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Date: Wed, 19 Jul 2017 16:25:56 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: 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>, Brian Gerst <brgerst@...il.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, 
	LKML <linux-kernel@...r.kernel.org>, 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>, linux-sparse@...r.kernel.org, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC 20/22] x86/relocs: Add option to generate 64-bit relocations

On Wed, Jul 19, 2017 at 4:08 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 07/19/17 15:47, Thomas Garnier wrote:
>> On Wed, Jul 19, 2017 at 3:33 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>> On 07/18/17 15:33, Thomas Garnier wrote:
>>>> The x86 relocation tool generates a list of 32-bit signed integers. There
>>>> was no need to use 64-bit integers because all addresses where above the 2G
>>>> top of the memory.
>>>>
>>>> This change add a large-reloc option to generate 64-bit unsigned integers.
>>>> It can be used when the kernel plan to go below the top 2G and 32-bit
>>>> integers are not enough.
>>>
>>> Why on Earth?  This would only be necessary if the *kernel itself* was
>>> more than 2G, which isn't going to happen for the forseeable future.
>>
>> Because the relocation integer is an absolute address, not an offset
>> in the binary. Next iteration, I can try using a 32-bit offset for
>> everyone.
>
> It is an absolute address *as the kernel was originally linked*, for
> obvious reasons.

Sure when the kernel was just above 0xffffffff80000000, it doesn't
work when it goes down to 0xffffffff00000000. That's why using an
offset might make more sense in general.

>
>         -hpa
>



-- 
Thomas.

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