Writing an OS in Rust (Second Edition)

Philipp Oppermann's blog

Testing

This post explores unit and integration testing in no_std executables. We will use Rust's support for custom test frameworks to execute test functions inside our kernel. To report the results out of QEMU, we will use different features of QEMU and the bootimage tool.

This blog is openly developed on GitHub. If you have any problems or questions, please open an issue there. You can also leave comments at the bottom. The complete source code for this post can be found in the post-04 branch.

Table of Contents

🔗 Requirements

This post replaces the (now deprecated) Unit Testing and Integration Tests posts. It assumes that you have followed the A Minimal Rust Kernel post after 2019-04-27. Mainly, it requires that you have a .cargo/config file that sets a default target and defines a runner executable.

🔗 Testing in Rust

Rust has a built-in test framework that is capable of running unit tests without the need to set anything up. Just create a function that checks some results through assertions and add the #[test] attribute to the function header. Then cargo test will automatically find and execute all test functions of your crate.

Unfortunately it's a bit more complicated for no_std applications such as our kernel. The problem is that Rust's test framework implicitly uses the built-in test library, which depends on the standard library. This means that we can't use the default test framework for our #[no_std] kernel.

We can see this when we try to run cargo xtest in our project:

> cargo xtest
   Compiling blog_os v0.1.0 (/…/blog_os)
error[E0463]: can't find crate for `test`

Since the test crate depends on the standard library, it is not available for our bare metal target. While porting the test crate to a #[no_std] context is possible, it is highly unstable and requires some hacks such as redefining the panic macro.

🔗 Custom Test Frameworks

Fortunately, Rust supports replacing the default test framework through the unstable custom_test_frameworks feature. This feature requires no external libraries and thus also works in #[no_std] environments. It works by collecting all functions annotated with a #[test_case] attribute and then invoking a user-specified runner function with the list of tests as argument. Thus it gives the implementation maximal control over the test process.

The disadvantage compared to the default test framework is that many advanced features such as should_panic tests are not available. Instead, it is up to the implementation to provide such features itself if needed. This is ideal for us since we have a very special execution environment where the default implementations of such advanced features probably wouldn't work anyway. For example, the #[should_panic] attribute relies on stack unwinding to catch the panics, which we disabled for our kernel.

To implement a custom test framework for our kernel, we add the following to our main.rs:

// in src/main.rs

#![feature(custom_test_frameworks)]
#![test_runner(crate::test_runner)]

#[cfg(test)]
fn test_runner(tests: &[&dyn Fn()]) {
    println!("Running {} tests", tests.len());
    for test in tests {
        test();
    }
}

Our runner just prints a short debug message and then calls each test function in the list. The argument type &[&dyn Fn()] is a slice of trait object references of the Fn() trait. It is basically a list of references to types that can be called like a function. Since the function is useless for non-test runs, we use the #[cfg(test)] attribute to include it only for tests.

When we run cargo xtest now, we see that it now succeeds. However, we still see our "Hello World" instead of the message from our test_runner. The reason is that our _start function is still used as entry point. The custom test frameworks feature generates a main function that calls test_runner, but this function is ignored because we use the #[no_main] attribute and provide our own entry point.

To fix this, we first need to change the name of the generated function to something different than main through the reexport_test_harness_main attribute. Then we can call the renamed function from our _start function:

// in src/main.rs

#![reexport_test_harness_main = "test_main"]

#[no_mangle]
pub extern "C" fn _start() -> ! {
    println!("Hello World{}", "!");

    #[cfg(test)]
    test_main();

    loop {}
}

We set the name of the test framework entry function to test_main and call it from our _start entry point. We use conditional compilation to add the call to test_main only in test contexts because the function is not generated on a normal run.

When we now execute cargo xtest, we see the "Running 0 tests" message from our test_runner on the screen. We are now ready to create our first test function:

// in src/main.rs

#[test_case]
fn trivial_assertion() {
    print!("trivial assertion... ");
    assert_eq!(1, 1);
    println!("[ok]");
}

When we run cargo xtest now, we see the following output:

QEMU printing "Hello World!", "Running 1 tests", and "trivial assertion... [ok]"

The tests slice passed to our test_runner function now contains a reference to the trivial_assertion function. From the trivial assertion... [ok] output on the screen we see that the test was called and that it succeeded.

After executing the tests, our test_runner returns to the test_main function, which in turn returns to our _start entry point function. At the end of _start, we enter an endless loop because the entry point function is not allowed to return. This is a problem, because we want cargo xtest to exit after running all tests.

