What is the difference between the kernel space and the user space

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

User · Answer

Trying to give a very simplified explanation

Virtual Memory is divided into kernel space and the user space. Kernel space is that area of virtual memory where kernel processes will run and user space is that area of virtual memory where user processes will be running.

This division is required for memory access protections.

Whenever a bootloader starts a kernel after loading it to a location in RAM, (on an ARM based controller typically)it needs to make sure that the controller is in supervisor mode with FIQ's and IRQ's disabled.

User · Answer

Each process has its own 4GB of virtual memory which maps to the physical memory through page tables  The virtual memory is mostly split in two parts  3 GB for the use of the process and 1 GB for the use of the Kernel  Most of the variables you create lie in the first part of the address space  That part is called user space  The last part is where the kernel resides and is common for all the processes   This is called Kernel space and most of this space is mapped to the starting locations of physical memory where the kernel image is loaded at boot time

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Kernel space  amp  virtual space are concepts of virtual memory    it doesn t mean Ram your actual memory  is divided into kernel  amp  User space  Each process is given virtual memory which is divided into kernel  amp  user space   So saying  The random access memory  RAM  can be divided into two distinct regions namely - the kernel space and the user space   is wrong    amp  regarding  kernel space vs user space  thing  When a  process is created and its virtual memory is divided into user-space and a kernel-space   where user space region contains data  code  stack  heap of the process  amp  kernel-space contains things such as the page table for the process  kernel data structures  and kernel code etc   To run kernel space code  control must shift to kernel mode using 0x80 software interrupt for system calls   amp  kernel stack is basically shared among all processes currently executing in kernel space

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Kernel Space and User Space are logical spaces   Most of the modern processors are designed to run in different privileged mode  x86 machines can run in 4 different privileged modes    And a particular machine instruction can be executed when in above particular privileged mode   Because of this design you are giving a system protection or sand-boxing the execution environment   Kernel is a piece of code  which manages your hardware and provide system abstraction  So it needs to have access for all the machine instruction  And it is most trusted piece of software  So i should be executed with the highest privilege  And Ring level 0 is the most privileged mode  So Ring Level 0 is also called as Kernel Mode   User Application are piece of software which comes from any third party vendor  and you can t completely trust them  Someone with malicious intent can write a code to crash your system if he had complete access to all the machine instruction  So application should be provided with access to limited set of instructions  And Ring Level 3 is the least privileged mode  So all your application run in that mode  Hence that Ring Level 3 is also called User Mode   Note  I am not getting Ring Levels 1 and 2  They are basically modes with intermediate privilege  So may be device driver code are executed with this privilege  AFAIK  linux uses only Ring Level 0 and 3 for kernel code execution and user application respectively   So any operation happening in kernel mode can be considered as kernel space  And any operation happening in user mode can be considered as user space

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Kernel space and user space is the separation of the privileged operating system functions and the restricted user applications  The separation is necessary to prevent user applications from ransacking your computer  It would be a bad thing if any old user program could start writing random data to your hard drive or read memory from another user program s memory space   User space programs cannot access system resources directly so access is handled on the program s behalf by the operating system kernel  The user space programs typically make such requests of the operating system through system calls   Kernel threads  processes  stack do not mean the same thing  They are analogous constructs for kernel space as their counterparts in user space

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The Random Access Memory  RAM  can be logically divided into two distinct regions namely - the kernel space and the user space  The Physical Addresses of the RAM are not actually divided only the Virtual Addresses  all this implemented by the MMU   The kernel runs in the part of memory entitled to it  This part of memory cannot be accessed directly by the processes of the normal users  while as the kernel can access all parts of the memory  To access some part of the kernel  the user processes have to use the predefined system calls i e  open  read  write etc  Also  the C library functions like printf call the system call write in turn    The system calls act as an interface between the user processes and the kernel processes  The access rights are placed on the kernel space in order to stop the users from messing up with the kernel  unknowingly   So  when a system call occurs  a software interrupt is sent to the kernel  The CPU may hand over the control temporarily to the associated interrupt handler routine  The kernel process which was halted by the interrupt resumes after the interrupt handler routine finishes its job

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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

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In Linux there are two space 1st is user space and another one is kernal space  user space consist of only user application which u want to run  as the kernal service there is process management  file management  signal handling  memory management  thread management  and so many services are present there  if u run the application from the user space that appliction interact with only kernal service  and that service is interact with device driver which is present between hardware and kernal         the main benefit of kernal space and user space seperation is  we can acchive a security by the virus bcaz of all user application present in user space  and service is present in kernal space  thats why linux doesn t affect from the virus

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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

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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

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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

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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

