[syslinux] Debugging

[Sebastian Herbszt]

> $ qemu -fda extlinux.144 -s -S

qemu.exe -L . -hda extlinux.img -s -S

Which qemu version are you using?
 
> Then in another terminal:
> 
> $ gdb extlinux.elf

Which gdb version?

> (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:

I don't get that. With qemu snapshot from 2006-06-23 and
gdb 6.8-debian i have:

$ gdb extlinux.elf
(gdb) target remote ip:1234
Remote debugging using ip:1234
0x0000fff0 in ?? ()
(gdb) set architecture i8086
The target architecture is assumed to be i8086

No kaboom.

> (gdb) display/10i $cs*16+$eip
> 1: x/10i $cs * 16 + $eip
> 0xffff0: ljmp   $0xf000,$0xe05b

(gdb) x/1i $cs*16+$eip
0xffff0:        ljmp   $0xf000,$0xe05b

> (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) break start
Breakpoint 1 at 0x7c5a: file extlinux.asm, line 199.

> (gdb) c
> Continuing.
> ^C
> Program received signal SIGINT, Interrupt.
> 0x000005a7 in ?? ()
> 1: x/10i $cs * 16 + $eip

(gdb) c
Continuing.

Breakpoint 1, start () at extlinux.asm:199
199                     cli                     ; No interrupts yet, please

(gdb) list
194     ;
195     ; Note we don't check the constraints above now; we did that at install
196     ; time (we hope!)
197     ;
198     start:
199                     cli                     ; No interrupts yet, please
200                     cld                     ; Copy upwards
201     ;
202     ; Set up the stack
203     ;


> 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?

(gdb) break getonesec
Breakpoint 2 at 0x7cf6: file extlinux.asm, line 330.

(gdb) c
Continuing.

Breakpoint 2, getonesec () at extlinux.asm:330
330                     mov bp,1                ; One sector

(gdb) bt
#0  getonesec () at extlinux.asm:330

> Otherwise, I would be interested to read how you debug com32 modules.

Same as above, but set the breakpoint on main and run the module, e.g.:

$ gdb pcitest.elf

(gdb) target remote ip:1234
Remote debugging using ip:1234
0x0000fff0 in ?? ()
(gdb) break main
Breakpoint 1 at 0x10112e: file pcitest.c, line 100.
(gdb) c
Continuing.

Run pcitest.c32.

Breakpoint 1, main (argc=1, argv=0x183dd4) at pcitest.c:100
100     {
(gdb) list
95               pci_bus_list->count,
96               pci_bus_list->count == 1 ? "" : "es");
97      }
98
99      int main(int argc, char *argv[])
100     {
101       struct pci_device_list pci_device_list;
102       struct pci_bus_list pci_bus_list;
103       openconsole(&dev_null_r, &dev_stdcon_w);
104

(gdb) break openconsole
Breakpoint 2 at 0x101cd0: file sys/openconsole.c, line 39.
(gdb) c
Continuing.

Breakpoint 2, openconsole (idev=0x1035d0, odev=0x1035b8) at sys/openconsole.c:39
39      {
(gdb) bt
#0  openconsole (idev=0x1035d0, odev=0x1035b8) at sys/openconsole.c:39
#1  0x00101150 in main (argc=<value optimized out>, argv=<value optimized out>) at pcitest.c:103
(gdb)

- Sebastian