/proc/cmdline contains the arguments («command line») with which the kernel was started. /proc/cmdline should also be found with dmesg. The following command prints, at least in my current Linux environment, the same line twice: $ cat /proc/cmdline && dmesg | grep -o -P '(?<=Command line: ).*'
init/main.c has the following comment: Unknown boot options get handed to init, unless they look like unused parameters (modprobe will find them in /proc/cmdline).
modprobe loads a module, it parses the command line. /proc/bootconfig