Oopsie, linkerscript markdown thing exists

Signed-off-by: vanten-s <vanten-s@vanten-s.com>

Makefile

@@ -4,7 +4,6 @@ all: build
 
 build:
 	rm -rf build
-	rm -rf kernel/target
 	mkdir build
 	i686-elf-as -o build/boot.o boot.s
 	cd kernel; cargo build

kernel/.cargo/config.toml

@@ -4,6 +4,3 @@ target = "i686-unknown-bare.json"
 [unstable]
 build-std-features = ["compiler-builtins-mem"]
 build-std = ["core", "compiler_builtins"]
-
-[target."i686-unknown-bare"]
-linker = "i686-elf-gcc"

log/2024-03-26.md

@@ -0,0 +1,365 @@
+# Writing an OS in rust
+
+# Introduction
+I have tried multiple times to write an OS, but have always failed. Time to fail again :). I will be writing down my progress here, and maybe share it some day. I have previously used [osdev](https://osdev.org/) to learn and now I also use [OS in Rust](https://os.phil-opp.com/).
+
+# Cross-compilation
+The first step to making an OS is of course installing the correct compilers.
+
+## binutils
+Binutils is needed for linking and assembling. To install binutils I began by downloading the latest version from [sourceware](https://sourceware.org/pub/binutils/snapshots/) into the repository root directory.
+After that I ran these commands in the repository:
+```bash
+tar -xzf binutils-x.y.z.tar.xz # Extract the source code
+mv binutils-x.y.z binutils # Move source into a binutils directory
+cd binutils
+mkdir build-binutils # Create build directory
+../configure --target=i686-elf --prefix=../../opt --with-sysroot --disable-nls --disable-werror
+make # Compile
+make install # Install
+```
+
+## gcc
+Gcc is needed for eventual C code and as a frontend for the linker. To install gcc I began by downloading the source code from [mirrorservice](https://mirrorservice.org/sites/sourceware.org/pub/gcc/snapshots/) into the repository directory. After I downloaded gcc I ran these commands:
+```bash
+tar -xzf gcc-x.y.z.tar.xz # Extract the source code
+mv gcc-x.y.z gcc # Move source into a binutils directory
+cd gcc
+mkdir build-gcc # Create build directory
+../configure --target=i686-elf --prefix=../../opt --disable-nls --enable-languages=c,c++ --without-headers
+make all-gcc # Compile gcc
+make all-target-libgcc # Compile libgcc
+make install-gcc # Install gcc
+make install-target-libgcc # Install libgcc
+```
+In order to add gcc and binutils to `$PATH` I added this line to `shellHook` in `shell.nix`:
+```bash
+export PATH="$PATH:$PWD/opt/bin"
+```
+
+## rustup
+[Rustup](https://rustup.rs/) is needed to install the correct target. To make anyone able to replicate my toolchain, I used [nix](https://nixos.org). Following [this](https://nixos.wiki/wiki/Rust) tutorial, I got rustup installed. I also added
+```bash
+rustup component add rust-analyzer
+rustup component add rust-src
+```
+to `shellHook` to add the necessary components for IDEs and for compiling to custom targets.
+
+In the end, `shell.nix` looked like this:
+```nix
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell rec {
+	buildInputs = with pkgs; [
+      clang
+      llvmPackages_17.bintools
+      rustup
+	  qemu
+      grub2
+      libisoburn
+      bison
+      flex
+      gmp
+      libmpc
+      mpfr
+      texinfo
+      isl
+    ];
+    RUSTC_VERSION = "nightly"; # Required for some experimental cargo features
+    hardeningDisable = [ "all" ]; # Required to compile gcc
+    LIBCLANG_PATH = pkgs.lib.makeLibraryPath [ pkgs.llvmPackages_latest.libclang.lib ];
+	shellHook = ''
+      rustup component add rust-analyzer
+      rustup component add rust-src # Needed for compiling to a custom target
+      export PATH=$PATH:''${CARGO_HOME:-~/.cargo}/bin
+      export PATH=$PATH:''${RUSTUP_HOME:-~/.rustup}/toolchains/nightly-x86_64-unknown-linux-gnu/bin/
+      export PATH="$PATH:$PWD/opt/bin"
+      '';
+    # Add glibc, clang, glib, and other headers to bindgen search path
+    BINDGEN_EXTRA_CLANG_ARGS =
+    # Includes normal include path
+    (builtins.map (a: ''-I"${a}/include"'') [
+      # add dev libraries here (e.g. pkgs.libvmi.dev)
+      pkgs.glibc.dev
+    ])
+    # Includes with special directory paths
+    ++ [
+      ''-I"${pkgs.llvmPackages_latest.libclang.lib}/lib/clang/${pkgs.llvmPackages_latest.libclang.version}/include"''
+      ''-I"${pkgs.glib.dev}/include/glib-2.0"''
+      ''-I${pkgs.glib.out}/lib/glib-2.0/include/''
+    ];
+}
+```
+
+# Bootstrap assembly
