Debugging QEMU with GDB: A Guide for Customizing and Debugging GRUB EFI Images

When it comes to customizing and debugging GRUB EFI images, having a reliable way to step through code and identify issues is crucial. In this article, we'll explore how to use QEMU's GDB server to debug GRUB EFI images while they're running in a virtual environment.

Setting Up the Environment

To begin, you'll need to have a vanilla GRUB image available for debugging. If you don't already have one, you can create a new one using your preferred method. For this example, we'll assume that you're using the Kernel Hacking Debian image as the basis for our VM.

Next, ensure that you have QEMU installed on your system. You can check this by running `qemu --version` in your terminal. If QEMU isn't already installed, you can download it from the official website and follow the installation instructions.

Loading the GRUB EFI Image

Once you've set up your environment, load the GRUB EFI image into QEMU using the following command:

`make qemu-nox-gdb`

This will launch QEMU in debug mode, allowing us to attach our GDB server. Make sure that you're running this command from the top-level directory of your XV6 installation.

Setting Breakpoints and Debugging

Now that we have QEMU running with the GRUB EFI image loaded, it's time to set some breakpoints and start debugging. Open up a new terminal window and run the following command:

`gdb -iex 'set auto-load safe-path scripts/gdb/' -ex 'target remote 127.0.0.1:23001' -ex c`

This will launch GDB in debug mode, connecting it to the QEMU instance running on port 23000. Once connected, you can set breakpoints using the `break` command:

`(gdb) break batadv_iv_send_outstanding_bat_ogm_packet`

This will create a breakpoint at the specified location in the batman-adv module.

Reloading Symbol Information

If you need to reload symbol information for your modules while debugging, use the following command:

`(gdb) lx-symbols /home/sven/tmp/qemu-batman/batman-adv/net/batman-adv/`

This will refresh your symbol table with the latest information from your module.

Customizing and Debugging GRUB EFI Images

With your breakpoints set and your GDB session established, you can now start customizing and debugging your GRUB EFI image. Use the `continue` command to step through code and identify any issues:

`(gdb) continue`

As you work through your customizations, remember that you can always reload symbol information or switch between different modules using the commands outlined above.

In this article, we've covered how to use QEMU's GDB server to debug GRUB EFI images while they're running in a virtual environment. By following these steps and using the `lx-*` helpers provided by QEMU, you'll be able to set breakpoints, reload symbol information, and customize your GRUB EFI image with confidence.

References

* QEMU documentation: 'https://www.qemu.org/docs/master/system-gdb.html'(https://www.qemu.org/docs/master/system-gdb.html)

* GDB documentation: 'https://sourceware.org/gdb/current/'(https://sourceware.org/gdb/current/)

Qemu gdb