Setting Up a VM to Boot with a Copied Kernel

This guide explains how to copy the running kernel on a Linux VM and configure the VM to boot using the copied kernel.

Prerequisites

A Linux-based virtual machine (e.g., Ubuntu, CentOS, or Debian).
Root or sudo access on the VM.
Basic familiarity with Linux commands and bootloader configuration.

Step 1: Identify the Running Kernel

Check the currently running kernel version:
```
uname -r
```
Example output: 5.15.0-83-generic

Locate the corresponding kernel files in /boot:

ls /boot/vmlinuz-$(uname -r)
ls /boot/initrd.img-$(uname -r)
ls /boot/config-$(uname -r)
ls /boot/System.map-$(uname -r)

Step 2: Copy the Kernel Files

Create a directory to store the copied kernel files:
```
sudo mkdir -p /boot/copied_kernel
```

Copy the kernel files to the new directory:

sudo cp /boot/vmlinuz-$(uname -r) /boot/copied_kernel/vmlinuz-copied
sudo cp /boot/initrd.img-$(uname -r) /boot/copied_kernel/initrd.img-copied
sudo cp /boot/config-$(uname -r) /boot/copied_kernel/config-copied
sudo cp /boot/System.map-$(uname -r) /boot/copied_kernel/System.map-copied

Step 3: Update the Bootloader (GRUB)

Open the GRUB configuration file for editing:
```
sudo nano /etc/default/grub
```

Add a new menu entry for the copied kernel. Append the following to the file:

menuentry 'Copied Kernel' {
    set root='hd0,msdos1' # Adjust based on your disk partition
    linux /boot/copied_kernel/vmlinuz-copied root=/dev/sda1 # Adjust root partition
    initrd /boot/copied_kernel/initrd.img-copied
}

Replace hd0,msdos1 and /dev/sda1 with the appropriate disk and partition details for your system.

Save and exit the editor.

Update GRUB to apply the changes:

sudo update-grub # For Debian/Ubuntu
sudo grub2-mkconfig -o /boot/grub2/grub.cfg # For CentOS/RHEL

Step 4: Reboot and Select the Copied Kernel

Reboot the VM:
```
sudo reboot
```
During boot, access the GRUB menu (usually by pressing Shift or Esc).
Select the “Copied Kernel” entry to boot using the copied kernel.

Step 5: Verify the Booted Kernel

After booting, verify that the VM is running the copied kernel:
```
uname -r
```
The output should match the kernel version you copied.

Step 6: Test the Kernel

Run tests or workloads to ensure the copied kernel functions as expected.
If issues arise, you can reboot and select the original kernel from the GRUB menu.

Conclusion

You have successfully copied the running kernel and configured the VM to boot using the copied kernel. This setup is useful for testing and validating kernel behavior in a controlled environment.

For advanced use cases, consider using tools like kexec for faster kernel switching or creating custom kernel builds for testing.