Writing an OS in Rust Philipp Oppermann's blog

Set Up Rust

In the previous posts we created a minimal Multiboot kernel and switched to Long Mode. Now we can finally switch to Rust code. Rust is a high-level language without runtime. It allows us to not link the standard library and write bare metal code. Unfortunately the setup is not quite hassle-free yet.

This blog post tries to set up Rust step-by-step and point out the different problems. If you have any questions, problems, or suggestions please file an issue or create a comment at the bottom. The code from this post is in a Github repository, too.

🔗 Installing Rust

We need a nightly compiler, as we will use many unstable features. To manage Rust installations I highly recommend rustup. It allows you to install nightly, beta, and stable compilers side-by-side and makes it easy to update them. To use a nightly compiler for the current directory, you can run rustup override add nightly.

The code from this post (and all following) is automatically tested every day and should always work for the newest nightly. If it doesn't, please file an issue.

🔗 Creating a Cargo project

Cargo is Rust's excellent package manager. Normally you would call cargo new when you want to create a new project folder. We can't use it because our folder already exists, so we need to do it manually. Fortunately we only need to add a cargo configuration file named Cargo.toml:

[package]
name = "blog_os"
version = "0.1.0"
authors = ["Philipp Oppermann <dev@phil-opp.com>"]

[lib]
crate-type = ["staticlib"]

The package section contains required project metadata such as the semantic crate version. The lib section specifies that we want to build a static library, i.e. a library that contains all of its dependencies. This is required to link the Rust project with our kernel.

Now we place our root source file in src/lib.rs:

#![feature(lang_items)]
#![no_std]

#[no_mangle]
pub extern fn rust_main() {}

#[lang = "eh_personality"] extern fn eh_personality() {}
#[lang = "panic_fmt"] #[no_mangle] pub extern fn panic_fmt() -> ! {loop{}}

Let's break it down:

🔗 Building Rust

We can now build it using cargo build, which creates a static library at target/debug/libblog_os.a. However, the resulting library is specific to our host operating system. This is undesirable, because our target system might be different.

Let's define some properties of our target system:

🔗 Target Specifications

Rust allows us to define custom targets through a JSON configuration file. A minimal target specification equal to x86_64-unknown-linux-gnu (the default 64-bit Linux target) looks like this:

{
  "llvm-target": "x86_64-unknown-linux-gnu",
  "data-layout": "e-m:e-i64:64-f80:128-n8:16:32:64-S128",
  "linker-flavor": "gcc",
  "target-endian": "little",
  "target-pointer-width": "64",
  "arch": "x86_64",
  "os": "linux"
}

The llvm-target field specifies the target triple that is passed to LLVM. Target triples are a naming convention that define the CPU architecture (e.g., x86_64 or arm), the vendor (e.g., apple or unknown), the operating system (e.g., windows or linux), and the ABI (e.g., gnu or msvc). For example, the target triple for 64-bit Linux is x86_64-unknown-linux-gnu and for 32-bit Windows the target triple is i686-pc-windows-msvc.

The data-layout field is also passed to LLVM and specifies how data should be laid out in memory. It consists of various specifications seperated by a - character. For example, the e means little endian and S128 specifies that the stack should be 128 bits (= 16 byte) aligned. The format is described in detail in the LLVM documentation but there shouldn't be a reason to change this string.

The linker-flavor field was recently introduced in #40018 with the intention to add support for the LLVM linker LLD, which is platform independent. In the future, this might allow easy cross compilation without the need to install a gcc cross compiler for linking.

The other fields are used for conditional compilation. This allows crate authors to use cfg variables to write special code for depending on the OS or the architecture. There isn't any up-to-date documentation about these fields but the corresponding source code is quite readable.

🔗 A Kernel Target Specification

For our target system, we define the following JSON configuration in a file named x86_64-blog_os.json:

{
  "llvm-target": "x86_64-unknown-none",
  "data-layout": "e-m:e-i64:64-f80:128-n8:16:32:64-S128",
  "linker-flavor": "gcc",
  "target-endian": "little",
  "target-pointer-width": "64",
  "arch": "x86_64",
  "os": "none",
  "disable-redzone": true,
  "features": "-mmx,-sse,+soft-float"
}

As llvm-target we use x86_64-unknown-none, which defines the x86_64 architecture, an unknown vendor, and no operating system (none). The ABI doesn't matter for us, so we just leave it off. The data-layout field is just copied from the x86_64-unknown-linux-gnu target. We also use the same values for the target-endian, target-pointer-width, and arch fields. For the os field we choose none, since our kernel runs on bare metal.

