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What are the differences between a monolithic kernel and a microkernel with respect to structure and security. My friend told me that Linux systems have monolithic kernel and thus are not easier to hack but I don't think he was actually correct.

Someone please enlighten me.

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For a bit of historical background, you can read the The Tanenbaum-Torvalds Debate from 1992. – Dennis Williamson Aug 14 '10 at 18:24
up vote 6 down vote accepted

The basic difference is that a micro kernel (MK) is very small and only provides a very minimal set of services. The majority of what are normally considered operating system services are provided by separate processes that execute outside of the kernel, and in a less privileged mode. These processes need to comunicate via some from of IPC, instead of just reading/writing to whatever data structure they want to access.

MKs tend to be fairly easy to port as the kernel is small, the porting effort is low.

Since a lot of the services provided are potentially running in userspace instead of kernel space, they can not easily trash other processes. This is a security plus.

E.g say the part of the networking stack was in userspace, and was compromised it might not be able to disrupt other processes/tasks. Whereas if that same part of the network stack in a monolithic kernel might be running in kernel space where if compromised it would be able to trash other processes, since it would be privileged.

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To illustrate Jason's talk, here is an illustration to see the difference between micro- hybrid- and monolithic-kernel :


Legible image source file on Wikipedia.

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