From: Martin Str|mberg <ams AT ludd DOT luth DOT se>
Message-Id: <200001061735.SAA06841@father.ludd.luth.se>
Subject: Re: The endless int31 hooking debugging continued
To: eliz AT is DOT elta DOT co DOT il (Eli Zaretskii)
Date: Thu, 6 Jan 100 18:35:53 +0100 (MET)
Cc: djgpp AT delorie DOT com (DJGPP)
In-Reply-To: <Pine.SUN.3.91.1000106191121.23317A@is> from Eli Zaretskii at "Jan 6, 0 07:13:42 pm"
X-Mailer: ELM [version 2.4ME+ PL15 (25)]
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Reply-To: djgpp AT delorie DOT com
Errors-To: dj-admin AT delorie DOT com
X-Mailing-List: djgpp AT delorie DOT com
X-Unsubscribes-To: listserv AT delorie DOT com
Precedence: bulk

According to Eli Zaretskii:
> 
> On Thu, 6 Jan 100, Martin Str|mberg wrote:
> 
> > > Or, better still, don't call any library functions from within the hook; 
> > > then you don't need to worry about SS and ESP.
> > 
> > Yes, but I need call cputs to get some debug information out.
> 
> It should be easy to throw together a function that copies the string 
> into a static buffer (together with any attribute byte, to make its tand 
> out), then blits it all to video RAM.  If you write this in assembly, you 
> can forget about the stack.

Well, that's far over my head. I just want to be able to hook INT31
and be able to get out some debug info (so I can see where the real
code goes wrong). So if you have any pointers to such a nice debug
routine I'll very much want to see them (and no, dbgcom.c doesn't help
as it doesn't bother to set up the ss=es=ds thing).

Alternatively perhaps I could use the
__dpmi_simulate_real_mode_... routines and write eveything in C, but
then there's the problem how to make it chain or just return depending
on the outcome in the C routine (whether a reusable selector was found
or not).


Here's the latest incarnation. This just hang after printing "esp =
0x9afb8".


Right,

							MartinS

---- Start of simple8.c. -----
#include <dpmi.h>
#include <go32.h>
#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#include <crt0.h>

int _crt0_startup_flags = _CRT0_FLAG_LOCK_MEMORY;

__dpmi_paddr old_addr, new_addr;

void handler (void);
void handler_end (void);

char chain_str[] = "Chaining.\n\r";

unsigned short my_ds, org_ss;
unsigned int org_eax, org_esp;
unsigned int my_esp;

__asm__ (".globl _handler
_handler:

/* Save  registers. */
        pushl %eax
        pushl %ds
        pushl %es

/* Restore segments to valid DJGPP state. */
        pushl %eax
        movw %cs:_my_ds, %ax
        movw %ax, %ds
        popl %eax
        movl %eax, _org_eax
        movw %ss, %ax
        movw %ax, _org_ss
        movw _my_ds, %ax
        movw %ax, %es
        movw %ax, %ss
        movl %esp, _org_esp
        movl _my_esp, %esp
        movl _org_eax, %eax

/* Jump to previous handler. */
chain:
        pusha
        pushf
        pushl $_chain_str
        call _cputs
        addl $4, %esp
        popf
        popa

        movl _org_esp, %esp
        movw _org_ss, %ax
        movw %ax, %ss
        popl %es
        popl %ds
        popl %eax
        ljmp %cs:_old_addr

        .globl _handler_end
_handler_end:
        nop");

int
main (void)
{
  int selector;
  __dpmi_paddr tmp_addr;

  my_ds = _my_ds();

  printf("cs = %hx, ds = %hx, ss= %hx.\n", _my_cs(), _my_ds(), _my_ss());

  if (__dpmi_get_protected_mode_interrupt_vector (0x31, &old_addr))
    {
      fprintf (stderr, "can not get interrupt\n");
      exit (EXIT_FAILURE);
    }

  printf ("got interrupt vector\n");

  asm("
        movl %esp, _my_esp

        ");
  printf("esp = 0x%x.\n", my_esp);
  my_esp -= 0x100;

  new_addr.selector = _my_cs ();
  new_addr.offset32 = (unsigned long) handler;
  if (__dpmi_set_protected_mode_interrupt_vector (0x31, &new_addr))
    {
      fprintf (stderr, "can not set interrupt\n");
      exit (EXIT_FAILURE);
    }

  printf ("set interrupt vector\n");

  selector = __dpmi_allocate_ldt_descriptors (1);
  if (selector == -1)
    fprintf (stderr, "can not allocate selector\n");
  else
    __dpmi_free_ldt_descriptor (selector);

  printf ("allocated LDT descriptor?\n");

  while (__dpmi_get_protected_mode_interrupt_vector (0x31, &tmp_addr)
         || (tmp_addr.selector != new_addr.selector)
         || (tmp_addr.offset32 != new_addr.offset32)
         || __dpmi_set_protected_mode_interrupt_vector (0x31, &old_addr))
    {
      fprintf (stderr, "can not restore interrupt\n");
      system ("");
    }

  printf ("restored interrupt vector\n");

  return EXIT_SUCCESS;
}

/*
   Local Variables:
   compile-command: "gcc -g -O2 -Wall -o simple8 simple8.c"
   End:
   */
---- End of simple8.c. -----