🔗 The Red Zone

The red zone is an optimization of the System V ABI that allows functions to temporary use the 128 bytes below its stack frame without adjusting the stack pointer:

stack frame with red zone

The image shows the stack frame of a function with n local variables. On function entry, the stack pointer is adjusted to make room on the stack for the local variables.

The red zone is defined as the 128 bytes below the adjusted stack pointer. The function can use this area for temporary data that's not needed across function calls. Thus, the two instructions for adjusting the stack pointer can be avoided in some cases (e.g. in small leaf functions).

However, this optimization leads to huge problems with exceptions or hardware interrupts. Let's assume that an exception occurs while a function uses the red zone:

red zone overwritten by exception handler

The CPU and the exception handler overwrite the data in red zone. But this data is still needed by the interrupted function. So the function won't work correctly anymore when we return from the exception handler. This might lead to strange bugs that take weeks to debug.

To avoid such bugs when we implement exception handling in the future, we disable the red zone right from the beginning. This is achieved by adding the "disable-redzone": true line to our target configuration file.

🔗 SIMD Extensions

The features field enables/disables target features. We disable the mmx and sse features by prefixing them with a minus and enable the soft-float feature by prefixing it with a plus. The mmx and sse features determine support for Single Instruction Multiple Data (SIMD) instructions, which simultaneously perform an operation (e.g. addition) on multiple data words. The x86 architecture supports the following standards:

By using such SIMD standards, programs can often speed up significantly. Good compilers are able to transform normal loops into such SIMD code automatically through a process called auto-vectorization.

However, the large SIMD registers lead to problems in OS kernels. The reason is that the kernel has to backup all registers that it uses on each hardware interrupt (we will look into this in the “Handling Exceptions” post). So if the kernel uses SIMD registers, it has to backup a lot more data, which noticably decreases performance. To avoid this performance loss, we disable the sse and mmx features (the avx feature is disabled by default).

As noted above, floating point operations on x86_64 use SSE registers, so floats are no longer usable without SSE. Unfortunately, the Rust core library already uses floats (e.g., it implements traits for f32 and f64), so we need an alternative way to implement float operations. The soft-float feature solves this problem by emulating all floating point operations through software functions based on normal integers.

🔗 Compiling

To build our kernel for our new target, we pass the configuration file's name as target argument:

cargo build --target=x86_64-blog_os

However, the following error occurs:

error[E0463]: can't find crate for `core`
  |
  = note: the `x86_64-blog_os` target may not be installed

The error tells us that the Rust compiler no longer finds the core library. The core library is implicitly linked to all no_std crates and contains things such as Result, Option, and iterators.

The problem is that the core library is distributed together with the Rust compiler as a precompiled library. So it is only valid for the host triple (e.g., x86_64-unknown-linux-gnu) but not for our custom target. If we want to compile code for other targets, we need to recompile core for these targets first.

🔗 Xargo

That's where xargo comes in. It is a wrapper for cargo that eases cross compilation. We can install it by executing:

cargo install xargo

Xargo depends on the rust source code, which we can install with rustup component add rust-src.

Xargo is “a drop-in replacement for cargo”, so every cargo command also works with xargo. You can do e.g. xargo --help, xargo clean, or xargo doc. However, the build command gains additional functionality: xargo build will automatically cross compile the core library when compiling for custom targets.

Let's try it:

> xargo build --target=x86_64-blog_os
   Compiling core v0.0.0 (file:///…/rust/src/libcore)
    Finished release [optimized] target(s) in 22.87 secs
   Compiling blog_os v0.1.0 (file:///…/blog_os/tags)
    Finished dev [unoptimized + debuginfo] target(s) in 0.29 secs

It worked! We see that xargo cross-compiled the core library for our new custom target and then continued to compile our blog_os crate. After compilation, we can find a static library at target/x86_64-blog_os/debug/libblog_os.a, which can be linked with our assembly kernel.

🔗 Integrating Rust

Let's try to integrate our Rust library into our assembly kernel so that we can call the rust_main function. For that we need to pass the libblog_os.a file to the linker, together with the assembly object files.

🔗 Adjusting the Makefile

To build and link the rust library on make, we extend our Makefile(full file):

# ...
target ?= $(arch)-blog_os
rust_os := target/$(target)/debug/libblog_os.a
# ...
.PHONY: all clean run iso kernel
# ...
$(kernel): kernel $(rust_os) $(assembly_object_files) $(linker_script)
    @ld -n -T $(linker_script) -o $(kernel) \
        $(assembly_object_files) $(rust_os)

kernel:
    @xargo build --target $(target)

We add a new kernel target that just executes xargo build and modify the $(kernel) target to link the created static lib. We also add the new kernel target to the .PHONY list, since it does not belong to a file with that name.