🔗 Exiting QEMU

Right now we have an endless loop at the end of our _start function and need to close QEMU manually on each execution of cargo xtest. This is unfortunate because we also want to run cargo xtest in scripts without user interaction. The clean solution to this would be to implement a proper way to shutdown our OS. Unfortunately this is relatively complex, because it requires implementing support for either the APM or ACPI power management standard.

Luckily, there is an escape hatch: QEMU supports a special isa-debug-exit device, which provides an easy way to exit QEMU from the guest system. To enable it, we need to pass a -device argument to QEMU. We can do so by adding a package.metadata.bootimage.test-args configuration key in our Cargo.toml:

# in Cargo.toml

[package.metadata.bootimage]
test-args = ["-device", "isa-debug-exit,iobase=0xf4,iosize=0x04"]

The bootimage runner appends the test-args to the default QEMU command for all test executables. For a normal cargo xrun, the arguments are ignored.

Together with the device name (isa-debug-exit), we pass the two parameters iobase and iosize that specify the I/O port through which the device can be reached from our kernel.

🔗 I/O Ports

There are two different approaches for communicating between the CPU and peripheral hardware on x86, memory-mapped I/O and port-mapped I/O. We already used memory-mapped I/O for accessing the VGA text buffer through the memory address 0xb8000. This address is not mapped to RAM, but to some memory on the VGA device.

In contrast, port-mapped I/O uses a separate I/O bus for communication. Each connected peripheral has one or more port numbers. To communicate with such an I/O port there are special CPU instructions called in and out, which take a port number and a data byte (there are also variations of these commands that allow sending an u16 or u32).

The isa-debug-exit devices uses port-mapped I/O. The iobase parameter specifies on which port address the device should live (0xf4 is a generally unused port on the x86's IO bus) and the iosize specifies the port size (0x04 means four bytes).

🔗 Using the Exit Device

The functionality of the isa-debug-exit device is very simple. When a value is written to the I/O port specified by iobase, it causes QEMU to exit with exit status (value << 1) | 1. So when we write 0 to the port QEMU will exit with exit status (0 << 1) | 1 = 1 and when we write 1 to the port it will exit with exit status (1 << 1) | 1 = 3.

Instead of manually invoking the in and out assembly instructions, we use the abstractions provided by the x86_64 crate. To add a dependency on that crate, we add it to the dependencies section in our Cargo.toml:

# in Cargo.toml

[dependencies]
x86_64 = "0.7.0"

Now we can use the Port type provided by the crate to create an exit_qemu function:

// in src/main.rs

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u32)]
pub enum QemuExitCode {
    Success = 0x10,
    Failed = 0x11,
}

pub fn exit_qemu(exit_code: QemuExitCode) {
    use x86_64::instructions::port::Port;

    unsafe {
        let mut port = Port::new(0xf4);
        port.write(exit_code as u32);
    }
}

The function creates a new Port at 0xf4, which is the iobase of the isa-debug-exit device. Then it writes the the passed exit code to the port. We use u32 because we specified the iosize of the isa-debug-exit device as 4 bytes. Both operations are unsafe, because writing to an I/O port can generally result in arbitrary behavior.

For specifying the exit status, we create a QemuExitCode enum. The idea is to exit with the success exit code if all tests succeeded and with the failure exit code otherwise. The enum is marked as #[repr(u32)] to represent each variant by an u32 integer. We use exit code 0x10 for success and 0x11 for failure. The actual exit codes do not matter much, as long as they don't clash with the default exit codes of QEMU. For example, using exit code 0 for success is not a good idea because it becomes (0 << 1) | 1 = 1 after the transformation, which is the default exit code when QEMU failed to run. So we could not differentiate a QEMU error from a successful test run.

We can now update our test_runner to exit QEMU after all tests ran:

fn test_runner(tests: &[&dyn Fn()]) {
    println!("Running {} tests", tests.len());
    for test in tests {
        test();
    }
    /// new
    exit_qemu(QemuExitCode::Success);
}

When we run cargo xtest now, we see that QEMU immediately closes after executing the tests. The problem is that cargo test interprets the test as failed even though we passed our Success exit code:

> cargo xtest
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
     Running target/x86_64-blog_os/debug/deps/blog_os-5804fc7d2dd4c9be
Building bootloader
   Compiling bootloader v0.5.3 (/home/philipp/Documents/bootloader)
    Finished release [optimized + debuginfo] target(s) in 1.07s
Running: `qemu-system-x86_64 -drive format=raw,file=/…/target/x86_64-blog_os/debug/
    deps/bootimage-blog_os-5804fc7d2dd4c9be.bin -device isa-debug-exit,iobase=0xf4,
    iosize=0x04`
error: test failed, to rerun pass '--bin blog_os'

The problem is that cargo test considers all error codes other than 0 as failure.