+To boot the operating system you need a bootloader. For this project I will just be using grub. But to use grub, grub needs to know how to boot your OS. To tell this to grub you need to have a header in your binary file. In order to achieve this I made an assembly file named `boot.s` containing this:
+```asm
+/* Declare constants for the multiboot header. */
+.set ALIGN,    1<<0             /* align loaded modules on page boundaries */
+.set MEMINFO,  1<<1             /* provide memory map */
+.set FLAGS,    ALIGN | MEMINFO  /* this is the Multiboot 'flag' field */
+.set MAGIC,    0x1BADB002       /* 'magic number' lets bootloader find the header */
+.set CHECKSUM, -(MAGIC + FLAGS) /* checksum of above, to prove we are multiboot */
+
+/*
+Declare a multiboot header that marks the program as a kernel. These are magic
+values that are documented in the multiboot standard. The bootloader will
+search for this signature in the first 8 KiB of the kernel file, aligned at a
+32-bit boundary. The signature is in its own section so the header can be
+forced to be within the first 8 KiB of the kernel file.
+*/
+.section .multiboot
+.align 4
+.long MAGIC
+.long FLAGS
+.long CHECKSUM
+
+/*
+The multiboot standard does not define the value of the stack pointer register
+(esp) and it is up to the kernel to provide a stack. This allocates room for a
+small stack by creating a symbol at the bottom of it, then allocating 16384
+bytes for it, and finally creating a symbol at the top. The stack grows
+downwards on x86. The stack is in its own section so it can be marked nobits,
+which means the kernel file is smaller because it does not contain an
+uninitialized stack. The stack on x86 must be 16-byte aligned according to the
+System V ABI standard and de-facto extensions. The compiler will assume the
+stack is properly aligned and failure to align the stack will result in
+undefined behavior.
+*/
+.section .bss
+.align 16
+stack_bottom:
+.skip 16384 # 16 KiB
+stack_top:
+
+/*
+The linker script specifies _start as the entry point to the kernel and the
+bootloader will jump to this position once the kernel has been loaded. It
+doesn't make sense to return from this function as the bootloader is gone.
+*/
+.section .text
+.global _start
+.type _start, @function
+_start:
+	/*
+	The bootloader has loaded us into 32-bit protected mode on a x86
+	machine. Interrupts are disabled. Paging is disabled. The processor
+	state is as defined in the multiboot standard. The kernel has full
+	control of the CPU. The kernel can only make use of hardware features
+	and any code it provides as part of itself. There's no printf
+	function, unless the kernel provides its own <stdio.h> header and a
+	printf implementation. There are no security restrictions, no
+	safeguards, no debugging mechanisms, only what the kernel provides
+	itself. It has absolute and complete power over the
+	machine.
+	*/
+
+	/*
+	To set up a stack, we set the esp register to point to the top of the
+	stack (as it grows downwards on x86 systems). This is necessarily done
+	in assembly as languages such as C cannot function without a stack.
+	*/
+	mov $stack_top, %esp
+
+	/*
+	This is a good place to initialize crucial processor state before the
+	high-level kernel is entered. It's best to minimize the early
+	environment where crucial features are offline. Note that the
+	processor is not fully initialized yet: Features such as floating
+	point instructions and instruction set extensions are not initialized
+	yet. The GDT should be loaded here. Paging should be enabled here.
+	C++ features such as global constructors and exceptions will require
+	runtime support to work as well.
+	*/
+
+	/*
+	Enter the high-level kernel. The ABI requires the stack is 16-byte
+	aligned at the time of the call instruction (which afterwards pushes
+	the return pointer of size 4 bytes). The stack was originally 16-byte
+	aligned above and we've pushed a multiple of 16 bytes to the
+	stack since (pushed 0 bytes so far), so the alignment has thus been
+	preserved and the call is well defined.
+	*/
+	call kernel_main
+
+	/*
+	If the system has nothing more to do, put the computer into an
+	infinite loop. To do that:
+	1) Disable interrupts with cli (clear interrupt enable in eflags).
+	   They are already disabled by the bootloader, so this is not needed.
+	   Mind that you might later enable interrupts and return from