But now xargo build is executed on every make, even if no source file was changed. And the ISO is recreated on every make iso/make run, too. We could try to avoid this by adding dependencies on all rust source and cargo configuration files to the kernel target, but the ISO creation takes only half a second on my machine and most of the time we will have changed a Rust file when we run make. So we keep it simple for now and let cargo do the bookkeeping of changed files (it does it anyway).

🔗 Calling Rust

Now we can call the main method in long_mode_start:

bits 64
long_mode_start:
    ...

    ; call the rust main
    extern rust_main     ; new
    call rust_main       ; new

    ; print `OKAY` to screen
    mov rax, 0x2f592f412f4b2f4f
    mov qword [0xb8000], rax
    hlt

By defining rust_main as extern we tell nasm that the function is defined in another file. As the linker takes care of linking them together, we'll get a linker error if we have a typo in the name or forget to mark the rust function as pub extern.

If we've done everything right, we should still see the green OKAY when executing make run. That means that we successfully called the Rust function and returned back to assembly.

🔗 Fixing Linker Errors

Now we can try some Rust code:

pub extern fn rust_main() {
    let x = ["Hello", "World", "!"];
    let y = x;
}

When we test it using make run, it fails with undefined reference to 'memcpy'. The memcpy function is one of the basic functions of the C library (libc). Usually the libc crate is linked to every Rust program together with the standard library, but we opted out through #![no_std]. We could try to fix this by adding the libc crate as extern crate. But libc is just a wrapper for the system libc, for example glibc on Linux, so this won't work for us. Instead we need to recreate the basic libc functions such as memcpy, memmove, memset, and memcmp in Rust.

🔗 rlibc

Fortunately there already is a crate for that: rlibc. When we look at its source code we see that it contains no magic, just some raw pointer operations in a while loop. To add rlibc as a dependency we just need to add two lines to the Cargo.toml:

...
[dependencies]
rlibc = "1.0"

and an extern crate definition in our src/lib.rs:

...
extern crate rlibc;

#[no_mangle]
pub extern fn rust_main() {
...

Now make run doesn't complain about memcpy anymore. Instead it will show a pile of new ugly linker errors:

target/x86_64-blog_os/debug/libblog_os.a(core-92335f822fa6c9a6.0.o):
    In function `_$LT$f32$u20$as$u20$core..num..dec2flt..
        rawfp..RawFloat$GT$::from_int::h50f7952efac3fdca':
    core.cgu-0.rs:(.text._ZN59_$LT$f32$u20$as$u20$core..num..dec2flt..
        rawfp..RawFloat$GT$8from_int17h50f7952efac3fdcaE+0x2):
    undefined reference to `__floatundisf'
target/x86_64-blog_os/debug/libblog_os.a(core-92335f822fa6c9a6.0.o):
    In function `_$LT$f64$u20$as$u20$core..num..dec2flt..rawfp..
        RawFloat$GT$::from_int::h12a81f175246914a':
    core.cgu-0.rs:(.text._ZN59_$LT$f64$u20$as$u20$core..num..dec2flt..rawfp..
        RawFloat$GT$8from_int17h12a81f175246914aE+0x2):
    undefined reference to `__floatundidf'
target/x86_64-blog_os/debug/libblog_os.a(core-92335f822fa6c9a6.0.o):
    In function `core::num::from_str_radix::h09b12650704e0508':
    core.cgu-0.rs:(.text._ZN4core3num14from_str_radix
        17h09b12650704e0508E+0xcf):
    undefined reference to `__muloti4'
...

🔗 --gc-sections

The new errors are linker errors about various missing functions such as __floatundisf or __muloti4. These functions are part of LLVM's compiler-rt builtins and are normally linked by the standard library. For no_std crates like ours, one has to link the compiler-rt library manually. Unfortunatly, this library is implemented in C and the build process is a bit cumbersome. Alternatively, there is the compiler-builtins crate that tries to port the library to Rust, but it isn't complete yet.

In our case, there is a much simpler solution, since our kernel doesn't really need any of those functions yet. So we can just tell the linker to remove unused program sections and hopefully all references to these functions will disappear. Removing unused sections is generally a good idea as it reduces kernel size. The magic linker flag for this is --gc-sections, which stands for “garbage collect sections”. Let's add it to the $(kernel) target in our Makefile:

$(kernel): xargo $(rust_os) $(assembly_object_files) $(linker_script)
    @ld -n --gc-sections -T $(linker_script) -o $(kernel) \
        $(assembly_object_files) $(rust_os)

Now we can do a make run again and it compiles without errors again. However, it doesn't boot anymore:

GRUB error: no multiboot header found.

