Learn User Space vs Kernel Space | Foundations of Device Drivers

In Linux and other modern operating systems, there is a strict separation between user space and kernel space. User space is where regular applications run, such as editors, browsers, or user programs. These applications cannot directly access hardware or critical system resources. Kernel space, on the other hand, is where the operating system kernel and its components—including device drivers—operate. The kernel has unrestricted access to hardware and manages system resources, memory, and process scheduling. This separation is enforced to protect the system from accidental or malicious interference by user applications, ensuring system stability and security.

syscall_bridge_example.c


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#include <stdio.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <errno.h>

// This function simulates a system call handler bridging user and kernel space.
// In reality, system call handlers are implemented in the kernel, but this
// example shows the flow from user space to the kernel and back.
int main() {
    long result;

    // SYS_getpid is a standard Linux system call to get the process ID.
    // This triggers a transition from user space to kernel space.
    result = syscall(SYS_getpid);

    if (result == -1) {
        perror("syscall");
        return 1;
    }

    printf("Process ID retrieved from kernel: %ld\n", result);
    return 0;
}

Device drivers operate in kernel space because they need direct access to hardware and must interact closely with the kernel’s subsystems. The system call mechanism, as illustrated above, is the primary way user space programs request services from the kernel. When you invoke a system call, the CPU switches from user mode to kernel mode, allowing the kernel to perform privileged operations safely on your behalf. Device drivers, running in kernel space, handle these requests by communicating with hardware, managing data transfer, and enforcing access controls. This design ensures that only trusted, thoroughly tested code can interact with sensitive system resources, reducing the risk of crashes or security breaches caused by faulty or malicious user applications.

user_kernel_transfer.c


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#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/syscall.h>

// A simple demonstration of passing data from user space to kernel space
// using the write system call, which is handled by a kernel driver.

int main() {
    const char *msg = "Hello from user space!\n";
    ssize_t bytes_written;

    // Write the message to standard output (file descriptor 1).
    // This invokes the kernel's write handler, transferring data from user to kernel space.
    bytes_written = syscall(SYS_write, 1, msg, strlen(msg));

    if (bytes_written == -1) {
        perror("syscall write");
        return 1;
    }

    printf("Bytes written by kernel: %zd\n", bytes_written);
    return 0;
}

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Section 1. Chapter 2

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syscall_bridge_example.c


              1234567891011121314151617181920212223
            
#include <stdio.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <errno.h>

// This function simulates a system call handler bridging user and kernel space.
// In reality, system call handlers are implemented in the kernel, but this
// example shows the flow from user space to the kernel and back.
int main() {
    long result;

    // SYS_getpid is a standard Linux system call to get the process ID.
    // This triggers a transition from user space to kernel space.
    result = syscall(SYS_getpid);

    if (result == -1) {
        perror("syscall");
        return 1;
    }

    printf("Process ID retrieved from kernel: %ld\n", result);
    return 0;
}

user_kernel_transfer.c


              123456789101112131415161718192021222324
            
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/syscall.h>

// A simple demonstration of passing data from user space to kernel space
// using the write system call, which is handled by a kernel driver.

int main() {
    const char *msg = "Hello from user space!\n";
    ssize_t bytes_written;

    // Write the message to standard output (file descriptor 1).
    // This invokes the kernel's write handler, transferring data from user to kernel space.
    bytes_written = syscall(SYS_write, 1, msg, strlen(msg));

    if (bytes_written == -1) {
        perror("syscall write");
        return 1;
    }

    printf("Bytes written by kernel: %zd\n", bytes_written);
    return 0;
}

Everything was clear?

Thanks for your feedback!

Section 1. Chapter 2