Re: Debugging GRUB2 with GDB and QEMU

[devShadow <devShadow@xacks.com>]

Hi, thanks for the help with debugging modules.  I have a module that I 
created and still need a little assistance in getting gdb to break in my 
  module's grub_cmd_*.  I am able to connect to QEMU with gdb and when I 
load the module, gdb loads the symbol table for my module.

But, when I set a breakpoint at the address of the first operation in my 
module, gdb doesn't break when I execute the command in GRUB.  If it has 
something to do with the aliases you mention at the bottom of your 
message, could you elaborate a little more or direct me in the right 
direction?

Thanks

David Johnson

Lubomir Kundrak wrote:
> I wrote some GDB macros that might be helpful for people
> that want to debug GRUB 2 modules with QEMU. [1]
> 
> [1] http://NetBSD.sk/~lkundrak/misc/grub2-gdb/
> 
> The file genmk.rb.diff is a patch to makefile-generator
> script, that makes the build system leave output files with
> debugging information. Of course, one has have ruby interpreter
> to regenerate the makefiles and compile GRUB with -g
> afterwards.
> 
> Second file is .gdbinit, the GDB initialization file.  It is
> commented, so there is no need to describe it here. It basically
> passes the information extracted from list headed by grub_dl_head to
> the perl script, gmodule.pl, and loads its output. It also
> sets a breakpoint whose command list contains macro for loading
> proper symbol file each time a module is loaded. (I know the work
> could be done without the help of a Perl script, but it would
> be far more complicated, I guess)
> 
> Another note worth mentioning is that you'll probably want
> to add aliases for some functions, so that all gdb facilities
> will work correctly. You will at least want to define the symbol
> 'main' for backtraces to stop at the right place. Some facilities
> also want the function 'malloc' to be defined. So, you'll
> probably have to add something like
> 
> .globl main
> main = codestart
> 
> to assembly language sources or
> 
> malloc() __attribute__ ((alias("grub_malloc")));
> 
> to C files.
> 
> I hope this will be useful to at least some of you. Best regards!
> Lubo.
>