linux-kernel - Re: WARNING: kernel stack frame pointer at ffffffff82e03f40 in swapper:0 has bad value (null)

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Date:   Tue, 13 Dec 2016 08:34:55 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86-ml <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: WARNING: kernel stack frame pointer at ffffffff82e03f40 in
 swapper:0 has bad value           (null)

On Mon, Dec 12, 2016 at 05:05:11PM -0600, Josh Poimboeuf wrote:
> On Mon, Dec 12, 2016 at 11:33:54PM +0100, Borislav Petkov wrote:
> > On Mon, Dec 12, 2016 at 04:11:47PM -0600, Josh Poimboeuf wrote:
> > > Yes, please.
> > 
> > Attached.
> 
> Thanks, I was able to recreate.  Will take a look tomorrow.

Figured it out.  Your config has CONFIG_PARAVIRT=n, which convinces gcc
to create the following preamble for x86_64_start_kernel():

  0000000000000124 <x86_64_start_kernel>:
   124:	4c 8d 54 24 08       	lea    0x8(%rsp),%r10
   129:	48 83 e4 f0          	and    $0xfffffffffffffff0,%rsp
   12d:	41 ff 72 f8          	pushq  -0x8(%r10)
   131:	55                   	push   %rbp
   132:	48 89 e5             	mov    %rsp,%rbp

It's an unusual pattern which aligns rsp (though in this case it's
already aligned) and saves the start_cpu() return address again on the
stack before storing the frame pointer.

The unwinder assumes the last stack frame header is at a certain offset,
but the above code breaks that assumption.  I still need to think about
the best way to fix it.

-- 
Josh