🔗 Success Exit Code

To work around this, bootimage provides a test-success-exit-code configuration key that maps a specified exit code to the exit code 0:

[package.metadata.bootimage]
test-args = […]
test-success-exit-code = 33         # (0x10 << 1) | 1

With this configuration, bootimage maps our success exit code to exit code 0, so that cargo xtest correctly recognizes the success case and does no count the test as failed.

Our test runner now automatically closes QEMU and correctly reports the test results out. We still see the QEMU window open for a very short time, but it does not suffice to read the results. It would be nice if we could print the test results to the console instead, so that we can still see them after QEMU exited.

🔗 Printing to the Console

To see the test output on the console, we need to send the data from our kernel to the host system somehow. There are various ways to achieve this, for example by sending the data over a TCP network interface. However, setting up a networking stack is a quite complex task, so we will choose a simpler solution instead.

🔗 Serial Port

A simple way to send the data is to use the serial port, an old interface standard which is no longer found in modern computers. It is easy to program and QEMU can redirect the bytes sent over serial to the host's standard output or a file.

The chips implementing a serial interface are called UARTs. There are lots of UART models on x86, but fortunately the only differences between them are some advanced features we don't need. The common UARTs today are all compatible to the 16550 UART, so we will use that model for our testing framework.

We will use the uart_16550 crate to initialize the UART and send data over the serial port. To add it as a dependency, we update our Cargo.toml and main.rs:

# in Cargo.toml

[dependencies]
uart_16550 = "0.2.0"

The uart_16550 crate contains a SerialPort struct that represents the UART registers, but we still need to construct an instance of it ourselves. For that we create a new serial module with the following content:

// in src/main.rs

mod serial;
// in src/serial.rs

use uart_16550::SerialPort;
use spin::Mutex;
use lazy_static::lazy_static;

lazy_static! {
    pub static ref SERIAL1: Mutex<SerialPort> = {
        let mut serial_port = unsafe { SerialPort::new(0x3F8) };
        serial_port.init();
        Mutex::new(serial_port)
    };
}

Like with the VGA text buffer, we use lazy_static and a spinlock to create a static writer instance. By using lazy_static we can ensure that the init method is called exactly once on its first use.

Like the isa-debug-exit device, the UART is programmed using port I/O. Since the UART is more complex, it uses multiple I/O ports for programming different device registers. The unsafe SerialPort::new function expects the address of the first I/O port of the UART as argument, from which it can calculate the addresses of all needed ports. We're passing the port address 0x3F8, which is the standard port number for the first serial interface.

To make the serial port easily usable, we add serial_print! and serial_println! macros:

#[doc(hidden)]
pub fn _print(args: ::core::fmt::Arguments) {
    use core::fmt::Write;
    SERIAL1.lock().write_fmt(args).expect("Printing to serial failed");
}

/// Prints to the host through the serial interface.
#[macro_export]
macro_rules! serial_print {
    ($($arg:tt)*) => {
        $crate::serial::_print(format_args!($($arg)*));
    };
}

/// Prints to the host through the serial interface, appending a newline.
#[macro_export]
macro_rules! serial_println {
    () => ($crate::serial_print!("\n"));
    ($fmt:expr) => ($crate::serial_print!(concat!($fmt, "\n")));
    ($fmt:expr, $($arg:tt)*) => ($crate::serial_print!(
        concat!($fmt, "\n"), $($arg)*));
}

The implementation is very similar to the implementation of our print and println macros. Since the SerialPort type already implements the fmt::Write trait, we don't need to provide our own implementation.

Now we can print to the serial interface instead of the VGA text buffer in our test code:

// in src/main.rs

#[cfg(test)]
fn test_runner(tests: &[&dyn Fn()]) {
    serial_println!("Running {} tests", tests.len());
    […]
}

#[test_case]
fn trivial_assertion() {
    serial_print!("trivial assertion... ");
    assert_eq!(1, 1);
    serial_println!("[ok]");
}

Note that the serial_println macro lives directly under the root namespace because we used the #[macro_export] attribute, so importing it through use crate::serial::serial_println will not work.

