Re: [Etherboot-developers] Adding real-mode stack for prot_to_real

[Eric W B. <ebi...@ln...>]

Marc Singer <el...@bu...> writes:

> On Fri, Sep 20, 2002 at 05:12:04PM -0600, Eric W Biederman wrote:
> > I have a confirmation for you.   mknbi fails to setup it's own stack
> > and then behaves badly if etherboot doesn't give it a stack.
> > 
> > I have triggered this bug in the development version by relocating
> > etherboot.
> > 
> > If you have the time to change mknbi to setup it's own stack 1K should
> > be sufficient, it would be appreciated.
> > 
> > The problem triggers in prot_to_real && real_to_prot.
> > 
> 
> If I understand it correctly, the problem is that the stack etherboot
> uses is greater than 64KiB away from the base address of mknbi.  So,
> when prot_to_real substracts $RELOC from the %esp and installs the
> real-mode %ss, the stack has moved and is probably trouncing something
> interesting.  

We also need to load the GDT, and segment registers with the values
mknbi assumes are in use.  A protected mode entry point can only
sanely assume a flat 32bit address space with a base address of 0.
I have played with the former and checked it into the developer etherboot
CVS tree.  But I had not identified the remaing stack issue.

The stack moving and trouncing something interesting is less of a problem.

> Now we could make real-mode %ss behave and leave the
> stack where it is, or we could setup a real-mode stack for the benefit
> of BIOS calls.

I would suggest making the protected mode stack behave.  We don't
need to touch it unless we return to etherboot.
 
> I've made a patch that I expect would work, but it doesn't.  This is
> against mknbi-1.2-7.  The behavior is a hard crash before I see
> anything from first32.c.  I've put the real_stack in .code32 and in
> .data with no change in behavior.  The odd thing is that if I only
> change the stack pointer to 0x00090000 before calling first() then
> I'll see the logon messages.  It must be something simple that I'm not
> noticing.  
> 
> Ideas?

I am not totally familiar with the usages in mknbi, but in etherboot
I seem to recall some of the callers passing values on the stack through
real_to_prot, and prot_to_real.  So switching stack may not work.

> BTW, I am aware that this won't work when we start in real-mode and
> first to protected mode first.  I think all we need to do is push an
> initial 32 bit stack pointer before the first call to real_to_prot and
> all should be good.

Starting in real mode actually appears to behave better, some better
in the existing cases.  A real mode stack at a semi random location is less
deadly than getting your segment registers wrong.

Eric