[syslinux] Debugging

[Ferenc Wagner]

Hi!

It's indeed better to open a new thread for this.  So, here's my test
session:

$ qemu -fda extlinux.144 -s -S

Then in another terminal:

$ gdb extlinux.elf
(gdb) set architecture i8086 
(gdb) target remote localhost:1234
Remote debugging using localhost:1234
0x0000fff0 in ?? () at localboot.inc:68
68			jmp kaboom			; If we returned, oh boy...

How does "jmp kaboom" get here?  We are at the BIOS entry point:

(gdb) display/10i $cs*16+$eip
1: x/10i $cs * 16 + $eip
0xffff0:	ljmp   $0xf000,$0xe05b
[...]
(gdb) info symbol 0x7c00
bootsec in section .text
(gdb) b bootsec
Note: breakpoint -2 also set at pc 0x7c00.
Breakpoint 1 at 0x7c00: file extlinux.asm, line 146.

Breakpoint -2?  That's new for me.  Anyway:

(gdb) c
Continuing.
^C
Program received signal SIGINT, Interrupt.
0x000005a7 in ?? ()
1: x/10i $cs * 16 + $eip
0xf05a7:	push   %bp
0xf05a8:	mov    %sp,%bp
0xf05aa:	push   %bx
0xf05ab:	push   %ds
0xf05ac:	mov    4(%bp),%ax
0xf05af:	movl   %ax,%ds
0xf05b1:	mov    6(%bp),%bx
0xf05b4:	mov    (%bx),%ax
0xf05b6:	pop    %ds
0xf05b7:	pop    %bx
(gdb) bt
#0  0x000005a7 in ?? ()
#1  0x00404dc6 in ?? ()
#2  0xfedf001c in ?? ()
#3  0x001ef000 in ?? ()
#4  0x003e001e in ?? ()
#5  0x7b66001e in ?? ()
#6  0x7b654c5c in ?? ()
#7  0x00007b64 in __stack_start ()
#8  0x11a8f000 in ?? ()
#9  0x00000000 in ?? ()

Now that doesn't buy me much...

(gdb) c
Continuing.
^C
Program received signal SIGINT, Interrupt.
pollchar () at conio.inc:289
289			mov dx,[SerialPort]
1: x/10i $cs * 16 + $eip
0x97e2 <pollchar+8>:	mov    11242,%dx
0x97e6 <pollchar+12>:	and    %dx,%dx
0x97e8 <pollchar+14>:	je     0x9801 <pollchar.done>
0x97ea <pollchar+16>:	add    $0x5,%dx
0x97ed <pollchar+19>:	in     (%dx),%al
0x97ee <pollchar+20>:	test   $0x1,%al
0x97f0 <pollchar+22>:	je     0x9801 <pollchar.done>
0x97f2 <pollchar+24>:	inc    %dx
0x97f3 <pollchar+25>:	mov    14758,%ah
0x97f7 <pollchar+29>:	in     (%dx),%al
(gdb) bt
#0  pollchar () at conio.inc:289

That doesn't either.  Do I ask too much from my good ol' GDB when I
need a stack trace?  Is there a way I can hint it perhaps?

Otherwise, I would be interested to read how you debug com32 modules.
-- 
Thanks,
Feri.