linux-kernel - Re: [PATCH] x86/vdso: Add prctl to set per-process VDSO load

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Message-ID: <CALCETrV7thL-0mST4O6YZgDNm1LyLqB=_bQ3vwwrAPYZCB=W+A@mail.gmail.com>
Date:	Fri, 19 Sep 2014 15:31:40 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Filipe Brandenburger <filbranden@...gle.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Greg Thelen <gthelen@...gle.com>,
	X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Michael Davidson <md@...gle.com>,
	Ingo Molnar <mingo@...hat.com>,
	Richard Larocque <rlarocque@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] x86/vdso: Add prctl to set per-process VDSO load

On Fri, Sep 19, 2014 at 3:19 PM, Filipe Brandenburger
<filbranden@...gle.com> wrote:
> Hi,
>
> On Fri, Sep 19, 2014 at 3:09 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Fri, Sep 19, 2014 at 3:02 PM, Filipe Brandenburger <filbranden@...gle.com> wrote:
>>> In case that's useful, I was looking at swapping the vvar page by
>>> changing the vm_special_mapping to change the pages array between the
>>> actual vvar page and the zero page and using zap_page_range to force
>>> the next access to go through a page fault that would remap it.
>>
>> That will do it globally, since the vm_special_mapping is global.  I
>> was just thinking of using remap_pfn_range.
>
> Sorry if I wasn't clear... That's the exact point of my patch, to make
> vm_special_mapping local to the task.
>
> I started with an approach of keeping a struct vm_special_mapping + a
> struct page * array per mm_struct.
>
> I was also looking at keeping two static vm_special_mapping structs,
> one with the actual vvar page and the other with the zero page and
> then swapping vma->vm_private_data to point to the appropriate one.

This sounds like it may be more complicated than necessary.  Is there
any reason that just doing remap_pfn_range on the vvar page isn't
enough?

Changing out the text is a whole can of worms involving self-modifying
code, although it may be completely safe if done through the page
tables.  But I don't think you can't use remap_pfn_range for that.

>
>> Give me another day or two to straighten out the vma stuff, although
>> it shouldn't impact your patches too much.  The main effect will be
>> that you'll be able to rely on mm->context.vdso (renamed to
>> vvar_vma_start, most likey) being correct.  Currently, you should
>> *not* rely on it, especially if CRIU is involved.
>
> Cool! As I said, let me know if you'd like to see some of my
> incomplete patches or if you'd like me to keep working on them to show
> you something more complete.

Maybe better: what exactly are you trying to do?

--Andy
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