linux-kernel - Re: [RFC 20/22] x86/relocs: Add option to generate 64-bit relocations

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Message-Id: <201707192346.v6JNk3QR002981@mail.zytor.com>
Date:   Wed, 19 Jul 2017 16:45:37 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Thomas Garnier <thgarnie@...gle.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>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        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>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Chris.Metcalf@...or.com
Subject: Re: [RFC 20/22] x86/relocs: Add option to generate 64-bit relocations

<cmetcalf@...lanox.com>,"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,Andrew Morton <akpm@...ux-foundation.org>,Christopher Li <sparse@...isli.org>,Dou Liyang <douly.fnst@...fujitsu.com>,Masahiro Yamada <yamada.masahiro@...ionext.com>,Daniel Borkmann <daniel@...earbox.net>,Markus Trippelsdorf <markus@...ppelsdorf.de>,Peter Foley <pefoley2@...oley.com>,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>
From: hpa@...or.com
Message-ID: <0EF6FAAA-A99C-4F0D-9E4A-AD25E93957FB@...or.com>

On July 19, 2017 4:25:56 PM PDT, Thomas Garnier <thgarnie@...gle.com> wrote:
>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
>>

What is the motivation for changing the pre linked address at all?
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