User Space
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In short
User space is the restricted area where ordinary programs run, kept separate from kernel space so a buggy or malicious app cannot take down the whole system.
What is user space in an operating system?
Modern operating systems split a computer into two worlds. Kernel space is where the kernel runs, with full access to memory and hardware. User space is where everything else runs: applications, shells, libraries, daemons, and even most of the graphical interface.
The split is enforced by the CPU itself. Processors have privilege levels, called rings on x86 and exception levels on ARM, and the kernel runs at the most privileged level while user programs run at the least. Memory pages are marked as kernel-only or user-accessible, so user code cannot read kernel memory, and privileged instructions, such as talking directly to devices, fail if user code tries them. To cross the boundary, a program makes a system call: the CPU switches into kernel mode, runs the requested kernel code, and returns to user mode with the result.
User space is like the public lobby of a bank, while kernel space is the vault. Customers move freely in the lobby but must ask a teller for anything in the vault. Thanks to this separation, a crashing app takes down only its own process, and security bugs in one program are much harder to turn into control of the whole machine. Some systems deliberately move work into user space for safety or speed, such as user-space file systems and drivers, while technologies like eBPF let user programs load small, verified programs into the kernel safely.
User space is often confused with user accounts. Even programs run by the root user or an administrator run in user space: they get more permission checks passed by the kernel, but they still have to ask the kernel through system calls. It also helps to separate the kernel from kernel space: the kernel is a program, while kernel space is the protected memory region and CPU mode in which it runs.
Key takeaways
- User space is where applications run with restricted privileges.
- Kernel space is reserved for the kernel, which has full hardware access.
- The CPU enforces the boundary with privilege levels and memory protection.
- Programs cross from user space into the kernel through system calls.
- Programs run by root still run in user space.
Example
# Count the system calls (user-to-kernel crossings) a command makes
strace -c ls > /dev/null
# Compare time spent in user space ("user") and in the kernel ("sys")
time find /usr -name "*.conf" > /dev/null 2>&1Readers ask
What is the difference between user space and kernel space?
Kernel space is the privileged area where the kernel runs and can access all memory and hardware. User space is the restricted area where applications run, and it must use system calls to ask the kernel for anything privileged.
Does running as root mean running in kernel space?
No. Root is a user account with extra permissions that the kernel honors, but root's programs still run in user space. Root can, however, load kernel modules, which do run in kernel space.
Why do programs run in user space?
Running applications with restricted privileges protects the system. A bug or attack in one program cannot directly overwrite the kernel or another process's memory.
See also
- KernelOperating Systems, p. 17A kernel is the core part of an operating system that manages the CPU, memory, and hardware devices and controls how programs get access to those resources.
- System CallOperating Systems, p. 31A system call is a request from a program to the operating system kernel to perform a privileged action, such as reading a file or starting a process.
- ProcessOperating Systems, p. 23A process is a running instance of a program, with its own memory space, resources, and at least one thread of execution managed by the operating system.
- Virtual MemoryOperating Systems, p. 37Virtual memory is an operating system technique that gives each process its own private address space and maps it to physical RAM or disk behind the scenes.
- Device DriverOperating Systems, p. 9A device driver is software that lets the operating system control a specific piece of hardware, translating generic OS requests into device-specific commands.
- File PermissionsOperating Systems, p. 11File permissions are rules stored with each file that decide which users may read it, change it, or run it, enforced by the operating system on every access.
- Kernel ModeOperating Systems, p. 18Kernel mode is the privileged CPU state in which the OS kernel has full access to hardware and memory, while ordinary programs run in restricted user mode.
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