CFCFG - tools for comparing Linux kernel configurations

Why

If you work with custom Linux kernels, you may need to compare two configurations.
For example, perhaps something broke after a change, or perhaps someone with a similar system
has a problem and you want to identify the difference between the configurations.

The Linux kernel source comes with a script diffconfig that provides a stripped-down
listing of changed items in alphabetical order, which lacks context compared to the menu-based
Linux make menuconfig tools.

cfcfg provides a more user-friendly comparison, using comments, indentation and colour
to reflect the organization of configuration items into menus and sub-menus.
It can optionally add the prompt text from the menus to annotate the configuration items.
Thus, where diffconfig might produce a listing including the lines:

 LOCALVERSION "-git" -> "-hp"
 LOCKD y -> m
 LOGO_LINUX_MONO n -> y
 LOG_CPU_MAX_BUF_SHIFT 13 -> 12
 LZ4_COMPRESS m -> y
 LZ4_DECOMPRESS m -> y
 MEDIA_DIGITAL_TV_SUPPORT y -> n
 MEDIA_SUPPORT y -> m
 MOUSE_APPLETOUCH n -> y
 MOUSE_BCM5974 n -> y
 MOUSE_PS2 y -> m
 MOUSE_SYNAPTICS_I2C n -> y
 MOUSE_SYNAPTICS_USB n -> y
 MSDOS_FS m -> n
 NETFILTER_INGRESS n -> y

cfcfg would produce:

# Linux/x86 6.4.3 Kernel Configuration ---> # Linux/x86 6.4.3 Kernel Configuration
   # General setup
|      LOCALVERSION="-git"--->"-hp"      # Local version - append to kernel release
|      DEFAULT_HOSTNAME="ryzen"--->"hp"          # Default hostname
+      CROSS_MEMORY_ATTACH=y     # Enable process_vm_readv/writev syscalls
+      AUDIT=y   # Auditing support
|      LOG_CPU_MAX_BUF_SHIFT=13--->12    # CPU kernel log buffer size contribution
-      BLK_CGROUP=y      # IO controller
-      CGROUP_FREEZER=y          # Freezer controller
+      CPUSETS=y         # Cpuset controller
-      CGROUP_DEVICE=y   # Device controller
+      PROC_PID_CPUSET=y         # Include legacy /proc/<pid>/cpuset file
+      TIME_NS=y         # TIME namespace
|      INITRAMFS_SOURCE="initramfs_list"--->"initramfs.list"     # Initramfs source
-      RD_ZSTD=y         # Support initial ramdisk/ramfs compressed using ZSTD
   # Processor type and features
-      X86_X2APIC=y      # Support x2apic
|      NR_CPUS=32--->4   # Maximum number of CPUs
       # Performance monitoring
-          PERF_EVENTS_INTEL_RAPL=y      # Intel/AMD rapl performance events

How

cfcfg is a POSIX shell script that uses GNU awk to preprocess kernel configuration files,
passes the results to GNU diff to produce a comparison, and finally tidies the results
with more awk. Linux kernel make configuration tools inset comments into the files
that reflect menu hierarchy, and the awk scripts use these to construct the relevent context around
changed values.

cfgsymbols is another POSIX shell script that again uses GNU awk to collect the
symbol names and the associated prompt text from the Linux kernel Kconfig files
which define the menu system (and much more). The resulting file typically contains
around 16,000 configuration items and associated prompts. cfcfg will use this file
if available to annotate the items listed.

Using the tools

Running cfcfg in the simplest way is:

| cfcfg oldconfig newconfig

such as:

| cfcfg .config.old .config

which will produce a colourful listing at the terminal.
You can of course redirect the output to file or pager program like less.
In these cases, the file is uncoloured unless you also specify the -c option:

| cfcfg -c .config.old .config | less

Optionally, create a symbol table using cfgsymbols - for example:

| cfgsymbols kernel-6.4.symbols /usr/src/linux-6.4.3

Note that this is quite slow process, as it has to find and parse a large number of
Kconfig files.
Note also that you need to update the symbol table for new kernel releases,
as they often get new symbols, for example for new hardware. They may also drop
old symbols, in which case you might wish to merge new and old symbol tables.
As the files are sorted, you can GNU sort to do this. For example:

| sort -m -k 1,1 -t = old.symbols kernel-6.4.symbols > new.symbols

Tell cfcfg about the symbol table using the CFGSYMBOLS environment variable:

| CFGSYMBOLS=new.symbols cfcfg /boot/config-6.3.1 /boot/config-6.4.3

Documentation

See the man pages for each tool:

• man 1 cfcfg
• man 1 cfgsymbols