Writing an OS in Rust

https://os.phil-opp.com

A Freestanding Rust Binary

(…) create a Rust executable that does not link the standard library. This makes it possible to run Rust code on the bare metal This means that we can’t use threads, files, heap memory, the network, random numbers, standard output, or any other features requiring OS abstractions or specific hardware.

there are a lot of Rust features that we can use. For example, we can use iterators, closures, pattern matching, option and result, string formatting, and of course the ownership system. These features make it possible to write a kernel in a very expressive, high level way without worrying about undefined behavior or memory safety.

we need to create an executable that can be run without an underlying operating system. Such an executable is often called a “freestanding” or “bare-metal” executable

Right now our crate implicitly links the standard library. Let’s try to disable this by adding the no_std attribute

You can’t (even) use println! with no_std!

The panic_handler attribute defines the function that the compiler should invoke when a panic occurs. The standard library provides its own panic handler function, but in a no_std environment we need to define it ourselves.

Rust has some special syntax for ‘diverging functions’, which are functions that do not return:

fn diverges() -> ! {
    panic!("This function never returns!");
}

The eh_personality language item marks a function that is used for implementing stack unwinding.

We don’t want the added overhead of an unwinding implementation, so we simply abort on panics instead.

One might think that the main function is the first function called when you run a program. However, most languages have a runtime system. In a typical Rust binary that links the standard library, execution starts in a C runtime library called crt0 (“C runtime zero”), which sets up the environment for a C application. This includes creating a stack and placing the arguments in the right registers. The C runtime then invokes the entry point of the Rust runtime, which is marked by the start language item. Rust only has a very minimal runtime, which takes care of some small things such as setting up stack overflow guards or printing a backtrace on panic. The runtime then finally calls the main function.

we need to tell the linker that it should not include the C runtime. We can do this either by passing a certain set of arguments to the linker or by building for a bare metal target.

To describe different environments, Rust uses a string called target triple. A host triple is x86_64-unknown-linux-gnu, which includes the CPU architecture (x86_64), the vendor (unknown), the operating system (linux), and the ABI (gnu). to avoid the linker errors, we can compile for a different environment with no underlying operating system. An example for such a bare metal environment is the thumbv7em-none-eabihf target triple, which describes an embedded ARM system. The details are not important, all that matters is that the target triple has no underlying operating system

A Minimal Rust Kernel

Almost all x86 systems have support for BIOS booting, including newer UEFI-based machines that use an emulated BIOS. This is great, because you can use the same boot logic across all machines from the last centuries. But this wide compatibility is at the same time the biggest disadvantage of BIOS booting, because it means that the CPU is put into a 16-bit compatibility mode called real mode before booting so that archaic bootloaders from the 1980s would still work.

looks for bootable disks. If it finds one, the control is transferred to its bootloader, which is a 512-byte portion of executable code stored at the disk’s beginning. Most bootloaders are larger than 512 bytes, so bootloaders are commonly split into a small first stage, which fits into 512 bytes, and a second stage, which is subsequently loaded by the first stage.

Writing a bootloader is a bit cumbersome as it requires assembly language and a lot of non insightful steps like “write this magic value to this processor register”. Therefore we don’t cover bootloader creation in this post and instead provide a tool named bootimage that automatically prepends a bootloader to your kernel.

the Free Software Foundation created an open bootloader standard called Multiboot in 1995. The standard defines an interface between the bootloader and operating system, so that any Multiboot compliant bootloader can load any Multiboot compliant operating system. The reference implementation is GNU GRUB.

Our goal is to create a disk image that prints a “Hello World!” to the screen when booted.

Rust allows us to define our own target through a JSON file. For example, a JSON file that describes the x86_64-unknown-linux-gnu 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",
    "arch": "x86_64",
    "target-endian": "little",
    "target-pointer-width": "64",
    "target-c-int-width": "32",
    "os": "linux",
    "executables": true,
    "linker-flavor": "gcc",
    "pre-link-args": ["-m64"],
    "morestack": false
}

We’re writing a kernel, so we’ll need to handle interrupts at some point. To do that safely, we have to disable a certain stack pointer optimization called the “red zone”, because it would cause stack corruptions otherwise.

The mmx and sse features determine support for Single Instruction Multiple Data (SIMD) instructions, which can often speed up programs significantly. However, using the large SIMD registers in OS kernels leads to performance problems. The reason is that the kernel needs to restore all registers to their original state before continuing an interrupted program. This means that the kernel has to save the complete SIMD state to main memory on each system call or hardware interrupt. Since the SIMD state is very large (512–1600 bytes) and interrupts can occur very often, these additional save/restore operations considerably harm performance. To avoid this, we disable SIMD for our kernel (not for applications running on top!).

How could the “applications running on top” use SIMD instructions/registers if the kernel doesn’t support/allow access to them?

The problem is that the core library is distributed together with the Rust compiler as a precompiled library. So it is only valid for supported host triples (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.

That’s where the build-std feature of cargo comes in. It allows to recompile core and other standard library crates on demand, instead of using the precompiled versions shipped with the Rust installation. This feature is very new and still not finished, so it is marked as “unstable” and only available on nightly Rust compilers.

The Rust compiler assumes that a certain set of built-in functions is available for all systems. Most of these functions are provided by the compiler_builtins crate that we just recompiled. However, there are some functions in that crate that are not enabled by default because they are normally provided by the C library on the system.

memcpy, memset, and memcmp; use rlibc as a replacement for now.

The easiest way to print text to the screen at this stage is the VGA text buffer. For printing “Hello World!”, we just need to know that the buffer is located at address 0xb8000 and that each character cell consists of an ASCII byte and a color byte.

The bootimage tool performs the following steps behind the scenes:

It compiles our kernel to an ELF file.
It compiles the bootloader dependency as a standalone executable.
It links the bytes of the kernel ELF file to the bootloader.

When booted, the bootloader reads and parses the appended ELF file. It then maps the program segments to virtual addresses in the page tables, zeroes the .bss section, and sets up a stack. Finally, it reads the entry point address (our _start function) and jumps to it.

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