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The maximum size of address space depends on the length of the address register on the CPU   On systems with 32-bit address registers  the maximum size of address space is 232 bytes  or 4 nbsp GiB  Similarly  on 64-bit systems  264 bytes can be addressed   Such address space is called virtual memory or virtual address space  It is not actually related to physical RAM size   On Linux platforms  virtual address space is divided into kernel space and user space   An architecture-specific constant called task size limit  or TASK SIZE  marks the position where the split occurs    the address range from 0 up to TASK SIZE-1 is allotted to user space  the remainder from TASK SIZE up to 232-1  or 264-1  is allotted to kernel space    On a particular 32-bit system for example  3 nbsp GiB could be occupied for user space and 1 nbsp GiB for kernel space   Each application program in a Unix-like operating system is a process  each of those has a unique identifier called Process Identifier  or simply Process ID  i e  PID   Linux provides two mechanisms for creating a process  1  the fork   system call  or 2  the exec   call   A kernel thread is a lightweight process and also a program under execution  A single process may consist of several threads sharing the same data and resources but taking different paths through the program code  Linux provides a clone   system call to generate threads   Example uses of kernel threads are  data synchronization of RAM  helping the scheduler to distribute processes among CPUs  etc

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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

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CPU rings are the most clear distinction  In x86 protected mode  the CPU is always in one of 4 rings  The Linux kernel only uses 0 and 3    0 for kernel 3 for users   This is the most hard and fast definition of kernel vs userland   Why Linux does not use rings 1 and 2  CPU Privilege Rings  Why rings 1 and 2 aren  39 t used   How is the current ring determined   The current ring is selected by a combination of    global descriptor table  a in-memory table of GDT entries  and each entry has a field Privl which encodes the ring   The LGDT instruction sets the address to the current descriptor table   See also  http   wiki osdev org Global Descriptor Table the segment registers CS  DS  etc   which point to the index of an entry in the GDT   For example  CS   0 means the first entry of the GDT is currently active for the executing code      What can each ring do   The CPU chip is physically built so that    ring 0 can do anything ring 3 cannot run several instructions and write to several registers  most notably    cannot change its own ring  Otherwise  it could set itself to ring 0 and rings would be useless   In other words  cannot modify the current segment descriptor  which determines the current ring  cannot modify the page tables  How does x86 paging work   In other words  cannot modify the CR3 register  and paging itself prevents modification of the page tables   This prevents one process from seeing the memory of other processes for security   ease of programming reasons  cannot register interrupt handlers  Those are configured by writing to memory locations  which is also prevented by paging   Handlers run in ring 0  and would break the security model   In other words  cannot use the LGDT and LIDT instructions  cannot do IO instructions like in and out  and thus have arbitrary hardware accesses   Otherwise  for example  file permissions would be useless if any program could directly read from disk   More precisely thanks to Michael Petch  it is actually possible for the OS to allow IO instructions on ring 3  this is actually controlled by the Task state segment   What is not possible is for ring 3 to give itself permission to do so if it didn t have it in the first place   Linux always disallows it  See also  Why doesn  39 t Linux use the hardware context switch via the TSS     How do programs and operating systems transition between rings    when the CPU is turned on  it starts running the initial program in ring 0  well kind of  but it is a good approximation   You can think this initial program as being the kernel  but it is normally a bootloader that then calls the kernel still in ring 0    when a userland process wants the kernel to do something for it like write to a file  it uses an instruction that generates an interrupt such as int 0x80 or syscall to signal the kernel  x86-64 Linux syscall hello world example    data hello world       ascii  hello world n      hello world len     - hello world  text  global  start  start         write        mov  1   rax     mov  1   rdi     mov  hello world   rsi     mov  hello world len   rdx     syscall         exit        mov  60   rax     mov  0   rdi     syscall   compile and run   as -o hello world o hello world S ld -o hello world out hello world o   hello world out   GitHub upstream   When this happens  the CPU calls an interrupt callback handler which the kernel registered at boot time  Here is a concrete baremetal example that registers a handler and uses it   This handler runs in ring 0  which decides if the kernel will allow this action  do the action  and restart the userland program in ring 3  x86 64  when the exec system call is used  or when the kernel will start  init   the kernel prepares the registers and memory of the new userland process  then it jumps to the entry point and switches the CPU to ring 3 If the program tries to do something naughty like write to a forbidden register or memory address  because of paging   the CPU also calls some kernel callback handler in ring 0   But since the userland was naughty  the kernel might kill the process this time  or give it a warning with a signal  When the kernel boots  it setups a hardware clock with some fixed frequency  which generates interrupts periodically   This hardware clock generates interrupts that run ring 0  and allow it to schedule which userland processes to wake up   This way  scheduling can happen even if the processes are not making any system calls    What is the point of having multiple rings   There are two major advantages of separating kernel and userland    it is easier to make programs as you are more certain one won t interfere with the other  E g   one