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

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Date:	Thu, 3 Mar 2016 17:29:52 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Brian Gerst <brgerst@...il.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 Thu, Mar 03, 2016 at 07:26:06AM -0800, H. Peter Anvin wrote:
> Why -8?

        GLOBAL(stack_start)
        .quad  init_thread_union+THREAD_SIZE-8
					    ^^^

But I don't see why it needed the -8 then. It came with a conglomerate
dump in 2002:

commit af53c7a2c81399b805b6d4eff887401a5e50feef
Author: Andi Kleen <ak@....de>
Date:   Fri Apr 19 20:23:17 2002 -0700

    [PATCH] x86-64 architecture specific sync for 2.5.8


-       /* Setup the first kernel stack (this instruction is modified by smpboot) */
-       .byte 0x48, 0xb8        /* movq *init_rsp,%rax */ 
-init_rsp:
-       .quad init_thread_union+THREAD_SIZE
-       movq    %rax, %rsp

...

-
-       /* SMP bootup changes this */   
+       /* SMP bootup changes these two */      
        .globl  initial_code
 initial_code:
        .quad   x86_64_start_kernel
+       .globl init_rsp
+init_rsp:
+       .quad  init_thread_union+THREAD_SIZE-8
+
---

But since we decrement first and then copy to stack ptr when we push, I
don't see why we need the -8.

Do you have a better clue?

-- 
Regards/Gruss,
    Boris.

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