🔗 QEMU Arguments

To see the serial output from QEMU, we need use the -serial argument to redirect the output to stdout:

# in Cargo.toml

[package.metadata.bootimage]
test-args = [
    "-device", "isa-debug-exit,iobase=0xf4,iosize=0x04", "-serial", "stdio"
]

When we run cargo xtest now, we see the test output directly in the console:

> cargo xtest
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
     Running target/x86_64-blog_os/debug/deps/blog_os-7b7c37b4ad62551a
Building bootloader
    Finished release [optimized + debuginfo] target(s) in 0.02s
Running: `qemu-system-x86_64 -drive format=raw,file=/…/target/x86_64-blog_os/debug/
    deps/bootimage-blog_os-7b7c37b4ad62551a.bin -device
    isa-debug-exit,iobase=0xf4,iosize=0x04 -serial stdio`
Running 1 tests
trivial assertion... [ok]

However, when a test fails we still see the output inside QEMU because our panic handler still uses println. To simulate this, we can change the assertion in our trivial_assertion test to assert_eq!(0, 1):

QEMU printing "Hello World!" and "panicked at 'assertion failed: (left == right) left: 0, right: 1', src/main.rs:55:5

We see that the panic message is still printed to the VGA buffer, while the other test output is printed to the serial port. The panic message is quite useful, so it would be useful to see it in the console too.

To exit QEMU with an error message on a panic, we can use conditional compilation to use a different panic handler in testing mode:

// our existing panic handler
#[cfg(not(test))] // new attribute
#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    println!("{}", info);
    loop {}
}

// our panic handler in test mode
#[cfg(test)]
#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    serial_println!("[failed]\n");
    serial_println!("Error: {}\n", info);
    exit_qemu(QemuExitCode::Failed);
    loop {}
}

For our test panic handler, we use serial_println instead of println and then exit QEMU with a failure exit code. Note that we still need an endless loop after the exit_qemu call because the compiler does not know that the isa-debug-exit device causes a program exit.

Now QEMU also exits for failed tests and prints a useful error message on the console:

> cargo xtest
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
     Running target/x86_64-blog_os/debug/deps/blog_os-7b7c37b4ad62551a
Building bootloader
    Finished release [optimized + debuginfo] target(s) in 0.02s
Running: `qemu-system-x86_64 -drive format=raw,file=/…/target/x86_64-blog_os/debug/
    deps/bootimage-blog_os-7b7c37b4ad62551a.bin -device
    isa-debug-exit,iobase=0xf4,iosize=0x04 -serial stdio`
Running 1 tests
trivial assertion... [failed]

Error: panicked at 'assertion failed: `(left == right)`
  left: `0`,
 right: `1`', src/main.rs:65:5

Since we see all test output on the console now, we no longer need the QEMU window that pops up for a short time. So we can hide it completely.

🔗 Hiding QEMU

Since we report out the complete test results using the isa-debug-exit device and the serial port, we don't need the QEMU window anymore. We can hide it by passing the -display none argument to QEMU:

# in Cargo.toml

[package.metadata.bootimage]
test-args = [
    "-device", "isa-debug-exit,iobase=0xf4,iosize=0x04", "-serial", "stdio",
    "-display", "none"
]

Now QEMU runs completely in the background and no window is opened anymore. This is not only less annoying, but also allows our test framework to run in environments without a graphical user interface, such as CI services or SSH connections.

🔗 Timeouts

Since cargo xtest waits until the test runner exits, a test that never returns can block the test runner forever. That's unfortunate, but not a big problem in practice since it's normally easy to avoid endless loops. In our case, however, endless loops can occur in various situations:

Since endless loops can occur in so many situations, the bootimage tool sets a timeout of 5 minutes for each test executable by default. If the test does not finish in this time, it is marked as failed and a "Timed Out" error is printed to the console. This feature ensures that tests that are stuck in an endless loop don't block cargo xtest forever.

You can try it yourself by adding a loop {} statement in the trivial_assertion test. When you run cargo xtest, you see that the test is marked as timed out after 5 minutes. The timeout duration is configurable through a test-timeout key in the Cargo.toml:

# in Cargo.toml

[package.metadata.bootimage]
test-timeout = 300          # (in seconds)

If you don't want to wait 5 minutes for the trivial_assertion test to time out, you can temporarily decrease the above value.

