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

[Doug Evans <dje@google.com>]

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.

Hmmm, amd64-linux-nat.c is not compiled even if I do
--enable-targets=all --enable-64-bit-bfd.
When does the above error get triggered?