blog_os/_drafts/rust-setup.md

layout, title

layout	title
post	Setup Rust in small steps

In the last posts we created a [minimal Multiboot kernel][multiboot post] and [switched to Long Mode][long mode post]. Now we can finally switch to sweet Rust code. [Rust] is a beautiful high-level language that has no 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 setup 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.

[multiboot post]: {% post_url 2015-08-18-multiboot-kernel %} [long mode post]: #TODO [Rust]: https://www.rust-lang.org/ [file an issue]: https://github.com/phil-opp/phil-opp.github.io/issues [Github repository]: https://github.com/phil-opp/blogOS/tree/rust_setup

Rust Setup

We need a nightly compiler, as we need to use many unstable features. To manage Rust installations I highly recommend multirust by brson. It allows you to install nightly, beta, and stable compilers side-by-side and makes it easy to update them. After installing you can just run multirust update nightly to install or update to the latest Rust nightly.

Creating a Rust project

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 will do it manually. Fortunately we just 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 basic project metadata and is identical to the Cargo.toml created by cargo new blog_os. The lib section specifies that we want to build a static library, i.e. a library that contains all of its dependencies.

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

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

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

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

Let's break it down:

• #! defines an attribute of the current module. Since we are at the root module, they apply to the crate itself.
• The features attribute is used to allow the specified feature-gated attributes in this crate. You can't do that in a stable/beta compiler, so this is one reason we need a Rust nighly.
• The no_std attribute prevents the automatic linking of the standard library. We can't use std because it relies on operating system features like files, system calls, and various device drivers. Remember that currently the only “feature” of our OS is printing OKAY :).
• A # without a ! afterwards defines an attribute for the following item (a function in our case).
• The no_mangle attribute disables the automatic name mangling that Rust uses to get unique function names. We want to do a call main from our assembly code, so this function name must stay as it is.
• We mark our main function as extern to make it compatible to the standard C calling convention.
• The lang attribute defines a Rust language item.
• The eh_personality function is used for Rust's unwinding on panic!. We can leave it empty since we don't have any unwinding support in our OS yet.
• The panic_fmt function is the entry point on panic. Right now we can't do anything useful, so we just make sure that it doesn't return (required by the ! return type).

Building Rust

We can now build it using cargo build. It creates a static library at target/debug/libblog_os.a that we can link with our assembly kernel. Let's extend our Makefile to do that. We add a new .PHONY target cargo and modify the $(kernel) target to link the created static lib (full file):

# ...
rust_os := target/debug/libblog_os.a
# ...
$(kernel): cargo $(rust_os) $(assembly_object_files) $(linker_script)
       @ld -n -T $(linker_script) -o $(kernel) $(assembly_object_files) $(rust_os)

cargo:
       @cargo build

Now cargo 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 cargo 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 it. 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 main     ; new
    call main       ; new

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

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

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

Testing

Let's play around with some Rust code:

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

When we test it using make run, it fails with undefined reference to 'memcpy'. This function is one of the basic functions of the C library (libc). Usually the libc crate is linked to every Rust program with the standard library but we opted out through #![no_std]. So 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 like memcpy, memmove, memset, and memcmp in Rust.

rlibc

Fortunately there already is a crate that does just 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. So let's add rlibc to our crate. We need to add a crates.io dependency in our Cargo.toml:

...
[dependencies]
rlibc = "*"

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

...
extern crate rlibc;

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

Now make run doesn't complain about memcpy anymore. Instead it will show a pile of fmod and fmodf errors. These functions are used for the modulo operation (%) on floating point numbers in libcore. The core library is added implicitly when using #![no_std] and provides basic standard library features like Option or Iterator. According to the documentation it is “dependency-free” but it actually has some dependencies, for example on fmod and fmodf.

--gc-sections

So how do we fix this problem? We don't use any floating point operations, so we could just provide our own implementations of fmod and fmodf that just do a loop{}. But there's a better way that doesn't fail silently when we use floats some day: We tell the linker to remove unused sections. That's generally a good idea as it reduces kernel size. And we don't have any references to fmod and fmodf anymore until we use floating point modulo. The magic linker flag is --gc-sections which stands for “garbage collect sections”. Let's add it to the $(kernel) target in our Makefile:

$(kernel): cargo $(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 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 that it should keep this section. We can do through the KEEP command in our linker.ld:

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

No everything should work (the green OKAY) again.

Unfortunately there is one problem left that gets triggered by the following code:

let mut a = 42;
a += 1;

When we add that code to main and test it using make run, the OS will constantly reboot itself. Let's try to debug it.