After this, we no longer need the trivial_assertion test, so we can delete it.

🔗 Testing the VGA Buffer

Now that we have a working test framework, we can create a few tests for our VGA buffer implementation. First, we create a very simple test to verify that println works without panicking:

// in src/vga_buffer.rs

#[cfg(test)]
use crate::{serial_print, serial_println};

#[test_case]
fn test_println_simple() {
    serial_print!("test_println... ");
    println!("test_println_simple output");
    serial_println!("[ok]");
}

The test just prints something to the VGA buffer. If it finishes without panicking, it means that the println invocation did not panic either. Since we only need the serial_println import in test mode, we add the cfg(test) attribute to avoid the unused import warning for a normal cargo xbuild.

To ensure that no panic occurs even if many lines are printed and lines are shifted off the screen, we can create another test:

// in src/vga_buffer.rs

#[test_case]
fn test_println_many() {
    serial_print!("test_println_many... ");
    for _ in 0..200 {
        println!("test_println_many output");
    }
    serial_println!("[ok]");
}

We can also create a test function to verify that the printed lines really appear on the screen:

// in src/vga_buffer.rs

#[test_case]
fn test_println_output() {
    serial_print!("test_println_output... ");

    let s = "Some test string that fits on a single line";
    println!("{}", s);
    for (i, c) in s.chars().enumerate() {
        let screen_char = WRITER.lock().buffer.chars[BUFFER_HEIGHT - 2][i].read();
        assert_eq!(char::from(screen_char.ascii_character), c);
    }

    serial_println!("[ok]");
}

The function defines a test string, prints it using println, and then iterates over the screen characters of the static WRITER, which represents the vga text buffer. Since println prints to the last screen line and then immediately appends a newline, the string should appear on line BUFFER_HEIGHT - 2.

By using enumerate, we count the number of iterations in the variable i, which we then use for loading the screen character corresponding to c. By comparing the ascii_character of the screen character with c, we ensure that each character of the string really appears in the vga text buffer.

As you can imagine, we could create many more test functions, for example a function that tests that no panic occurs when printing very long lines and that they're wrapped correctly. Or a function for testing that newlines, non-printable characters, and non-unicode characters are handled correctly.

For the rest of this post, however, we will explain how to create integration tests to test the interaction of different components together.

🔗 Integration Tests

The convention for integration tests in Rust is to put them into a tests directory in the project root (i.e. next to the src directory). Both the default test framework and custom test frameworks will automatically pick up and execute all tests in that directory.

All integration tests are their own executables and completely separate from our main.rs. This means that each test needs to define its own entry point function. Let's create an example integration test named basic_boot to see how it works in detail:

// in tests/basic_boot.rs

#![no_std]
#![no_main]
#![feature(custom_test_frameworks)]
#![test_runner(crate::test_runner)]
#![reexport_test_harness_main = "test_main"]

use core::panic::PanicInfo;

#[no_mangle] // don't mangle the name of this function
pub extern "C" fn _start() -> ! {
    test_main();

    loop {}
}

fn test_runner(tests: &[&dyn Fn()]) {
    unimplemented!();
}

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    loop {}
}

Since integration tests are separate executables, we need to provide all the crate attributes (no_std, no_main, test_runner, etc.) again. We also need to create a new entry point function _start, which calls the test entry point function test_main. We don't need any cfg(test) attributes because integration test executables are never built in non-test mode.

We use the unimplemented macro that always panics as a placeholder for the test_runner function and just loop in the panic handler for now. Ideally, we want to implement these functions exactly as we did in our main.rs using the serial_println macro and the exit_qemu function. The problem is that we don't have access to these functions since tests are built completely separately of our main.rs executable.

If you run cargo xtest at this stage, you will get an endless loop because the panic handler loops endlessly. You need to use the Ctrl+c keyboard shortcut for exiting QEMU.

🔗 Create a Library

To make the required functions available to our integration test, we need to split off a library from our main.rs, which can be included by other crates and integration test executables. To do this, we create a new src/lib.rs file:

// src/lib.rs

#![no_std]

Like the main.rs, the lib.rs is a special file that is automatically recognized by cargo. The library is a separate compilation unit, so we need to specify the #![no_std] attribute again.

