Making System Calls in Rust: Requesting Services from the Kernel

Requesting Services from Kernel (System Calls) using Rust

Operating systems rely on the kernel as a crucial intermediary that manages hardware resources and facilitates communication between applications and the underlying system. In this role, the kernel acts as a bridge between hardware and user applications, ensuring that resources are allocated efficiently and that communication flows smoothly. With Rust, developers can leverage its safety and concurrency features to interact with the kernel more effectively, creating robust applications that manage system resources while minimizing the risk of common programming errors.

Moreover, applications utilize system calls as the essential interface to request services from the kernel. These system calls enable a variety of operations, including file manipulation, process control, and network communication. By providing a standardized method for applications to interact with system resources, system calls not only enhance functionality but also promote consistency and reliability across different applications.

While developers predominantly use C for kernel development, my familiarity with Rust has led me to choose it for this article. This article serves as an introduction for those interested in managing system calls using Rust.

1.0 System Call

A system call is a programmatic mechanism that enables a computer program to request services from the kernel of the operating system it runs on. These calls act as the sole entry points into the kernel and execute in kernel mode, allowing programs to interact with system resources effectively.

Furthermore, user programs utilize system calls to communicate with the operating system and request various services. In response, the operating system processes these requests by invoking the appropriate system calls to fulfill them.

Moreover, system calls play a vital role in the effective functioning of an operating system by providing a standardized interface for programs to access system resources. Without system calls, each application would have to create its own methods for interacting with hardware and system services, leading to inconsistent behavior and a higher likelihood of errors. Consequently, by centralizing this functionality, system calls ensure reliability and efficiency across all applications.

1.1 Functions Provided by System Calls

System calls provide essential services for operating systems, including process creation and management, where Rust’s std::process::Command module can be used to spawn processes. Main memory management allows allocation and freeing of memory, utilizing Rust’s Box and Rc types.

File access and system management enable operations like creating and reading files through the std::fs module, while device handling involves I/O operations with functions from the std::io module.

Protection services ensure proper access rights, and networking capabilities are available via Rust’s std::net module for TCP and UDP connections. Additionally, process control manages processes and memory, and communication services facilitate inter-process communication using channels from std::sync::mpsc. These system call services are vital for user programs to interact with system resources effectively.

1.2 Types of System Calls

The services offered by an operating system encompass a wide range of operations that user programs can perform, such as creating, terminating, forking processes, moving files, and facilitating communication. To streamline these functionalities, similar operations are organized into distinct categories of system calls.

Process Control

Unix FunctionsDescription
Fork()Creates a new process by duplicating the calling process. The new process is referred to as the child process.
Exit()Terminates the calling process and returns an exit status to the parent process.
Wait()Makes the calling process wait until one of its child processes terminates.

File Management

Unix FunctionsDescription
Open()Opens a file and returns a file descriptor for subsequent operations.
Read()Reads data from a file associated with a file descriptor into a buffer.
Write()Writes data from a buffer to a file associated with a file descriptor.
Close()Closes a file descriptor, releasing any resources associated with it.

Device Management

Unix FunctionsDescription
Ioctl()Performs device-specific input/output operations on file descriptors.
Read()Reads data from a device file, similar to reading from a regular file.
Write()Writes data to a device file, similar to writing to a regular file.

Information Retrieval

Unix FunctionsDescription
Getpid()Returns the process ID of the calling process.
Alarm()Sets a timer that sends a signal to the process after a specified number of seconds.
Sleep()Suspends the execution of the calling process for a specified number of seconds.

Inter-Process Communication

Unix FunctionsDescription
Pipe()Creates a unidirectional data channel that can be used for inter-process communication.
Shmget()Allocates a shared memory segment and returns an identifier for it.
Mmap()Maps files or devices into memory, allowing for file I/O through memory access.

Access Control

Unix FunctionsDescription
Chmod()Changes the file mode (permissions) of a specified file.
Umask()Sets the file mode creation mask, determining the default permissions for newly created files.
Chown()Changes the ownership of a file or directory to a specified user and/or group.

2.0 Getting Started with System Calls in Rust

To get started with system calls in Rust, you typically use the libc crate, which provides bindings to the C standard library and enables you to make direct system calls. This approach offers the flexibility to access kernel-level functionality without depending on the abstractions provided by the Rust standard library.