What happened? Well, the linker removed unused sections. And since we don't use the Multiboot section anywhere, ld removes it, too. So we need to tell the linker explicitely that it should keep this section. The KEEP command does exactly that, so we add it to the linker script (linker.ld):

.boot :
{
    /* ensure that the multiboot header is at the beginning */
    KEEP(*(.multiboot_header))
}

Now everything should work again (the green OKAY). But there is another linking issue, which is triggered by some other example code.

🔗 panic = "abort"

The following snippet still fails:

    ...
    let test = (0..3).flat_map(|x| 0..x).zip(0..);

The error is a linker error again (hence the ugly error message):

target/x86_64-blog_os/debug/libblog_os.a(blog_os-b5a29f28b14f1f1f.0.o):
    In function `core::ptr::drop_in_place<core::iter::Zip<
        core::iter::FlatMap<core::ops::Range<i32>, core::ops::Range<i32>,
        closure>, core::ops::RangeFrom<i32>>>':
        /…/rust/src/libcore/ptr.rs:66:
    undefined reference to `_Unwind_Resume'
target/x86_64-blog_os/debug/libblog_os.a(blog_os-b5a29f28b14f1f1f.0.o):
    In function `core::iter::iterator::Iterator::zip<core::iter::FlatMap<
        core::ops::Range<i32>, core::ops::Range<i32>, closure>,
        core::ops::RangeFrom<i32>>':
        /…/rust/src/libcore/iter/iterator.rs:389:
    undefined reference to `_Unwind_Resume'
...

So the linker can't find a function named _Unwind_Resume that is referenced e.g. in iter/iterator.rs:389 in libcore. This reference is not really there at line 389 of libcore's iterator.rs. Instead, it is a compiler inserted landing pad, which is used for panic handling.

By default, the destructors of all stack variables are run when a panic occurs. This is called unwinding and allows parent threads to recover from panics. However, it requires a platform specific gcc library, which isn't available in our kernel.

Fortunately, Rust allows us to disable unwinding for our target. For that we add the following line to our x86_64-blog_os.json file:

{
  "...",
  "panic-strategy": "abort"
}

By setting the panic strategy to abort instead of the default unwind, we disable all unwinding in our kernel. Let's try make run again:

   Compiling core v0.0.0 (file:///…/rust/src/libcore)
    Finished release [optimized] target(s) in 22.24 secs
    Finished dev [unoptimized + debuginfo] target(s) in 0.5 secs
target/x86_64-blog_os/debug/libblog_os.a(blog_os-b5a29f28b14f1f1f.0.o):
    In function `core::ptr::drop_in_place<…>':
    /…/src/libcore/ptr.rs:66:
    undefined reference to `_Unwind_Resume'
...

We see that xargo recompiles the core crate, but the _Unwind_Resume error still occurs. This is because our blog_os crate was not recompiled somehow and thus still references the unwinding function. To fix this, we need to force a recompile using cargo clean:

> cargo clean
> make run
   Compiling rlibc v1.0.0
   Compiling blog_os v0.1.0 (file:///home/philipp/Documents/blog_os/tags)
warning: unused variable: `test` […]

    Finished dev [unoptimized + debuginfo] target(s) in 0.60 secs

It worked! We no longer see linker errors and our kernel prints OKAY again.

🔗 Hello World!

Finally, it's time for a Hello World! from Rust:

#[no_mangle]
pub extern fn rust_main() {
    // ATTENTION: we have a very small stack and no guard page

    let hello = b"Hello World!";
    let color_byte = 0x1f; // white foreground, blue background

    let mut hello_colored = [color_byte; 24];
    for (i, char_byte) in hello.into_iter().enumerate() {
        hello_colored[i*2] = *char_byte;
    }

    // write `Hello World!` to the center of the VGA text buffer
    let buffer_ptr = (0xb8000 + 1988) as *mut _;
    unsafe { *buffer_ptr = hello_colored };

    loop{}
}

Some notes:

🔗 Stack Overflows

Since we still use the small 64 byte stack from the last post, we must be careful not to overflow it. Normally, Rust tries to avoid stack overflows through guard pages: The page below the stack isn't mapped and such a stack overflow triggers a page fault (instead of silently overwriting random memory). But we can't unmap the page below our stack right now since we currently use only a single big page. Fortunately the stack is located just above the page tables. So some important page table entry would probably get overwritten on stack overflow and then a page fault occurs, too.

🔗 What's next?

Until now we write magic bits to some memory location when we want to print something to screen. In the next post we create a abstraction for the VGA text buffer that allows us to print strings in different colors and provides a simple interface.



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