Fixing a non booting kernel based on custom kernel config file

Recently I was trying to migrate my kernel config across Virtual Box, VMware and Qemu based VM’s and noticed that in some of the cases, I was not able to boot into the kernel at all. I did enable vmware, virtual box and qemu specific options in the kernel but still the kernel was not booting up (It was booting up on my machine but not on a friend’s machine). Looking deep into the problem I noticed that the versions of vmware and qemu were different than that of my machine.

Now how to solve this issue?

Well, I cannot adjust the config for every version of vmware, qemu or virtual box, so we need to find a way so that the person using this config can fix it by themselves. Thats where localmodconfig comes in

So make has a target called localmodconfig that enables all the modules that are currently being used by the kernel. It will also deselect the modules which are currently not being used by kernel which in turn allows us to create a minimal kernel config. Not only that, localmodconfig can also use the .config file present in the kernel build directory. This allows us to have a base kernel with necessary options already enabled.

So lets begin fixing our config

Step 1 : Cleaning the Kernel

We will start with a fresh build of the kernel so reboot into the default kernel and switch to the kernel build directory and run make mrproper. Invoking this target will ensure that all the build artifacts generated during the build will be removed (including the config file).

Step 2 : Copying the config file

Copy the config file into the kernel build directory and rename it to .config. After that run make nconfig or make menuconfig and press escape (and save the config if a prompt appears). Doing this will update the config file for the kernel version being used.

Step 3 : Running localmodconfig

Run make localmodconfig to enable modules presently being used by the kernel. If there are no warnings or errors proceed with the kernel compilation (You can run menuconfig or nconfig to see if important options are enabled).

But if there are some warnings saying that the config option is disabled or not present go to Step 4

Step 4 : Setting additional config options

Looking at the messages given by local modconfig, we can see that the last word of these messages contains the config option (starting with CONFIG) that needs to be set in the .config file. So just open the file using your favourite text editor and add/edit the config options mentioned before and set it to y (or m).Run make menuconfig or make nconfig and save the config file.

If there are too many options to be configured, you can use awk and sed to add the config options automatically

• First save the output of localmodconfig in a text file
  1	make localmodconfig > add_opts.txt 2>&1

• Now run the awk command to get the last column representing the config option and save it to a new file
  1	awk '{print $(NF)}' add_opts.txt > add_opts_cleaned.txt

• Now to put =y in front of these configs we use the sed command
  1	sed -i s/$/=y/ add_opts_cleaned.txt

• Finally open the file, take a look at if everything is correct and copy paste the entire content of the file into .config file
• Now run make menuconfig or make nconfig. These targets will automatically set the additional config options required. Now press ESC key on your keyboard and save the configuration.

Step 5 : Continuing the Compilation

Continue the kernel compilation by running make -j$(nproc)

Conclusion

Using this method will help you to boot into the kernel. If you want to get a complete list of the kernel modules required (for running everything as per your requirements, you can have a look at modprobed-db

Additional References

Introduction to Awk : Learning Awk Is Essential For Linux Users

Introduction to Sed : Intro to Sed