Additionally, using the libc crate allows you to interact with system calls in a more granular way, giving you control over low-level operations while still benefiting from Rust’s safety features.

2.1 Setting Up Your Environment

To create a new Rust project, start by using Cargo, Rust’s package manager and build system. You can accomplish this by running the command below in your terminal.

cargo new project_name

Change the directory to your project and edit the Cargo.toml file for dependencies. After this process, your Cargo.toml file should look like the one below.

[dependencies]
libc = "0.2"

After setting up the dependency, you can import the necessary functions from the libc crate in your main.rs file. This will enable you to use system calls directly in your Rust code, allowing for low-level interactions with the operating system.

2.2 Overview of the Standard C Library

libc refers to the standard C library in Rust, allowing us to use standard C functions in our Rust code. However, calling C functions can lead to undefined behavior if not used correctly, which is why an unsafe block is necessary when using them.

extern crate libc;

use libc::printf;
use std::ffi::CString;

fn main() {
    unsafe {
        let message = CString::new("Here it is!\n").expect("CString::new failed");
        printf(message.as_ptr());
    }
}

The provided Rust code demonstrates how to call the C printf function using the libc crate, which provides bindings to the standard C library.

CString is necessary here because native Rust strings do not include a null terminator by default. CString ensures that the string is properly null-terminated, which is essential for compatibility with C functions.

3.0 File Management in Rust via System Calls

3.1 Open and Close Files

To work with files at a low level in Rust, we need to utilize system calls. For this, we will use two important types: CString and RawFd.

RawFd is a type that represents a raw file descriptor in Unix-like operating systems. A file descriptor is a non-negative integer that uniquely identifies an open file or other I/O resource within a process.

extern crate libc;

use std::ffi::CString;
use std::os::unix::io::RawFd;

fn open_file(path: &str) -> RawFd {
    let c_path = CString::new(path).unwrap();
    unsafe { libc::open(c_path.as_ptr(), libc::O_CREAT | libc::O_RDWR, 0o644) }
}

fn close_file(f: RawFd) {
    unsafe { libc::close(f) };
}

fn main() {
    let f = open_file("file.txt");
    close_file(f);
}

This script has no error handling. We are using the unwrap function for the CString conversion, which will panic if it fails. Additional error handling can be added.

// Change c_path string from native Rust string to C string
let c_path = CString::new(path).unwrap();

To use the path string in the open function, we need to convert it from a Rust string to a C string, as we mentioned before.

unsafe { libc::open(c_path.as_ptr(), libc::O_CREAT | libc::O_RDWR, 0o644) }

libc::O_CREAT: This flag tells the operating system to create the file if it does not already exist. If the file does exist, this flag has no effect on the existing file.

libc::O_RDWR: This flag indicates that the file should be opened for both reading and writing. If the file is opened successfully, you can read from and write to it.

The 0o644 is an octal representation of the file permissions that should be set when a new file is created. The 0o prefix indicates that the number is in octal format (similar to chmod).

3.2 Writing and Reading File

In this section, we will demonstrate how to open a file, write data to it, and subsequently close the file. This process is fundamental in file management and allows us to store information persistently.

extern crate libc;

use std::ffi::CString;
use std::os::unix::io::RawFd;

fn write_file(f: RawFd, data: &[u8]) {
    let bytes_written = unsafe { libc::write(f, data.as_ptr() as *const _, data.len()) };
    assert!(bytes_written >= 0, "Failed to write to file");
}

fn main() {
    let path = "file.txt";
    let f = unsafe {
        libc::open(CString::new(path).unwrap().as_ptr(), libc::O_CREAT | libc::O_WRONLY | libc::O_TRUNC, 0o644)
    };
    assert!(f >= 0, "Failed to open file");

    write_file(f, b"System calls used for this text!");
    unsafe { libc::close(f) };
}

This script opens a file in write mode and writes data to it using system calls.

fn write_file(f: RawFd, data: &[u8]) {
    let bytes_written = unsafe { libc::write(fd, data.as_ptr() as *const _, data.len()) };
    assert!(bytes_written >= 0, "Failed to write to file");
}

