What is the difference between the kernel space an

Question

What is the difference between the kernel space and the user space? Do kernel space, kernel threads, kernel processes and kernel stack mean the same thing? Also, why do we need this differentiation?

Answer 1

Memory get's divided into two distinct areas:

• The user space, which is a set of locations where normal user processes run (i.e everything other than the kernel). The role of the kernel is to manage applications running in this space from messing with each other, and the machine.
• The kernel space, which is the location where the code of the kernel is stored, and executes under.

Processes running under the user space have access only to a limited part of memory, whereas the kernel has access to all of the memory. Processes running in user space also don't have access to the kernel space. User space processes can only access a small part of the kernel via an interface exposed by the kernel - the system calls.If a process performs a system call, a software interrupt is sent to the kernel, which then dispatches the appropriate interrupt handler and continues its work after the handler has finished.

Answer 2

The really simplified answer is that the kernel runs in kernel space, and normal programs run in user space. User space is basically a form of sand-boxing -- it restricts user programs so they can't mess with memory (and other resources) owned by other programs or by the OS kernel. This limits (but usually doesn't entirely eliminate) their ability to do bad things like crashing the machine.

The kernel is the core of the operating system. It normally has full access to all memory and machine hardware (and everything else on the machine). To keep the machine as stable as possible, you normally want only the most trusted, well-tested code to run in kernel mode/kernel space.

The stack is just another part of memory, so naturally it's segregated right along with the rest of memory.

Answer 3

The kernel space means a memory space can only be touched by kernel. On 32bit linux it is 1G(from 0xC0000000 to 0xffffffff as virtual memory address).Every process created by kernel is also a kernel thread, So for one process, there are two stacks: one stack in user space for this process and another in kernel space for kernel thread.

the kernel stack occupied 2 pages(8k in 32bit linux), include a task_struct(about 1k) and the real stack(about 7k). The latter is used to store some auto variables or function call params or function address in kernel functions. Here is the code(Processor.h (linux\include\asm-i386)):

#define THREAD_SIZE (2*PAGE_SIZE)
#define alloc_task_struct() ((struct task_struct *) __get_free_pages(GFP_KERNEL,1))
#define free_task_struct(p) free_pages((unsigned long) (p), 1)

__get_free_pages(GFP_KERNEL,1)) means alloc memory as 2^1=2 pages.

But the process stack is another thing, its address is just bellow 0xC0000000(32bit linux), the size of it can be quite bigger, used for the user space function calls.

So here is a question come for system call, it is running in kernel space but was called by process in user space, how does it work? Will linux put its params and function address in kernel stack or process stack? Linux's solution: all system call are triggered by software interruption INT 0x80. Defined in entry.S (linux\arch\i386\kernel), here is some lines for example:

ENTRY(sys_call_table)
.long SYMBOL_NAME(sys_ni_syscall)   /* 0  -  old "setup()" system call*/
.long SYMBOL_NAME(sys_exit)
.long SYMBOL_NAME(sys_fork)
.long SYMBOL_NAME(sys_read)
.long SYMBOL_NAME(sys_write)
.long SYMBOL_NAME(sys_open)     /* 5 */
.long SYMBOL_NAME(sys_close)

Answer 4

Briefly : Kernel runs in Kernel Space, the kernel space has full access to all memory and resources, you can say the memory divide into two parts, part for kernel , and part for user own process, (user space) runs normal programs, user space cannot access directly to kernel space so it request from kernel to use resources. by syscall (predefined system call in glibc)

there is a statement that simplify the different "User Space is Just a test load for the Kernel " ...

To be very clear : processor architecture allow CPU to operate in two mode, Kernel Mode and User Mode, the Hardware instruction allow switching from one mode to the other.

memory can be marked as being part of user space or kernel space.

When CPU running in User Mode, the CPU can access only memory that is being in user space, while cpu attempts to access memory in Kernel space the result is a "hardware exception", when CPU running in Kernel mode, the CPU can access directly to both kernel space and user space ...

Answer 5

By Sunil Yadav, on Quora:

The Linux Kernel refers to everything that runs in Kernel mode and is made up of several distinct layers. At the lowest layer, the Kernel interacts with the hardware via the HAL. At the middle level, the UNIX Kernel is divided into 4 distinct areas. The first of the four areas handles character devices, raw and cooked TTY and terminal handling. The second area handles network device drivers, routing protocols and sockets. The third area handles disk device drivers, page and buffer caches, file system, virtual memory, file naming and mapping. The fourth and last area handles process dispatching, scheduling, creation and termination as well as signal handling. Above all this we have the top layer of the Kernel which includes system calls, interrupts and traps. This level serves as the interface to each of the lower level functions. A programmer uses the various system calls and interrupts to interact with the features of the operating system.

Answer 6

IN short kernel space is the portion of memory where linux kernel runs (top 1 GB virtual space in case of linux) and user space is the portion of memory where user application runs( bottom 3 GB of virtual memory in case of Linux. If you wanna know more the see the link given below :)

http://learnlinuxconcepts.blogspot.in/2014/02/kernel-space-and-user-space.html