about kernel
 

Hai LQ folks
My question is that why kernel is mounted as read-only during booting and after booting / partion is has rw permissions in fstab?

While a filesystem is mounted read-only, you can still run tools like 'fsck' on them without worrying too much about accidental damage.

OP is asking about the lines "mounting root partition as read-only" during the initialization of the kernel, and then "re-mounting root partition as rw" and during poweroff, "re-mounting root partition as read-only"

This is to prevent, at the operating system level, from destroying critical system data. There is no need for the kernel itself to modify any data on disk, so that precaution is taken to make absolutely certain that any malicious code that may have gotten into the kernel cannot mess with it or any other data on disk at that time. When you configure your boot-loader, you also have the option to allow the root drive containing the boot image to be read-only or read-write, and the universally-recommended choice is read-only.