Here this function takes file path and data (&[u8] is a type that represents a slice of bytes) and write this data to file.

  let f = unsafe {
        libc::open(CString::new(path).unwrap().as_ptr(), libc::O_CREAT | libc::O_WRONLY | libc::O_TRUNC, 0o644)
    };

We’ve added 2 different two flags O_WRONLY and O_TRUNC in open function. First flag indicates that the file should be opened for writing only and second one tells the operating system to truncate the file to zero length if it already exists.

extern crate libc;

use std::ffi::CString;
use std::os::unix::io::RawFd;

fn read_file(f: RawFd, buffer: &mut [u8]) -> usize {
    unsafe { libc::read(f, buffer.as_mut_ptr() as *mut _, buffer.len()) as usize }
}

fn main() {
    let path = "file.txt";
    let c_path = CString::new(path).unwrap();
    let f = unsafe { libc::open(c_path.as_ptr(), libc::O_RDONLY) };

    let mut buffer = [0u8; 50]; 
    let bytes_read = read_file(f, &mut buffer);
    let contents = String::from_utf8_lossy(&buffer[..bytes_read]);
    println!("file content: {}", contents);

    unsafe { libc::close(f) };
}

This script is designed to open a file in read mode and read data into a buffer from that file.

fn read_file(f: RawFd, buffer: &mut [u8]) -> usize {
    unsafe { libc::read(f, buffer.as_mut_ptr() as *mut _, buffer.len()) as usize }
}

In the read_file function, we added the mut keyword to buffer. In Rust, the mut keyword represents a mutable reference to a slice of bytes, which means that you can modify the data.

4.0 Process Control in Rust

Let’s switch to process control functions. These functions are used for managing the execution of processes in an operating system.

They allow you to create new processes, terminate existing ones, wait for processes to finish, and control their execution environment.

These functions are essential for multitasking and resource management in applications that require concurrent execution of tasks.

extern crate libc;

use std::ffi::CString;

fn main() {
    unsafe {
        let pid = libc::fork();
        if pid < 0 {
            eprintln!("Fork failed!");
            return;
        } else if pid == 0 {

            println!("Child process created (PID: {})", libc::getpid());
            let command = CString::new("/bin/ls").unwrap();
            let args = [
                CString::new("-l").unwrap(),
                CString::new("-a").unwrap(),
                CString::new("-h").unwrap(),
            ];
            let mut argv: Vec<*const i8> = args.iter().map(|arg| arg.as_ptr()).collect();
            argv.push(std::ptr::null()); 
            
            println!("Executed command: /bin/ls -l -a -h");
            libc::execvp(command.as_ptr(), argv.as_ptr() as *const _);
            eprintln!("Exec failed!");
            libc::_exit(1);
        } else {

            println!("Parent process (PID: {}) is waiting for child process (PID: {}) to finish.", libc::getpid(), pid);
            libc::waitpid(pid, std::ptr::null_mut(), 0);
            println!("Child process has finished.");
        }
    }
}

This Rust program demonstrates basic process control by forking a new process and executing a command in the child process.

let pid = libc::fork();

Forking is a fundamental concept in operating systems that allows a process to create a new process. When a process forks, it creates a child process that is a duplicate of the parent process.

libc::execvp(command.as_ptr(), argv.as_ptr() as *const _);

The exec function is used to replace the current process’s memory space with a new program. This means that the original program’s code, data, and stack are replaced by those of the new program.

libc::_exit(1);

The _exit function terminates the calling process immediately without performing any cleanup operations.

An exit status of 0 usually means that the process finished successfully without any issues. On the other hand, a non-zero exit status, such as 1, indicates that the process ran into a problem or didn’t complete as expected so we used 1 here.

libc::waitpid(pid, std::ptr::null_mut(), 0);

This function is used to wait for a specific child process to change state, typically to terminate. It causes the parent process to pause its execution until the child process has finished and its status has been updated.

Conclusion

In my opinion, this article provides a comprehensive introduction to managing system calls in Rust. We explored various types of system calls, including process control, file management, device management, and inter-process communication.

Looking ahead, this article will be updated to cover additional system call functions, further enhancing your understanding of Rust in system programming.

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