Date: Mon, 12 Dec 2016 16:11:47 -0600
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Borislav Petkov <bp@alien8.de>
Cc: x86-ml <x86@kernel.org>, lkml <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>
Subject: Re: WARNING: kernel stack frame pointer at ffffffff82e03f40 in
 swapper:0 has bad value           (null)
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On Mon, Dec 12, 2016 at 10:34:46PM +0100, Borislav Petkov wrote:
> On Mon, Dec 12, 2016 at 03:16:27PM -0600, Josh Poimboeuf wrote:
> > I still can't figure out what could cause this, nor can I recreate it.
> 
> Want my .config?

Yes, please.

> > Andy, any idea?  I'm trying to figure out why a stack trace of the
> > initial task, early in start_kernel(), would show start_cpu() on the
> > stack *twice*.  The start_cpu() entry on the stack at ffffffffbce03f50
> > is right where it's supposed to be.  But then there's another
> > start_cpu() entry at 0xffffffffbce03f48 which is pointed to by the frame
> > pointer chain.  I can't figure out where that one came from and why the
> > stack is offset by a word, compared to all the other idle task stacks
> > I've seen.
> 
> Btw, why do you have:
> 
>         call    1f              # put return address on stack for unwinder
> 
> there in start_cpu() instead of
> 
> 	push $start_cpu
> 
> or so? That CALL looks strange there. If you want to put the return
> address, just push start_cpu's address and that's it.
> 
> Or am I missing something?

Yeah, it's kind of obtuse.

The problem with "push $start_cpu" is that it will show up on the stack
trace as:

  secondary_startup_64+0x90/0x90

instead of what you would expect:

  start_cpu+0x0/0x14

That's because the printk '%pB' modifier is smart enough to know that
the beginning of a function isn't a valid function call return address.
The only way such an address could end up on the stack would be if the
previous function made a tail call.  So it shows the end of the previous
function instead.

That said, the code could probably be made a little clearer by changing
"call 1f" to "push $1f" and then move the '1' label to after the lretq
instruction, like:

	pushq	$1f		# put return address on stack for unwinder
	xorq	%rbp, %rbp	# clear frame pointer
	movq	initial_code(%rip), %rax
	pushq	$__KERNEL_CS	# set correct cs
	pushq	%rax		# target address in negative space
	lretq
1:
ENDPROC(start_cpu)

That shows:

  start_cpu+0x14/0x14

Which is more accurate anyway.  I'll make a patch.

-- 
Josh