Date: Wed, 2 Mar 2016 17:55:16 +0100
From: Borislav Petkov <bp@alien8.de>
To: Mika =?utf-8?B?UGVudHRpbMOk?= <mika.penttila@nextfour.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: [RFC PATCH] x86: Make sure verify_cpu has a good stack
Message-ID: <20160302165516.GC8362@pd.tnic>
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On Wed, Mar 02, 2016 at 06:38:15PM +0200, Mika Penttilä wrote:
> 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 :)

Here it is :-)

$ readelf -a vmlinux | grep stack_start
 70526: ffffffff81cbabf8     0 NOTYPE  GLOBAL DEFAULT   14 stack_start

0xffffffff81cbabf8 - __START_KERNEL_map =
0xffffffff81cbabf8 - 0xffffffff80000000 =
0x1cbabf8

(gdb) x/x 0x1cbabf8
0x1cbabf8:      0xffffffff81c03ff8

(You don't need gdb for that - you can hexdump or objdump vmlinux).

Now stack_start is:

        GLOBAL(stack_start)
        .quad  init_thread_union+THREAD_SIZE-8

which is

$ readelf -a vmlinux | grep init_thread_union
 82491: ffffffff81c00000 16384 OBJECT  GLOBAL DEFAULT   14 init_thread_union

so init_thread_union+THREAD_SIZE-8 = 0xffffffff81c00000 + 4*4096-8 = 0xffffffff81c03ff8

So you have to subtract __START_KERNEL_map again because it has there a
virtual address and we haven't enabled paging yet:

0xffffffff81c03ff8 - 0xffffffff80000000 = 0x1c03ff8.

Makes sense?

-- 
Regards/Gruss,
    Boris.

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