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

[Josh Poimboeuf]

On Mon, Dec 12, 2016 at 12:10:25PM -0600, Josh Poimboeuf wrote:
> On Mon, Dec 12, 2016 at 06:50:23PM +0100, Borislav Petkov wrote:
> > [    0.000000] ffffffffbce03f40: 0000000000000000 ...
> > [    0.000000] ffffffffbce03f48: ffffffffbc0001b5 (start_cpu+0x5/0x14)
> > [    0.000000] ffffffffbce03f50: ffffffffbc0001b5 (start_cpu+0x5/0x14)
> > [    0.000000] ffffffffbce03f58: 0000000000000000 ...
> > [    0.000000] Linux version 4.9.0-rc8+ (root@gondor) (gcc version 6.2.0 20161109 (Debian 6.2.0-13) ) #2 SMP PREEMPT Mon Dec 12 18:36:48 CET 2016
> 
> Something funky is going on in the head code.  It looks like start_cpu
> is getting executed twice somehow.  Need to do some more head
> scratching...

I still can't figure out what could cause this, nor can I recreate it.

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.

Boris, what kind of CPU is it?

-- 
Josh