Has Linux always separated User and Kernel space?

Question

I was thinking back to my introduction to programming recently and remembered writing a C++ program that deliberately read and wrote to memory addresses at random. I did this to see what would happen.

To my surprise, on my Windows 98 PC, my program would create some really weird side effects. Occasionally it would toggle OS settings, or create graphical glitches. More often than not it would do nothing or just crash the entire system.

I later learned this was because Windows 98 didn't restrict what a user process had access to. I could read and write to RAM used by other processes and even the OS.

It is my understanding that this changed with Windows NT (though I think it took a while to get right). Now Windows prevents you from poking around in RAM that doesn't belong to your process.

I vaguely remember running my program on a Linux system later on and not getting nearly as many entertaining results. If I understand correctly this is, at least in part, due to the separation of User and Kernel space.

So, my question is:

Was there a time when Linux did not separate User and Kernel space? In other words, was there a time when my rogue program could have caused similar havoc to a Linux system?

Answer 1

Linux has always protected the kernel by preventing user space from directly accessing the memory it uses; it has also always protected processes from directly accessing each others’ memory. Programs can only access memory through a virtual address space which gives access to memory mapped for them by the kernel; access outside allocated memory results in a segmentation fault. (Programs can access the kernel through system calls and drivers, including the infamous /dev/mem and /dev/kmem; they can also share memory with each other.)

Is the MMU inside of Unix/Linux kernel? or just in a hardware device with its own memory? explains how the kernel/user separation is taken care of in Linux nowadays (early releases of Linux handled this differently; see Linux Memory Management Overview and 80386 Memory Management for details).

Some Linux-related projects remove this separation; for example the Embeddable Linux Kernel Subset is a subset of Linux compatible with the 8086 CPU, and as a result it doesn’t provide hardware-enforced protection. µClinux provides support for embedded systems with no memory management unit, and its core “ingredients” are now part of the mainline kernel, but such configurations aren’t possible on “PC” architectures.

Answer 2

Was there a time when Linux did not separate User and Kernel space?

That depends on how you define the terms "Linux", "user space", and "kernel space".

Remember how Linus Torvalds originally created Linux. Linus saved some money to buy himself an (at the time state-of-the-art) PC with an Intel 80386 CPU. He wanted to understand how the 80386 works, and he thought the best way to do that would be to write some low-level hardware code in 80386 assembly. At the same time, he was also dissatisfied with the performance of the terminal emulator running under Minix he was using to log into the university.

So, he decided to write a terminal emulator in 80386 assembly which you could boot directly. For this, he needed to write a bootloader, a keyboard driver, a (character) display driver, a serial driver, and a driver for whatever protocol he used to connect to the university.

Soon, he found that he also wanted to download files from the university, so he had to implement some file transfer protocol (probably ZMODEM, but maybe he chose XMODEM for simplicity) and also a hard disk driver, partition table parser, and a filesystem driver for the Minix filesystem. Because he also wanted to continue working while the terminal emulator was performing some long-running operation (such as a file download), he implemented multithreading.

This is the point where he realized that he had already implemented some significant portions of an Operating System, and so he thought it would be another fun project to turn the terminal emulator into one.

At some point after that, he accidentally mistyped a command and overwrote his Minix partition with a backup. Now he had a choice to make: reinstall Minix or finish his Operating System and use that instead.

When he came to a point where his Operating System was capable of running simple programs, he decided to upload it to the university's FTP server, and named it Freax (he thought that naming something after himself was pretentious and arrogant). The sysadmin of the FTP server didn't like the name, though, and decided to rename the file to Linux, which he thought sounded better.

Another while later, Linus made the very first public mention of Linux in his famous message where he stated that Linux was so tied to the 80386 that it would never be portable to anything else and where he predicted that Linux would never be big and professional.

Now, the question is: at which point in this journey did "Linux" become "Linux" and at which point in this journey did Linux become a "kernel" so that talking about separation of user and kernel space even makes sense?

As I said at the beginning: it really depends on how you define those terms.

In other words, was there a time when my rogue program could have caused similar havoc to a Linux system?

There was certainly a time in its evolution where the piece of software which later became an OS and which was later called "Linux" had no protection, had complex enough services that you might get away with calling it a "kernel" and had independent enough subsystems that you might get away with calling those subsystems "programs". For example, in a "real" Unix system, the terminal emulator and the file transfer would typically be two separate programs and the thing that coordinates the two and which houses the code for accessing the serial port, the hard disk, the screen, the keyboard, and the filesystem would be the kernel.

But was this piece of software "Linux"? I will leave that to you to decide.

Answer 3

Yes, Linux has always need an MMU to work for memory protection. Many people have ported it to small embedded systems without MMU but there won't be complete memory protection anymore, so a process can read/write pretty much everything

• Is the MMU inside of Unix/Linux kernel? or just in a hardware device with its own memory?
• Is the MMU (Memory Management Unit) chip necessary for a processor to have virtual memory support?
• Memory Management