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Re: [Xen-devel] [RFC 21/22] x86/module: Add support for mcmodel large and PLTs
- To: Brian Gerst <brgerst@xxxxxxxxx>
- From: Thomas Garnier <thgarnie@xxxxxxxxxx>
- Date: Wed, 19 Jul 2017 08:58:32 -0700
- Cc: Michal Hocko <mhocko@xxxxxxxx>, kvm list <kvm@xxxxxxxxxxxxxxx>, Radim Krčmář <rkrcmar@xxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Catalin Marinas <catalin.marinas@xxxxxxx>, Christopher Li <sparse@xxxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>, Pavel Machek <pavel@xxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Kernel Hardening <kernel-hardening@xxxxxxxxxxxxxxxxxx>, Christoph Lameter <cl@xxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Chris Metcalf <cmetcalf@xxxxxxxxxxxx>, linux-arch <linux-arch@xxxxxxxxxxxxxxx>, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>, Daniel Borkmann <daniel@xxxxxxxxxxxxx>, Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>, Joerg Roedel <joro@xxxxxxxxxx>, Peter Foley <pefoley2@xxxxxxxxxxx>, Christian Borntraeger <borntraeger@xxxxxxxxxx>, linux-sparse@xxxxxxxxxxxxxxx, Matthias Kaehlcke <mka@xxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Borislav Petkov <bp@xxxxxxx>, Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx>, Len Brown <len.brown@xxxxxxxxx>, Kees Cook <keescook@xxxxxxxxxxxx>, Arnd Bergmann <arnd@xxxxxxxx>, linux-pm <linux-pm@xxxxxxxxxxxxxxx>, "H . J . Lu" <hjl.tools@xxxxxxxxx>, Steven Rostedt <rostedt@xxxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxx>, Josh Poimboeuf <jpoimboe@xxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Dou Liyang <douly.fnst@xxxxxxxxxxxxxx>, Paul Bolle <pebolle@xxxxxxxxxx>, "Paul E . McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>, Baoquan He <bhe@xxxxxxxxxx>, Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>, Jiri Kosina <jkosina@xxxxxxx>, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>, "Rafael J . Wysocki" <rjw@xxxxxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Daniel Micay <danielmicay@xxxxxxxxx>, "linux-crypto@xxxxxxxxxxxxxxx" <linux-crypto@xxxxxxxxxxxxxxx>, Rob Landley <rob@xxxxxxxxxxx>, Tejun Heo <tj@xxxxxxxxxx>, Paolo Bonzini <pbonzini@xxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>, "David S . Miller" <davem@xxxxxxxxxxxxx>, "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
- Delivery-date: Wed, 19 Jul 2017 15:58:39 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
On Tue, Jul 18, 2017 at 8:59 PM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
> On Tue, Jul 18, 2017 at 9:35 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>> On 07/18/17 15:33, Thomas Garnier wrote:
>>> With PIE support and KASLR extended range, the modules may be further
>>> away from the kernel than before breaking mcmodel=kernel expectations.
>>>
>>> Add an option to build modules with mcmodel=large. The modules generated
>>> code will make no assumptions on placement in memory.
>>>
>>> Despite this option, modules still expect kernel functions to be within
>>> 2G and generate relative calls. To solve this issue, the PLT arm64 code
>>> was adapted for x86_64. When a relative relocation go outside its range,
>>> a dynamic PLT entry is used to correctly jump to the destination.
>>
>> Why large as opposed to medium or medium-PIC?
>
> Or for that matter, why not small-PIC? We aren't changing the size of
> the kernel to be larger than 2G text or data. Small-PIC would still
> allow it to be placed anywhere in the address space, and would
> generate far better code.
My understanding was that small=PIC and medium=PIC assume that the
module code is in the lower 2G of memory. I will do additional testing
on the modules to confirm that.
>
> --
> Brian Gerst
--
Thomas
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