Example: Writing a Kernel in About 100 Lines of Safe Rust
To give you a sense of how Asterinas OSTD enables writing kernels in safe Rust, we will show a new kernel in about 100 lines of safe Rust.
Our new kernel will be able to run the following Hello World program.
# SPDX-License-Identifier: MPL-2.0
.global _start # entry point
.section .text # code section
_start:
mov $1, %rax # syscall number of write
mov $1, %rdi # stdout
mov $message, %rsi # address of message
mov $message_end, %rdx
sub %rsi, %rdx # calculate message len
syscall
mov $60, %rax # syscall number of exit, move it to rax
mov $0, %rdi # exit code, move it to rdi
syscall
.section .rodata # read only data section
message:
.ascii "Hello, world\n"
message_end:
The assembly program above can be compiled with the following command.
gcc -static -nostdlib hello.S -o hello
The user program above requires our kernel to support three main features:
- Loading a program as a process image in user space;
- Handling the write system call;
- Handling the exit system call.
A sample implementation of the kernel in safe Rust is given below. Comments are added to highlight how the APIs of Asterinas OSTD enable safe kernel development.
// SPDX-License-Identifier: MPL-2.0
#![no_std]
#![deny(unsafe_code)]
extern crate alloc;
use align_ext::AlignExt;
use core::str;
use alloc::sync::Arc;
use alloc::vec;
use ostd::arch::qemu::{exit_qemu, QemuExitCode};
use ostd::cpu::UserContext;
use ostd::mm::{
CachePolicy, FallibleVmRead, FallibleVmWrite, FrameAllocOptions, PageFlags, PageProperty,
Vaddr, VmIo, VmSpace, VmWriter, PAGE_SIZE,
};
use ostd::prelude::*;
use ostd::task::{Task, TaskOptions};
use ostd::user::{ReturnReason, UserMode, UserSpace};
/// The kernel's boot and initialization process is managed by OSTD.
/// After the process is done, the kernel's execution environment
/// (e.g., stack, heap, tasks) will be ready for use and the entry function
/// labeled as `#[ostd::main]` will be called.
#[ostd::main]
pub fn main() {
let program_binary = include_bytes!("../hello");
let user_space = create_user_space(program_binary);
let user_task = create_user_task(Arc::new(user_space));
user_task.run();
}
fn create_user_space(program: &[u8]) -> UserSpace {
let nbytes = program.len().align_up(PAGE_SIZE);
let user_pages = {
let segment = FrameAllocOptions::new(nbytes / PAGE_SIZE)
.alloc_contiguous()
.unwrap();
// Physical memory pages can be only accessed
// via the `Frame` or `Segment` abstraction.
segment.write_bytes(0, program).unwrap();
segment
};
let user_address_space = {
const MAP_ADDR: Vaddr = 0x0040_0000; // The map addr for statically-linked executable
// The page table of the user space can be
// created and manipulated safely through
// the `VmSpace` abstraction.
let vm_space = VmSpace::new();
let mut cursor = vm_space.cursor_mut(&(MAP_ADDR..MAP_ADDR + nbytes)).unwrap();
let map_prop = PageProperty::new(PageFlags::RWX, CachePolicy::Writeback);
for frame in user_pages {
cursor.map(frame, map_prop);
}
drop(cursor);
Arc::new(vm_space)
};
let user_cpu_state = {
const ENTRY_POINT: Vaddr = 0x0040_1000; // The entry point for statically-linked executable
// The user-space CPU states can be initialized
// to arbitrary values via the UserContext
// abstraction.
let mut user_cpu_state = UserContext::default();
user_cpu_state.set_rip(ENTRY_POINT);
user_cpu_state
};
UserSpace::new(user_address_space, user_cpu_state)
}
fn create_user_task(user_space: Arc<UserSpace>) -> Arc<Task> {
fn user_task() {
let current = Task::current().unwrap();
// Switching between user-kernel space is
// performed via the UserMode abstraction.
let mut user_mode = {
let user_space = current.user_space().unwrap();
UserMode::new(user_space)
};
loop {
// The execute method returns when system
// calls or CPU exceptions occur or some
// events specified by the kernel occur.
let return_reason = user_mode.execute(|| false);
// The CPU registers of the user space
// can be accessed and manipulated via
// the `UserContext` abstraction.
let user_context = user_mode.context_mut();
if ReturnReason::UserSyscall == return_reason {
handle_syscall(user_context, current.user_space().unwrap());
}
}
}
// Kernel tasks are managed by the Framework,
// while scheduling algorithms for them can be
// determined by the users of the Framework.
Arc::new(
TaskOptions::new(user_task)
.user_space(Some(user_space))
.data(0)
.build()
.unwrap(),
)
}
fn handle_syscall(user_context: &mut UserContext, user_space: &UserSpace) {
const SYS_WRITE: usize = 1;
const SYS_EXIT: usize = 60;
match user_context.rax() {
SYS_WRITE => {
// Access the user-space CPU registers safely.
let (_, buf_addr, buf_len) =
(user_context.rdi(), user_context.rsi(), user_context.rdx());
let buf = {
let mut buf = vec![0u8; buf_len];
// Copy data from the user space without
// unsafe pointer dereferencing.
let current_vm_space = user_space.vm_space();
let mut reader = current_vm_space.reader(buf_addr, buf_len).unwrap();
reader
.read_fallible(&mut VmWriter::from(&mut buf as &mut [u8]))
.unwrap();
buf
};
// Use the console for output safely.
println!("{}", str::from_utf8(&buf).unwrap());
// Manipulate the user-space CPU registers safely.
user_context.set_rax(buf_len);
}
SYS_EXIT => exit_qemu(QemuExitCode::Success),
_ => unimplemented!(),
}
}