Debugging

Such a boot loop is most likely caused by some CPU exception. When these exceptions aren't handled, a Triple Fault occurs and the processor resets itself. We can look at generated CPU interrupts/exceptions using QEMU:

> qemu-system-x86_64 -d int -no-reboot -hda build/os-x86_64.iso
SMM: enter
...
SMM: after RSM
...
check_exception old: 0xffffffff new 0x6
     0: v=06 e=0000 i=0 cpl=0 IP=0008:0000000000100200 pc=0000000000100200
     SP=0010:0000000000102fd0 env->regs[R_EAX]=0000000080010010
...
check_exception old: 0xffffffff new 0xd
     1: v=0d e=0062 i=0 cpl=0 IP=0008:0000000000100200 pc=0000000000100200
     SP=0010:0000000000102fd0 env->regs[R_EAX]=0000000080010010
...
check_exception old: 0xd new 0xd
     2: v=08 e=0000 i=0 cpl=0 IP=0008:0000000000100200 pc=0000000000100200
     SP=0010:0000000000102fd0 env->regs[R_EAX]=0000000080010010
...
check_exception old: 0x8 new 0xd

Let me first explain the QEMU arguments: The -d int logs CPU interrupts to the console and the -no-reboot flag closes QEMU instead of constant rebooting. But what does the cryptical output mean? I already removed most of it as we don't need it here. Let's break down the rest:

• The first two blocks, SMM: enter and SMM: after RSM are created before our OS boots, so we just ignore them.
• The next block, check_exception old: 0xffffffff new 0x6 is the interesting one. It says: “a new CPU exception with number 0xe occurred“.
• The last blocks indicate further exceptions. They were thrown because we didn't handle the 0x6 exception, so we're going to ignore them, too.

So let's look at the first exception: old:0xffffffff means that the CPU wasn't handling an interrupt when the exception occurred. The register dump tells us that the current instruction was 0x100200 (in IP (instruction pointer) or pc (program counter)). By looking at an exception table we learn that the number 0x6 indicates a Invalid Opcode fault. So the instruction at 0x100200 seems to be invalid. Let's look at it using objdump:

> objdump -D build/kernel-x86_64.bin | grep "100200:"
100200:	0f 28 05 49 01 00 00 	movaps 0x149(%rip),%xmm0 ...

Through objdump -D we disassemble our whole kernel and grep picks the relevant line. The instruction at 100200 seems to be a valid movaps instruction. It's a SSE instruction that moves 128 bit between memory and SSE-registers (e.g. xmm0). But why the Invalid Opcode exception? The answer is hidden behind the movaps link: The section Protected Mode Exceptions lists the conditions for the various exceptions. The short code of the Invalid Opcode is #UD, so the exceptions occurs:

For an unmasked Streaming SIMD Extensions 2 instructions numeric exception (CR4.OSXMMEXCPT =0). If EM in CR0 is set. If OSFXSR in CR4 is 0. If CPUID feature flag SSE2 is 0.

The rough translation of this cryptic low-level code is: If SSE isn't enabled. So apparently Rust uses SSE instructions by default and we didn't enable SSE before. Let's fix this:

Enabling SSE

To enable SSE we need to return to assembly. We need to add a function that checks if SSE is available and enables it then. Else we want to print an error message. But we can't use our existing error function because it uses (now invalid) 32-bit instructions. So we need a new one (in long_mode_init.asm):

; Prints `ERROR: ` and the given error code to screen and hangs.
; parameter: error code (in ascii) in al
error:
    mov rbx, 0x4f4f4f524f524f45
    mov [0xb8000], rbx
    mov rbx, 0x4f204f204f3a4f52
    mov [0xb8008], rbx
    mov byte [0xb800e], al
    hlt
    jmp error

It's the nearly the same as the 32-bit code in the last post (instead of ERR: we print ERROR: here). Now we can add a function that checks for SSE and enables it:

; Check for SSE and enable it. If it's not supported throw error "a".
setup_SSE:
    ; check for SSE
    mov rax, 0x1
    cpuid
    test edx, 1<<25
    jz .no_SSE

    ; enable SSE
    mov rax, cr0
    and ax, 0xFFFB      ; clear coprocessor emulation CR0.EM
    or ax, 0x2          ; set coprocessor monitoring  CR0.MP
    mov cr0, rax
    mov rax, cr4
    or ax, 3 << 9       ; set CR4.OSFXSR and CR4.OSXMMEXCPT at the same time
    mov cr4, rax

    ret
.no_SSE:
    mov al, "a"
    jmp error

Notice that we set/unset exactly the bits that can cause the Invalid Opcode exception. Now we can insert a call setup_SSE right before calling main and our Rust code will finally work. TODO _Unwind_Resume

“OS returned!”

Now that we're editing assembly anyway, we should change the OKAY message to something more meaningful. My suggestion is a red OS returned!:

...
call main

.os_returned:
    ; rust main returned, print `OS returned!`
    mov rax, 0x4f724f204f534f4f
    mov [0xb8000], rax
    mov rax, 0x4f724f754f744f65
    mov [0xb8008], rax
    mov rax, 0x4f214f644f654f6e
    mov [0xb8010], rax
    hlt

Testing

• it works now
• provocate stack overflow -> increase stack