linux-kernel - Re: [PATCH 3/4] x86,mm: Improve _install_special

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Date:	Tue, 20 May 2014 10:24:49 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	X86 ML <x86@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Dave Jones <davej@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 3/4] x86,mm: Improve _install_special_mapping and fix x86
 vdso naming

On Tue, May 20, 2014 at 10:21 AM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Mon, May 19, 2014 at 03:58:33PM -0700, Andy Lutomirski wrote:
>> Using arch_vma_name to give special mappings a name is awkward.  x86
>> currently implements it by comparing the start address of the vma to
>> the expected address of the vdso.  This requires tracking the start
>> address of special mappings and is probably buggy if a special vma
>> is split or moved.
>>
>> Improve _install_special_mapping to just name the vma directly.  Use
>> it to give the x86 vvar area a name, which should make CRIU's life
>> easier.
>>
>> As a side effect, the vvar area will show up in core dumps.  This
>> could be considered weird and is fixable.  Thoughts?
>>
>> Cc: Cyrill Gorcunov <gorcunov@...nvz.org>
>> Cc: Pavel Emelyanov <xemul@...allels.com>
>> Signed-off-by: Andy Lutomirski <luto@...capital.net>
>
> Hi Andy, thanks a lot for this! I must confess I don't yet know how
> would we deal with compat tasks but this is 'must have' mark which
> allow us to detect vvar area!

Out of curiosity, how does CRIU currently handle checkpointing a
restored task?  In current kernels, the "[vdso]" name in maps goes
away after mremapping the vdso.

I suspect that you'll need kernel changes for compat tasks, since I
think that mremapping the vdso on any reasonably modern hardware in a
32-bit task will cause sigreturn to blow up.  This could be fixed by
making mremap magical, although adding a new prctl or arch_prctl to
reliably move the vdso might be a better bet.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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