Re: stack_used() not accurate?

[Jim Keniston <jkenisto at us dot ibm dot com>]

On Mon, 2008-06-02 at 17:23 -0400, Mike Snitzer wrote:
> On Mon, Jun 2, 2008 at 4:29 PM, Frank Ch. Eigler <fche@redhat.com> wrote:
...
> >>
> >> So, given that the calculated "free" is incorrect, this says to me
> >> that the incorrect value is getting stored in CONTEXT->regs->esp on
> >> x86 (or the value is stale)?
> >
> > Why do you think the latter is incorrect?  The asm "esp"-related one
> > should give you the stack pointer at the moment of the probe handler
> > execution.  The CONTEXT->regs->esp value should give the stack pointer
> > at the moment of the probe hit - displaced in time (earlier) and
> > potentially in space (different stack).  This is the dimension of
> > imagination.  It is an area which we call the twilight zone.
> 
> I understand that CONTEXT->regs->esp has the properties you described;
> completely explains why it is so important to use CONTEXT->regs.  But
> the problem, on x86, is that if I use CONTEXT->regs->esp I get a
> negative values for "free" or <100 Bytes free.  I know that not to be
> the case...
> 
> Mike

Sorry, I haven't been following this thread for a while, so maybe this
has already been mentioned.  But keep in mind that on i386, when your
breakpoint trap happens in kernel code, esp and ss aren't saved on the
stack.  So regs->esp and regs->ss contain the top of the pre-trap stack,
and the pre-trap stack pointer is &regs->esp, not regs->esp.

Jim