Re: [Qemu-devel] Trouble with GDB & Some 'Can it be done' Debugging questions

[Mulyadi Santosa <a_mulyadi@softhome.net>]

Hi Steve...

> Hi -
>
> I'm having a bit of trouble getting gdb to do what I was hoping it
> would with qemu. Following the instructions in the docs:
>
> #1) I launch qemu with -S -s flags ( since I want to trace the
> bootloader code )
> It says: Waiting gdb connection on port 1234 - which is correct, and
> it opens the monitor window.
>
> #2) I open a second terminal window and type gdb vmlinux
>[cut]...
> "i386-redhat-linux-gnu"...vmlinux: No such file or directory.

This message obviously said: either you don't actually have "vmlinux" 
file or you don't give correct path to the vmlinux file.  Can you 
confirm that you had given correct path? Also, it is possible that its 
name isn't vmlinux (since one is free to rename it)...

> #3) Anytime I try to dump the instruction at the current IP such as:
> (gdb) x /10i $eip
>
> I get this - which means it's not actually reading or displaying the
> memory properly, since those look to be what you would see if it was
> all 0 in memory (or maybe it's all 0xff - whichever).l

are you sure you had executed this command in gdb?:
target remote localhost:1234

Seems like gdb is dumping a wrong address space...

> This leads to my next question:
>
> #4) Can you use gdb to debug and set breakpoints on binary code you
> don't have any source code or other file for the binary, except the
> binary file itself? Everything I've read so far on GDB (and
> especially any GDB Gui front end) seems to suggest it's not possible.
> That would really suck.

Well, you can, but of course you can't set the breakpoint at certain 
source code's line, but instead put the breakpoint explicitly as memory 
address.

Anyway, i really suggest to read more about gdb by typing:
info gdb
in your shell prompt. It will display the complete gdb manual.

Don't be hesitate to ask (we're all still learning after all)...

regards,

Mulyadi