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

[Marc S. <el...@bu...>]

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.  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'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?

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.

============================================================

diff -u -r ../mknbi-1.2.7-original/start32.S ../mknbi-1.2.7/start32.S
--- ../mknbi-1.2.7-original/start32.S	2002-03-04 04:44:53.000000000 -0800
+++ ../mknbi-1.2.7/start32.S	2002-09-20 21:07:54.000000000 -0700
@@ -350,6 +350,7 @@
 	movl	%eax,%es
 	movl	%eax,%ss
 	addl	$RELOC,%esp		/* Fix up stack pointer */
+	popl	%esp			/* Restore 32 bit stack */
 	xorl	%eax,%eax
 	movl	%eax,%fs
 	movl	%eax,%gs
@@ -367,6 +368,10 @@
 	popl	%eax
 	subl	$RELOC,%eax		/* Adjust return address */
 	pushl	%eax
+	movl	%esp, %edx
+	lea	real_stack, %eax
+	movl	%eax, %esp
+	pushl	%edx			/* Save the 32 bit stack pointer */
 	subl	$RELOC,%esp		/* Adjust stack pointer */
 #ifdef	GAS291
 	ljmp	$REAL_CODE_SEG,$1f-RELOC	/* jump to a 16 bit segment */
@@ -437,9 +442,15 @@
 
 /* Other variables */
 #ifdef	FIRST32PM
+.global initsp
 initsp:	.long	0
 #else
+.global initsp
 initss:	.word	0
 initsp:	.word	0
 days:	.long	0
 #endif
+
+.data
+real_stack_end:	.fill	1024,1,0	/* Real-mode only stack */
+real_stack: