Re: Debugging 64-bit programs using 32-bit GDB

[Pedro Alves <palves at redhat dot com>]

On 07/24/2013 05:09 PM, Doug Evans wrote:
> On Wed, Jul 24, 2013 at 7:33 AM, Pedro Alves <palves@redhat.com> wrote:
>> On 07/09/2013 11:06 AM, Nikita Karetnikov wrote:
>>> Is it possible to debug a 64-bit executable ('bin/guile' [1]) using
>>> 32-bit GDB?  (I use GNU/Linux.)
>>
>> I don't think so.  The ptrace registers interfaces allow retrieving
>> the right register sets (at least the modern interfaces), but
>> things like breakpoints, etc. won't work, as the 32-bit GDB won't
>> be able to handle the inferior's 64-bit addresses (due to how ptrace
>> works).  Last I checked, Power had extra ptrace interfaces for that.
>> Don't know about MIPS.
>>
>> (you can however connect a 32-bit GDB to a 64-bit GDBserver, and use
>> that to debug a 64-bit program, though that's more trouble
>> than just using a 64-bit GDB.).
>>
>>>
>>> I've tried to configure GDB 7.6 with '--enable-64-bit-bfd' (see [2]),
>>> but it still shows "mips-tdep.c:709: internal-error: bad register
>>> size".
>>
>> The x86_64 port catches the case early and bails out (amd64_linux_read_description):
>>
>>   if (sizeof (void *) == 4 && is_64bit && !is_x32)
>>     error (_("Can't debug 64-bit process with 32-bit GDB"));
> 
> [filed for reference sake]
> 
> Alas basic 32-bit linux builds of gdb don't compile amd64-linux-nat.c.
> It would be really nice to print a better error message than the
> following for 32-bit gdb debugging 64-bit executable:
> 
> "/tmp/hello.x64": not in executable format: File format not recognized
> 
> It's a not uncommon failure mode.

Agree, but not sure how much clearer can it get.  Maybe:

 "/tmp/hello.x64": not in recognized executable file format.

?

I'd just suggest building 32-bit GDB with 64-bit support.  Even if
it can't debug 64-bit programs natively, it can remote debug
them, and debug 64-bit cores, in addition to getting rid of that
failure mode.

> Hmmm, amd64-linux-nat.c is not compiled even if I do
> --enable-targets=all --enable-64-bit-bfd.

Oh, you're right, of course.  Guess I was thinking of gdbserver,
where linux-x86.c is shared between 32-bit and 64-bit, and it has
the similar handling:

  if (sizeof (void *) == 4)
    {
      if (is_elf64 > 0)
       error (_("Can't debug 64-bit process with 32-bit GDBserver"));
#ifndef __x86_64__
      else if (machine == EM_X86_64)
       error (_("Can't debug x86-64 process with 32-bit GDBserver"));
#endif
    }

Two weeks vacation can swap out a lot of context.  :-)

> When does the above error get triggered?

Looks like it doesn't.

-- 
Pedro Alves