To make our library work with cargo xtest, we need to also add the test functions and attributes:

// in src/lib.rs

#![cfg_attr(test, no_main)]
#![feature(custom_test_frameworks)]
#![test_runner(crate::test_runner)]
#![reexport_test_harness_main = "test_main"]

use core::panic::PanicInfo;

pub fn test_runner(tests: &[&dyn Fn()]) {
    serial_println!("Running {} tests", tests.len());
    for test in tests {
        test();
    }
    exit_qemu(QemuExitCode::Success);
}

pub fn test_panic_handler(info: &PanicInfo) -> ! {
    serial_println!("[failed]\n");
    serial_println!("Error: {}\n", info);
    exit_qemu(QemuExitCode::Failed);
    loop {}
}

/// Entry point for `cargo xtest`
#[cfg(test)]
#[no_mangle]
pub extern "C" fn _start() -> ! {
    test_main();
    loop {}
}

#[cfg(test)]
#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    test_panic_handler(info)
}

To make our test_runner available to executables and integration tests, we don't apply the cfg(test) attribute to it and make it public. We also factor out the implementation of our panic handler into a public test_panic_handler function, so that it is available for executables too.

Since our lib.rs is tested independently of our main.rs, we need to add a _start entry point and a panic handler when the library is compiled in test mode. By using the cfg_attr crate attribute, we conditionally enable the no_main attribute in this case.

We also move over the QemuExitCode enum and the exit_qemu function and make them public:

// in src/lib.rs

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u32)]
pub enum QemuExitCode {
    Success = 0x10,
    Failed = 0x11,
}

pub fn exit_qemu(exit_code: QemuExitCode) {
    use x86_64::instructions::port::Port;

    unsafe {
        let mut port = Port::new(0xf4);
        port.write(exit_code as u32);
    }
}

Now executables and integration tests can import these functions from the library and don't need to define their own implementations. To also make println and serial_println available, we move the module declarations too:

// in src/lib.rs

pub mod serial;
pub mod vga_buffer;

We make the modules public to make them usable from outside of our library. This is also required for making our println and serial_println macros usable, since they use the _print functions of the modules.

Now we can update our main.rs to use the library:

// src/main.rs

#![no_std]
#![no_main]
#![feature(custom_test_frameworks)]
#![test_runner(blog_os::test_runner)]
#![reexport_test_harness_main = "test_main"]

use core::panic::PanicInfo;
use blog_os::println;

#[no_mangle]
pub extern "C" fn _start() -> ! {
    println!("Hello World{}", "!");

    #[cfg(test)]
    test_main();

    loop {}
}

/// This function is called on panic.
#[cfg(not(test))]
#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    println!("{}", info);
    loop {}
}

#[cfg(test)]
#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    blog_os::test_panic_handler(info)
}

The library is usable like a normal external crate. It is called like our crate, which is blog_os in our case. The above code uses the blog_os::test_runner function in the test_runner attribute and the blog_os::test_panic_handler function in our cfg(test) panic handler. It also imports the println macro to make it available to our _start and panic functions.

At this point, cargo xrun and cargo xtest should work again. Of course, cargo xtest still loops endlessly (you can exit with ctrl+c). Let's fix this by using the required library functions in our integration test.

🔗 Completing the Integration Test

Like our src/main.rs, our tests/basic_boot.rs executable can import types from our new library. This allows us to import the missing components to complete our test.

// in tests/basic_boot.rs

#![test_runner(blog_os::test_runner)]

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    blog_os::test_panic_handler(info)
}

Instead of reimplementing the test runner, we use the test_runner function from our library. For our panic handler, we call the blog_os::test_panic_handler function like we did in our main.rs.

Now cargo xtest exits normally again. When you run it, you see that it builds and runs the tests for our lib.rs, main.rs, and basic_boot.rs separately after each other. For the main.rs and the basic_boot integration test, it reports "Running 0 tests" since these files don't have any functions annotated with #[test_case].

We can now add tests to our basic_boot.rs. For example, we can test that println works without panicking, like we did in the vga buffer tests:

// in tests/basic_boot.rs

use blog_os::{println, serial_print, serial_println};

#[test_case]
fn test_println() {
    serial_print!("test_println... ");
    println!("test_println output");
    serial_println!("[ok]");
}

When we run cargo xtest now, we see that it finds and executes the test function.

The test might seem a bit useless right now since it's almost identical to one of the VGA buffer tests. However, in the future the _start functions of our main.rs and lib.rs might grow and call various initialization routines before running the test_main function, so that the two tests are executed in very different environments.

