What is the first program that is usually started, at boot time, by the Linux kernel when using SysV init?

/lib/init.so is a shared library, not an executable program, and therefore cannot be directly started as the initial system process.

In a System V init environment, the kernel's final action during boot is to load and execute the `/sbin/init` program. This program is responsible for bringing the system to a defined runlevel by executing scripts in `/etc/rc.d/` or similar directories, and managing all other processes.

/etc/rc.d/rcinit (or similar rc scripts) are typically executed by `init` at various runlevels, not started directly by the kernel itself.

/proc/sys/kernel/init is a kernel parameter file used for configuration, not an executable program that the kernel starts.

/boot/init is not a standard path for the init executable; `/sbin/init` is the conventional and expected location for the System V init process.