The moment the .config kernel configuration file is to be written to, the kbuild system checks whether it already exists, and if so, it backs it up with the name .config.old. Knowing this, we can always differentiate the two to see the changes we have wrought. However, using your typical diff(1) utility to do so makes the differences quite hard to interpret. The kernel helpfully provides a better way, a console-based script that specializes in doing precisely this. The scripts/diffconfig script (within the kernel source tree) is really useful for this. To see why, let's just run its help screen first:
$ scripts/diffconfig --help
Usage: diffconfig [-h] [-m] [<config1> <config2>]
Diffconfig is a simple utility for comparing two .config files.
Using standard diff to compare .config files often includes extraneous and
distracting information. This utility produces sorted output with...
