GDB for m68k in Qemu Problems

Question

I'm trying to use GDB to debug Qemu running on m68k architecture. I am attempting to make some changes to Qemu source to add some functionality to m68k support, and I am trying to use GDB to test changes I have currently made. Important to note is that I am using Qemu v5.2.0-rc1, as that is the version of Qemu currently utilized in a larger system that I am concurrently working with. Currently, I am using qemu-system-m68k to run buildroot (uClinux kernel) on that architecture, using the "-s -S" options for gdb. In a separate terminal, I am trying to run GDB on that remote target using (gdb) target remote localhost:1234, but I am getting the following error message:

warning: Architecture rejected target-supplied description

The connection to the uClinux Qemu kernel seems to go okay, but I cannot connect to the remote host.

I have tried finding any possible solutions to this problem, including setting the GDB architecture (currently says i386, not sure if this is right), which does not work. I have no idea how to get GDB running on this m68k Qemu emulated Linux kernel, and any help would be great. Thanks!

Answer 1

The error message means that QEMU's gdb stub sent gdb a description of what the CPU is (mostly a list of registers), and gdb didn't understand it. One common cause of this is trying to debug with a gdb for the wrong architecture.

You need to use a gdb which is aware of the the target architecture. On Ubuntu there is a 'gdb-multiarch' package which has a gdb-multiarch binary that knows about more than just x86. Otherwise you might need to build gdb from source. You can check if your gdb knows about m68k by running it with no arguments and typing the command set arch m68k. If it replies The target architecture is assumed to be m68k then it's OK (and your issue is something more complicated); if it gives you an error (eg Undefined item: "m68k") then you need to use a different gdb.