From: Brian Gerst <brgerst@gmail.com>
Subject: Re: [RFC 21/22] x86/module: Add support for mcmodel large and PLTs
Date: Wed, 19 Jul 2017 13:34:46 -0400
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References: <20170718223333.110371-1-thgarnie@google.com> <20170718223333.110371-22-thgarnie@google.com>
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Cc: "H. Peter Anvin" <hpa@zytor.com>, Herbert Xu <herbert@gondor.apana.org.au>,
	"David S . Miller" <davem@davemloft.net>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>, Josh Poimboeuf <jpoimboe@redhat.com>,
	Arnd Bergmann <arnd@arndb.de>, Matthias Kaehlcke <mka@chromium.org>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>, Juergen Gross <jgross@suse.com>,
	Paolo Bonzini <pbonzini@redhat.com>, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
	Joerg Roedel <joro@8bytes.org>, Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>, Borislav Petkov <bp@suse.de>,
	Christian Borntraeger <borntraeger@de.ibm.com>, "Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Len Brown <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>, Tejun Heo <tj@kernel.org>,
	Christ
To: Thomas Garnier <thgarnie@google.com>
In-Reply-To: <CAJcbSZFphkF8bcodkcwpK-oi3D7GTQXyTkOLPkmtWSPLPkvB0Q@mail.gmail.com>

On Wed, Jul 19, 2017 at 11:58 AM, Thomas Garnier <thgarnie@google.com> wrote:
> On Tue, Jul 18, 2017 at 8:59 PM, Brian Gerst <brgerst@gmail.com> wrote:
>> On Tue, Jul 18, 2017 at 9:35 PM, H. Peter Anvin <hpa@zytor.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.

That is only for small/medium absolute (non-PIC) code.  Think about
userspace shared libraries.  They are not limited to being mapped in
the lower 2G of the address space.

--
Brian Gerst