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Date: Wed, 19 Jul 2017 15:44:48 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Brian Gerst <brgerst@...il.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>, 
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	Joerg Roedel <joro@...tes.org>, Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>, 
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, Borislav Petkov <bp@...e.de>, 
	Christian Borntraeger <borntraeger@...ibm.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>, Kees Cook <keescook@...omium.org>, 
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	Masahiro Yamada <yamada.masahiro@...ionext.com>, Daniel Borkmann <daniel@...earbox.net>, 
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	Steven Rostedt <rostedt@...dmis.org>, Tim Chen <tim.c.chen@...ux.intel.com>, 
	Ard Biesheuvel <ard.biesheuvel@...aro.org>, Catalin Marinas <catalin.marinas@....com>, 
	Matthew Wilcox <mawilcox@...rosoft.com>, Michal Hocko <mhocko@...e.com>, Rob Landley <rob@...dley.net>, 
	Jiri Kosina <jkosina@...e.cz>, "H . J . Lu" <hjl.tools@...il.com>, Paul Bolle <pebolle@...cali.nl>, 
	Baoquan He <bhe@...hat.com>, Daniel Micay <danielmicay@...il.com>, 
	"the arch/x86 maintainers" <x86@...nel.org>, 
	"linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, xen-devel@...ts.xenproject.org, 
	kvm list <kvm@...r.kernel.org>, linux-pm <linux-pm@...r.kernel.org>, 
	linux-arch <linux-arch@...r.kernel.org>, linux-sparse@...r.kernel.org, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC 06/22] kvm: Adapt assembly for PIE support

On Wed, Jul 19, 2017 at 3:27 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 07/19/17 08:40, Thomas Garnier wrote:
>>>
>>> This doesn't look right.  It's accessing a per-cpu variable.  The
>>> per-cpu section is an absolute, zero-based section and not subject to
>>> relocation.
>>
>> PIE does not respect the zero-based section, it tries to have
>> everything relative. Patch 16/22 also adapt per-cpu to work with PIE
>> (while keeping the zero absolute design by default).
>>
>
> This is silly.  The right thing is for PIE is to be explicitly absolute,
> without (%rip).  The use of (%rip) memory references for percpu is just
> an optimization.

I agree that it is odd but that's how the compiler generates code. I
will re-explore PIC options with mcmodel=small or medium, as mentioned
on other threads.

>
>         -hpa
>



-- 
Thomas

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