linux-kernel - Re: [RFC PATCH] x86: Make sure verify

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Date:	Wed, 2 Mar 2016 18:38:15 +0200
From:	Mika Penttilä <mika.penttila@...tfour.com>
To:	Borislav Petkov <bp@...en8.de>
CC:	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [RFC PATCH] x86: Make sure verify_cpu has a good stack


On 02.03.2016 18:15, Borislav Petkov wrote:
> On Wed, Mar 02, 2016 at 05:55:14PM +0200, Mika Penttilä wrote:
>>> +	/* Setup a stack for verify_cpu */
>>> +	movq    stack_start - __START_KERNEL_map, %rsp
>>> +	subq	$__START_KERNEL_map, %rsp
>>> +
>> You subtract __START_KERNEL_map twice ?
> Yes. That's not very obvious and it took me a while. I probably should
> add a comment.
>
> Want to stare at it a little bit more and try to figure it out or should
> I explain?
>
> :-)
>

I actually looked at it a while too...

The
  movq stack_start - __START_KERNEL_map, %rsp

turns into (objdump disassembly)

  mov    0x0,%rsp

with relocation
0000000000000004 R_X86_64_32S      stack_start+0x0000000080000000

Now stack_start is at ffffffff81ef3380, so the relocation gives 1ef3380 which would be correct, so why the
second subq ?

You may explain :)

--Mika