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Date: Thu, 3 Mar 2016 12:22:06 -0800 From: "H. Peter Anvin" <hpa@...or.com> To: Borislav Petkov <bp@...en8.de> 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 03/03/16 08:29, Borislav Petkov wrote: > 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? > The only thing I can think of is that the -8 creates a null pointer that terminates a stack trace. -hpa
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