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Comparing Linux kernels

The main focus of DiffKemp is on Red Hat Enterprise Linux (RHEL), whose kernel includes a list of functions (KABI -- Kernel Application Binary Interface) whose semantics should remain the same across the lifetime of a single major release. In this example, we will show how to compare two versions of the kernel.

1. Downloading and preparing the kernels

Because getting and comparing the kernels requires additional dependencies we will use Nix development environment. After entering the development shell using nix develop command, we need to build DiffKemp. Then we can use rhel-kernel-get script for downloading and preparing the kernels. We will compare RHEL/CentOS 8.2 (4.18.0-193.el8) and 8.3 (4.18.0-240.el8) versions, which we can get by:

mkdir -p kernel
rhel-kernel-get --kabi 4.18.0-193.el8 -o kernel
rhel-kernel-get --kabi 4.18.0-240.el8 -o kernel

The --kabi option specifies that we want to download the list of functions whose semantics should remain the same, and the -o kernel specifies the folder to which we want to download the kernels. After the kernels are downloaded and prepared, we can create snapshots from them and compare them.

2. Creating snapshots of the kernels

For creating snapshots of the kernels we won't be using diffkemp build but diffkemp build-kernel command which is specifically made for the kernel:

mkdir -p snapshots
diffkemp build-kernel kernel/linux-4.18.0-193.el8/ snapshots/linux-4.18.0-193.el8/ kernel/linux-4.18.0-193.el8/kabi_whitelist_x86_64
diffkemp build-kernel kernel/linux-4.18.0-240.el8/ snapshots/linux-4.18.0-240.el8/ kernel/linux-4.18.0-240.el8/kabi_whitelist_x86_64

The kernel/linux-4.18.0-XXX.el8/ directories are directories with the downloaded and prepared 8.2 and 8.3 kernels, the snapshots/linux-4.18.0-XXX.el8/ are directories where the created snapshots will be saved and kernel/linux-4.18.0-XXX.el8/kabi_whitelist_x86_64 files are files that contain the list of KABI symbols.

When running the command, we are informed which symbols were already found and compiled into LLVM IR (LLVM IR is used for the comparison). For some symbols we can get source not found note, this is normal, so do not worry.

Note

If you get source not found note for all symbols, there is a problem. It could be caused by cancelling the build-kernel command prematurely which could result in the cscope cross-referencer, used by DiffKemp, not being run for all symbols. Removing the kernel/linux-4.18.0-XXX.el8/cscope.files file (replace XXX with 193 or 240 based on what is the problematic kernel) should fix it, but we will need to rerun the build-kernel command.

3. Comparing the kernels

Now we can use compare command to compare the created snapshots (the versions of the kernel). It will compare functions from the KABI list.

diffkemp compare snapshots/linux-4.18.0-193.el8 snapshots/linux-4.18.0-240.el8 -o diff-linux

The command will notify us on standard output about functions for which it was unable to determine semantic equivalency (e.g. because the compared symbol was found only in one compared version). The command creates diff-linux directory containing information about the found semantic differences. The structure of the directory is the same as was described in the library example.

4. Viewing the found differences

We can also visualise the differences with the result viewer using the following command:

diffkemp view diff-linux

The command will launch a web application that we can access in our browser. The viewer is more described in the library example

5. Summary

That's all for the example of the kernel comparison. We learned, that:

  • We can use Nix development environment, which contains the necessary dependencies for preparing and comparing the kernels.
  • The environment contains rhel-kernel-get script, which can be used to download and prepare the kernels.
  • For creating snapshots of the kernels we need to use build-kernel command.
  • We can compare the snapshots with the compare command and visualise the differences with view command.