Switching into (Linux) Kernel Mode

Question

Linux n00b here. How does one switch from User Mode to Kernel Mode? I'm running Linux Ubuntu 12.10. Is there an interrupt that I can call using inline assembly code that will do this? If not, how can it be done?

I'm asking this question because I am wishing to write a SCTP (network)protocol stack which has access to the kernel and runs in the background constantly though the UI cannot directly access the kernel. Never done anything like this before so tips from pros would definitely be appreciated.

Answer 1

All switches to kernel mode are made via system calls. In the case of network protocols these system calls are socket, listen, accept, ioctl, read, write, recvmsg, etc.

You write a Linux kernel module. There is already a SCTP protocol stack for Linux though. You would likely be better off modifying it to do what you want.

Once you have written and compiled your module you can load it into the kernel using insmod and rmmod. In my experience you rarely get a chance to use rmmod because if you made a mistake the system crashes or freezes. So use a virtual machine for your testing. It is faster to reboot, you lose less data, and it is easier to hook up a virtual serial console for debugging.

I am sure this question is a duplicate by the way. You can find a lot of questions on this topic.