, 
	Arnd Bergmann <arnd@...db.de>, Matthias Kaehlcke <mka@...omium.org>, 
	Boris Ostrovsky <boris.ostrovsky@...cle.com>, Juergen Gross <jgross@...e.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Radim Krčmář <rkrcmar@...hat.com>, 
	Joerg Roedel <joro@...tes.org>, Tom Lendacky <thomas.lendacky@....com>, 
	Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...e.de>, Brian Gerst <brgerst@...il.com>, 
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, "Rafael J . Wysocki" <rjw@...ysocki.net>, 
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>, Tejun Heo <tj@...nel.org>, 
	Christoph Lameter <cl@...ux.com>, Paul Gortmaker <paul.gortmaker@...driver.com>, 
	Chris Metcalf <cmetcalf@...lanox.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, Nicolas Pitre <nicolas.pitre@...aro.org>, 
	Christopher Li <sparse@...isli.org>, "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>, 
	Lukas Wunner <lukas@...ner.de>, Mika Westerberg <mika.westerberg@...ux.intel.com>, 
	Dou Liyang <douly.fnst@...fujitsu.com>, Daniel Borkmann <daniel@...earbox.net>, 
	Alexei Starovoitov <ast@...nel.org>, Masahiro Yamada <yamada.masahiro@...ionext.com>, 
	Markus Trippelsdorf <markus@...ppelsdorf.de>, Steven Rostedt <rostedt@...dmis.org>, 
	Rik van Riel <riel@...hat.com>, David Howells <dhowells@...hat.com>, Waiman Long <longman@...hat.com>, 
	Kyle Huey <me@...ehuey.com>, Peter Foley <pefoley2@...oley.com>, 
	Tim Chen <tim.c.chen@...ux.intel.com>, Catalin Marinas <catalin.marinas@....com>, 
	Ard Biesheuvel <ard.biesheuvel@...aro.org>, Michal Hocko <mhocko@...e.com>, 
	Matthew Wilcox <mawilcox@...rosoft.com>, "H . J . Lu" <hjl.tools@...il.com>, 
	Paul Bolle <pebolle@...cali.nl>, Rob Landley <rob@...dley.net>, Baoquan He <bhe@...hat.com>, 
	Daniel Micay <danielmicay@...il.com>, "the arch/x86 maintainers" <x86@...nel.org>, 
	Linux Crypto Mailing List <linux-crypto@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	xen-devel <xen-devel@...ts.xenproject.org>, kvm list <kvm@...r.kernel.org>, 
	Linux PM list <linux-pm@...r.kernel.org>, linux-arch <linux-arch@...r.kernel.org>, 
	Sparse Mailing-list <linux-sparse@...r.kernel.org>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, 
	Borislav Petkov <bp@...en8.de>
Subject: Re: x86: PIE support and option to extend KASLR randomization

On Fri, Sep 22, 2017 at 11:38 AM, H. Peter Anvin <hpa@...or.com> wrote:
> We lose EBX on 32 bits, but we don't lose RBX on 64 bits - since x86-64
> has RIP-relative addressing there is no need for a dedicated PIC register.

FWIW, since gcc 5, the PIC register isn't totally lost. It is now
reusable, and that seems to have improved performance:
https://gcc.gnu.org/gcc-5/changes.html

-Kees

-- 
Kees Cook
Pixel Security

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