By testing println in a basic_boot environment without calling any initialization routines in _start, we can ensure that println works right after booting. This is important because we rely on it e.g. for printing panic messages.

🔗 Future Tests

The power of integration tests is that they're treated as completely separate executables. This gives them complete control over the environment, which makes it possible to test that the code interacts correctly with the CPU or hardware devices.

Our basic_boot test is a very simple example for an integration test. In the future, our kernel will become much more featureful and interact with the hardware in various ways. By adding integration tests, we can ensure that these interactions work (and keep working) as expected. Some ideas for possible future tests are:

As you can imagine, many more tests are possible. By adding such tests, we can ensure that we don't break them accidentally when we add new features to our kernel or refactor our code. This is especially important when our kernel becomes larger and more complex.

🔗 Testing Our Panic Handler

Another thing that we can test with an integration test is that our panic handler is called correctly. The idea is to deliberately cause a panic in the test function and exit with a success exit code in the panic handler.

Since we exit from our panic handler, the panicking test never returns to the test runner. For this reason, it does not make sense to add more than one test because subsequent tests are never executed. For cases like this, where only a single test function exists, we can disable the test runner completely and run our test directly in the _start function.

🔗 No Harness

The harness flag defines whether a test runner is used for an integration test. When it's set to false, both the default test runner and the custom test runner feature are disabled, so that the test is treated like a normal executable.

Let's create a panic handler test with a disabled harness flag. First, we create a skeleton for the test at tests/panic_handler.rs:

// in tests/panic_handler.rs

#![no_std]
#![no_main]

use core::panic::PanicInfo;
use blog_os::{QemuExitCode, exit_qemu};

#[no_mangle]
pub extern "C" fn _start() -> ! {
    exit_qemu(QemuExitCode::Failed);
    loop {}
}

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    exit_qemu(QemuExitCode::Failed);
    loop {}
}

The code is similar to the basic_boot test with the difference that no test attributes are needed and no runner function is called. We immediately exit with an error from the _start entry point and the panic handler for now and first try to get it to compile.

If you run cargo xtest now, you will get an error that the test crate is missing. This error occurs because we didn't set a custom test framework, so that the compiler tries to use the default test framework, which is unavailable for our panic. By setting the harness flag to false for the test in our Cargo.toml, we can fix this error:

# in Cargo.toml

[[test]]
name = "panic_handler"
harness = false

Now the test compiles fine, but fails of course since we always exit with an error exit code.

🔗 Implementing the Test

Let's complete the implementation of our panic handler test:

// in tests/panic_handler.rs

use blog_os::{serial_print, serial_println, QemuExitCode, exit_qemu};

const MESSAGE: &str = "Example panic message from panic_handler test";
const PANIC_LINE: u32 = 14; // adjust this when moving the `panic!` call

#[no_mangle]
pub extern "C" fn _start() -> ! {
    serial_print!("panic_handler... ");
    panic!(MESSAGE); // must be in line `PANIC_LINE`
}

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    serial_println!("[ok]");
    exit_qemu(QemuExitCode::Success);
    loop {}
}

We immediately panic in our _start function with a MESSAGE. In the panic handler, we exit with a success exit code. We don't need a qemu_exit call at the end of our _start function, since the Rust compiler knows for sure that the code after the panic is unreachable. If we run the test with cargo xtest --test panic_handler now, we see that it succeeds as expected.

We will need the MESSAGE and PANIC_LINE constants in the next section. The PANIC_LINE constant specifies the line number that contains the panic! invocation, which is 14 in our case (but it might be different for you).

🔗 Checking the PanicInfo

To ensure that the given PanicInfo is correct, we can extend the panic function to check that the reported message and file/line information are correct:

// in tests/panic_handler.rs

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    check_message(info);
    check_location(info);

    // same as before
    serial_println!("[ok]");
    exit_qemu(QemuExitCode::Success);
    loop {}
}

We will show the implementation of check_message and check_location in a moment. Before that, we create a fail helper function that can be used to print an error message and exit QEMU with an failure exit code:

// in tests/panic_handler.rs

fn fail(error: &str) -> ! {
    serial_println!("[failed]");
    serial_println!("{}", error);
    exit_qemu(QemuExitCode::Failed);
    loop {}
}

Now we can implement our check_location function:

// in tests/panic_handler.rs

fn check_location(info: &PanicInfo) {
    let location = info.location().unwrap_or_else(|| fail("no location"));
    if location.file() != file!() {
        fail("file name wrong");
    }
    if location.line() != PANIC_LINE {
        fail("file line wrong");
    }
}

The function takes queries the location information from the PanicInfo and fails if it does not exist. It then checks that the reported file name is correct by comparing it with the output of the compiler-provided file! macro. To check the reported line number, it compares it with the PANIC_LINE constant that we manually defined above.

