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Message-ID: <CAJcbSZFphkF8bcodkcwpK-oi3D7GTQXyTkOLPkmtWSPLPkvB0Q@mail.gmail.com>
Date: Wed, 19 Jul 2017 08:58:32 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: Brian Gerst <brgerst@...il.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, Herbert Xu <herbert@...dor.apana.org.au>,
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Subject: Re: [RFC 21/22] x86/module: Add support for mcmodel large and PLTs
On Tue, Jul 18, 2017 at 8:59 PM, Brian Gerst <brgerst@...il.com> wrote:
> On Tue, Jul 18, 2017 at 9:35 PM, H. Peter Anvin <hpa@...or.com> 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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