How do you communicate between userspace and kernel space?

There are couple of well known methods to communicate from user space to kernel space.

Virtual file system like /proc , /sys , /configfs , /debugfs Standard.
system call like read() , write() , open() , close() , fork()
ioctl for char drivers.
netlink socket – mostly used by network subsystem.

What is the difference between userspace and kernel space?

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.

Is Linux kernel a process?

The kernel doesn’t really behave like a process at all. It doesn’t get scheduled, it either runs on behalf of a process (so called process-context or user-context), or runs as a result of an interrupt or exception (so called interrupt-context).

How do I access user space from kernel?

Whilst a user-space program is not allowed to access kernel memory, it is possible for the kernel to access user memory. However, the kernel must never execute user-space memory and it must also never access user-space memory without explicit expectation to do so.

What is user mode and kernel mode in OS?

The User mode is normal mode where the process has limited access. While the Kernel mode is the privileged mode where the process has unrestricted access to system resources like hardware, memory, etc. System Calls are the only way through which a process can go into kernel mode from user mode.

What are Netlink sockets?

The Netlink socket family is a Linux kernel interface used for inter-process communication (IPC) between both the kernel and userspace processes, and between different userspace processes, in a way similar to the Unix domain sockets. Originally, Netlink used the AF_NETLINK socket family.

Are drivers in kernel space?

User space drivers run in user space. Kernel drivers run in kernel space. What are the limitations of both of them? The kernel driver can do anything the kernel can, so you could say it has no limitations.

Is kernel thread a process?

A kernel thread is a kernel entity, like processes and interrupt handlers; it is the entity handled by the system scheduler. A kernel thread runs within a process, but can be referenced by any other thread in the system.

How does copy to user work?

A copy to or from user space is executed by the kernel code that is executing on behalf of the process and actually it’s the memory mapping (i.e. page tables) of that process that are in-use during the copy. This takes place while execution is in kernel mode – i.e. privileged/supervisor mode in x86 language.

Why do we use copy_to_user in kernel programs?

The copy_to_user function copies a block of data from the kernel into user space. This function accepts a pointer to a user space buffer, a pointer to a kernel buffer, and a length defined in bytes. The function returns zero on success or non-zero to indicate the number of bytes that weren’t transferred.

Can a user mapping be used in kernel address space?

You cannot use any mapping inside your process address space to help you during the exploitation process. You could say that the combined user and kernel address space approach is best: to be efficient, the separated approach needs some help from the underlying architecture, as happens with the context registers on UltraSPARC machines.

How big is the address space in Linux?

For example, on a 32-bit x86 machine running Linux, the kernel resides in the 0xc00000000–0xffffffff range (the “top” gigabyte of virtual memory), whereas each process is free to use all the addresses beneath this range (the “lower” 3GB of virtual memory).

How to replicate the kernel page table entries?

This is achieved by replicating the kernel page table entries over every process’s page tables.

How do you communicate between userspace and kernel space?