userland process does not have to worry about overwriting the memory of another program because of paging  nor about putting hardware in an invalid state for another process  it is more secure  E g  file permissions and memory separation could prevent a hacking app from reading your bank data  This supposes  of course  that you trust the kernel    How to play around with it   I ve created a bare metal setup that should be a good way to manipulate rings directly  https   github com cirosantilli x86-bare-metal-examples  I didn t have the patience to make a userland example unfortunately  but I did go as far as paging setup  so userland should be feasible  I d love to see a pull request   Alternatively  Linux kernel modules run in ring 0  so you can use them to try out privileged operations  e g  read the control registers  How to access the control registers cr0 cr2 cr3 from a program  Getting segmentation fault  Here is a convenient QEMU   Buildroot setup to try it out without killing your host   The downside of kernel modules is that other kthreads are running and could interfere with your experiments  But in theory you can take over all interrupt handlers with your kernel module and own the system  that would be an interesting project actually   Negative rings  While negative rings are not actually referenced in the Intel manual  there are actually CPU modes which have further capabilities than ring 0 itself  and so are a good fit for the  negative ring  name   One example is the hypervisor mode used in virtualization   For further details see    https   security stackexchange com questions 129098 what-is-protection-ring-1 https   security stackexchange com questions 216527 ring-3-exploits-and-existence-of-other-rings   ARM  In ARM  the rings are called Exception Levels instead  but the main ideas remain the same   There exist 4 exception levels in ARMv8  commonly used as    EL0  userland EL1  kernel   supervisor  in ARM terminology    Entered with the svc instruction  SuperVisor Call   previously known as swi before unified assembly  which is the instruction used to make Linux system calls  Hello world ARMv8 example   hello S   text  global  start  start         write        mov x0  1     ldr x1   msg     ldr x2   len     mov x8  64     svc 0         exit        mov x0  0     mov x8  93     svc 0 msg       ascii  hello syscall v8 n  len     - msg   GitHub upstream   Test it out with QEMU on Ubuntu 16 04   sudo apt-get install qemu-user gcc-arm-linux-gnueabihf arm-linux-gnueabihf-as -o hello o hello S arm-linux-gnueabihf-ld -o hello hello o qemu-arm hello   Here is a concrete baremetal example that registers an SVC handler and does an SVC call  EL2  hypervisors  for example Xen   Entered with the hvc instruction  HyperVisor Call    A hypervisor is to an OS  what an OS is to userland   For example  Xen allows you to run multiple OSes such as Linux or Windows on the same system at the same time  and it isolates the OSes from one another for security and ease of debug  just like Linux does for userland programs   Hypervisors are a key part of today s cloud infrastructure  they allow multiple servers to run on a single hardware  keeping hardware usage always close to 100  and saving a lot of money   AWS for example used Xen until 2017 when its move to KVM made the news  EL3  yet another level  TODO example   Entered with the smc instruction  Secure Mode Call    The ARMv8 Architecture Reference Model DDI 0487C a - Chapter D1 - The AArch64 System Level Programmer s Model - Figure D1-1 illustrates this beautifully     The ARM situation changed a bit with the advent of ARMv8 1 Virtualization Host Extensions  VHE   This extension allows the kernel to run in EL2 efficiently     VHE was created because in-Linux-kernel virtualization solutions such as KVM have gained ground over Xen  see e g  AWS  move to KVM mentioned above   because most clients only need Linux VMs  and as you can imagine  being all in a single project  KVM is simpler and potentially more efficient than Xen  So now the host Linux kernel acts as the hypervisor in those cases   Note how ARM  maybe due to the benefit of hindsight  has a better naming convention for the privilege levels than x86  without the need for negative levels  0 being the lower and 3 highest  Higher levels tend to be created more often than lower ones   The current EL can be queried with the MRS instruction  what is the current execution mode exception level  etc   ARM does not require all exception levels to be present to allow for implementations that don t need the feature to save chip area  ARMv8  Exception levels  says      An implementation might not include all of the Exception levels  All implementations must include EL0 and EL1    EL2 and EL3 are optional    QEMU for example defaults to EL1  but EL2 and EL3 can be enabled with command line options  qemu-system-aarch64 entering el1 when emulating a53 power up  Code snippets tested on Ubuntu 18 10

User · Answer

The correct answer is  There is no such thing as kernel space and user space  The processor instruction set has special permissions to set destructive things like the root of the page table map  or access hardware device memory  etc   Kernel code has the highest level privileges  and user code the lowest  This prevents user code from crashing the system  modifying other programs  etc   Generally kernel code is kept under a different memory map than user code  just as user spaces are kept in different memory maps than each other   This is where the  kernel space  and  user space  terms come from  But that is not a hard and fast rule  For example  since the x86 indirectly requires its interrupt trap handlers to be mapped at all times  part  or some OSes all  of the kernel must be mapped into user space  Again  this does not mean that such code has user privileges   Why is the kernel user divide necessary  Some designers disagree that it is  in fact  necessary  Microkernel architecture is based on the idea that the highest privileged sections of code should be as small as possible  with all significant operations done in user privileged code  You would need to study why this might be a good idea  it is not a simple concept  and is famous for both having advantages and drawbacks

[linux-kernel] What is the difference between the kernel space and the user space?

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