Can 32bit gdb debug 64bit binary? - gdb

Question

Can 32bit gdb debug 64bit binary?

Answer 1

The answer above is incorrect. You need a 64-bit debugger to debug a 64-bit process. That's precisely the reason why gdb forks a 64-bit copy of itself behind the scenes.

Answer 2

Yes. A 32 bit gdb debugs 64 bit binaries fine on Solaris, at least on the latest release.
$ cat /etc/release
Oracle Solaris 11 Express snv_151a X86
Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
Assembled 04 November 2010
$ file /usr/bin/gdb
/usr/bin/gdb: ELF 32-bit LSB executable 80386 Version 1 [FPU], dynamically linked, not stripped, no debugging information available
$ file a
a: ELF 64-bit LSB executable AMD64 Version 1, dynamically linked, not stripped
$ gdb a
GNU gdb 6.8
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i386-pc-solaris2.11"...
(gdb) b main
Breakpoint 1 at 0x400e9c: file a.c, line 3.
(gdb) run
Starting program: /tmp/a
Breakpoint 1, main () at a.c:3
3 printf("hello world !");
(gdb) quit
The program is running. Exit anyway? (y or n) y
$
However, if you look closer, this 32 gdb is launching a 64 bit gdb under the hood:
$ truss -f -t execve /usr/bin/gdb a
1793: execve("/usr/bin/gdb", 0x0804755C, 0x08047568) argc = 2
1793: execve("/usr/bin/amd64/gdb", 0x0804755C, 0x08047568) argc = 2
GNU gdb 6.8
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i386-pc-solaris2.11"...
(gdb)
It does it regardless of whether the binary debugged is 32 or 64 bit.
I still think the way a debugger interact with a process under Solaris is size independent so technically, a 32 bit only binary debugger should be able to debug a 64 bit program.
A 64 bit gdb is able to debug both 32 bit and 64 bit binaries but a 32 bit gdb cannot debug 64 bit ones. This is what you are experiencing.