linux-kernel - Re: [RFC 06/22] kvm: Adapt assembly for PIE support

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Message-Id: <201707192348.v6JNm8Rg003916@mail.zytor.com>
Date:   Wed, 19 Jul 2017 16:47:45 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
CC:     Thomas Garnier <thgarnie@...gle.com>,
        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>,
        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>,
        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@...or.com
Subject: Re: [RFC 06/22] kvm: Adapt assembly for PIE support

<paul.gortmaker@...driver.com>,Chris Metcalf <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>,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 <linux-sparse@...r.kernel.org>,Kernel Hardening <kernel-hardening@...ts.openwall.com>
From: hpa@...or.com
Message-ID: <83BA7600-BC8D-4C91-812C-DD2A0BF4474B@...or.com>

On July 19, 2017 3:58:07 PM PDT, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
>On 19 July 2017 at 23:27, 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.
>>
>
>Sadly, there is an issue in binutils that may prevent us from doing
>this as cleanly as we would want.
>
>For historical reasons, bfd.ld emits special symbols like
>__GLOBAL_OFFSET_TABLE__ as absolute symbols with a section index of
>SHN_ABS, even though it is quite obvious that they are relative like
>any other symbol that points into the image. Unfortunately, this means
>that binutils needs to emit R_X86_64_RELATIVE relocations even for
>SHN_ABS symbols, which means we lose the ability to use both absolute
>and relocatable symbols in the same PIE image (unless the reloc tool
>can filter them out)
>
>More info here:
>https://sourceware.org/bugzilla/show_bug.cgi?id=19818

The reloc tool already has the ability to filter symbols.
-- 
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