🔗 Checking the Panic Message

Checking the reported panic message is a bit more complicated. The reason is that the PanicInfo::message function returns a fmt::Arguments instance that can't be compared with our MESSAGE string directly. To work around this, we need to create a CompareMessage struct:

// in tests/panic_handler.rs

use core::fmt;

/// Compares a `fmt::Arguments` instance with the `MESSAGE` string
///
/// To use this type, write the `fmt::Arguments` instance to it using the
/// `write` macro. If the message component matches `MESSAGE`, the `expected`
/// field is the empty string.
struct CompareMessage {
    expected: &'static str,
}

impl fmt::Write for CompareMessage {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        if self.expected.starts_with(s) {
            self.expected = &self.expected[s.len()..];
        } else {
            fail("message not equal to expected message");
        }
        Ok(())
    }
}

The trick is to implement the fmt::Write trait like we did for our VGA buffer writer. The write_str method is called with a &str parameter that we can compare with the expected message. An important detail is that the method is called multiple times with the individual string components. For example, when we do print!("{}z", "xy") the method on our VGA buffer writer is invoked once with the string "xy" and once with the string "z".

This means that we can't directly compare the s argument with the expected message, since it might only be a substring. Instead, we use the starts_with method to verify that the given string component is a substring of the expected message. Then we use string slicing to remove the already printed characters from the expected string. If the expected field is an empty string after writing the panic message, it means that it matches the expected message.

With the CompareMessage type, we can finally implement our check_message function:

// in tests/panic_handler.rs

#![feature(panic_info_message)] // at the top of the file

use core::fmt::Write;

fn check_message(info: &PanicInfo) {
    let message = info.message().unwrap_or_else(|| fail("no message"));
    let mut compare_message = CompareMessage { expected: MESSAGE };
    write!(&mut compare_message, "{}", message)
        .unwrap_or_else(|_| fail("write failed"));
    if !compare_message.expected.is_empty() {
        fail("message shorter than expected message");
    }
}

The function uses the PanicInfo::message function to get the panic message. If no message is reported, it calls fail to fail the test. Since the function is unstable, we need to add the #![feature(panic_info_message)] attribute at the top of our test file. Note that you need to adjust the PANIC_INFO line number after adding the attribute and the imports on top.

After querying the message, the function constructs a CompareMessage instance with the expected field set to the MESSAGE string. Then it writes the message to it using the write! macro. After the write, it reads the expected field and fails the test if it is not the empty string.

Now we can run the test using cargo xtest --test panic_handler. We see that it passes, which means that the reported panic info is correct. If we use a wrong line number in PANIC_LINE or panic with an additional character through panic!("{}x", MESSAGE), we see that the test indeed fails.

🔗 Summary

Testing is a very useful technique to ensure that certain components have a desired behavior. Even if they cannot show the absence of bugs, they're still an useful tool for finding them and especially for avoiding regressions.

This post explained how to set up a test framework for our Rust kernel. We used the custom test frameworks feature of Rust to implement support for a simple #[test_case] attribute in our bare-metal environment. By using the isa-debug-exit device of QEMU, our test runner can exit QEMU after running the tests and report the test status out. To print error messages to the console instead of the VGA buffer, we created a basic driver for the serial port.

After creating some tests for our println macro, we explored integration tests in the second half of the post. We learned that they live in the tests directory and are treated as completely separate executables. To give them access to the exit_qemu function and the serial_println macro, we moved most of our code into a library that can be imported by all executables and integration tests. Since integration tests run in their own separate environment, they make it possible to test the interactions with the hardware or Rust's panic system.

We now have a test framework that runs in a realistic environment inside QEMU. By creating more tests in future posts, we can keep our kernel maintainable when it becomes more complex.

🔗 What's next?

In the next post, we will explore CPU exceptions. These exceptions are thrown by the CPU when something illegal happens, such as a division by zero or an access to an unmapped memory page (a so-called “page fault”). Being able to catch and examine these exceptions is very important for debugging future errors. Exception handling is also very similar to the handling of hardware interrupts, which is required for keyboard support.



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