+	   kernel_main (which is sort of nonsensical to do).
+	2) Wait for the next interrupt to arrive with hlt (halt instruction).
+	   Since they are disabled, this will lock up the computer.
+	3) Jump to the hlt instruction if it ever wakes up due to a
+	   non-maskable interrupt occurring or due to system management mode.
+	*/
+	cli
+1:	hlt
+	jmp 1b
+
+/*
+Set the size of the _start symbol to the current location '.' minus its start.
+This is useful when debugging or when you implement call tracing.
+*/
+.size _start, . - _start
+```
+This file creates a multiboot header and calls `kernel_main`.
+
+To assemble this I used the GNU assembler we compiled earlier:
+
+`i686-elf-as -o boot.o boot.s`
+
+# Linking
+## What is it?
+In order to combine `boot.s` and the kernel we need to do something called "linking". This will combine all the object files into a raw binary. An object file is a like a half done executable. It will contain references in ascii to for example `printf`. This can't be ran directly, because it doesn't know where `printf` is. When you link the object file, it combines your object file with `stdio`, which contains `printf`.
+
+## For an operating system
+I have to combine `boot.s` and the kernel in a specific way to preserve the multiboot header. To do this I can create a *linker script* that tells the linker how to link the object files. To do this I created a file called `linker.ld` in the repository root, which contains this:
+```linkerscript
+/* The bootloader will look at this image and start execution at the symbol
+   designated as the entry point. */
+ENTRY(_start)
+ 
+/* Tell where the various sections of the object files will be put in the final
+   kernel image. */
+SECTIONS
+{
+	/* Start offset */
+	. = 2M;
+ 
+	/* First put the multiboot header, as it is required to be put very early
+	   in the image or the bootloader won't recognize the file format.
+	   Next we'll put the .text section. */
+	.text BLOCK(4K) : ALIGN(4K)
+	{
+		*(.multiboot)
+		*(.text)
+	}
+ 
+	/* Read-only data. */
+	.rodata BLOCK(4K) : ALIGN(4K)
+	{
+		*(.rodata)
+	}
+ 
+	/* Read-write data (initialized) */
+	.data BLOCK(4K) : ALIGN(4K)
+	{
+		*(.data)
+	}
+ 
+	/* Read-write data (uninitialized) and stack */
+	.bss BLOCK(4K) : ALIGN(4K)
+	{
+		*(COMMON)
+		*(.bss)
+	}
+ 
+	/* The compiler may produce other sections, by default it will put them in
+	   a segment with the same name. Simply add stuff here as needed. */
+}
+
+```
+I specified that I want to use this script in `i686-bare-metal.json`.
+
+# Kernel
+The last thing that `boot.s` does is that it calls `kernel_main`. This is defined in a rust file. To setup the rust project I used `cargo new kernel`. This creates an (not entirely) empty rust project called `kernel`. 
+
+## Target
+By default rust will compile to x86_64-unknown-linux-gnu, but to make it compile to x86 without an OS and with our own linker script I needed to define a custom target. In `kernel/i686-bare-metal.json` I put this
+```json
+{
+  "llvm-target": "i686-unknown-none",
+  "data-layout": "e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-i128:128-f64:32:64-f80:32-n8:16:32-S128",
+  "arch": "x86",
+  "target-endian": "little",
+  "target-pointer-width": "32",
+  "target-c-int-width": "32",
+  "os": "none",
+  "executables": true,
+  "linker-flavor": "gcc",
+  "linker": "i686-elf-gcc",
+  "panic-strategy": "abort",
+  "disable-redzone": true,
+  "features": "-sse,+soft-float",
+  "dynamic-linking": false,
+  "relocation-model": "pic",
+  "code-model": "kernel",
+  "exe-suffix": ".elf",
+  "has-rpath": false,
+  "no-default-libraries": true,
+  "position-independent-executables": false,
+  "pre-link-args": {
+    "gcc": ["-T", "../linker.ld", "-ffreestanding", "-nostdlib", "-lgcc", "../build/boot.o"]
+  }
+}
+```
+This tells the compiler to compile to 32-bit x86:
+```json
+"llvm-target": "i686-unknown-none",
+"arch": "x86",
+"target-pointer-width": "32",
+"target-c-int-width": "32",
+```
+To use gcc for linking:
+```json
+"linker-flavor": "gcc", # Tells the compiler that we're using gcc
+"linker": "i686-elf-gcc", # Tells the compiler what gcc executable we want to use
+```
+And this tells rust to link this with `boot.s`, without a standard library, freestanding and using my build script `../../linker.ld`.
+
+To use this custom target I needed to create a cargo configuration. To do this I wrote this in `kernel/.cargo/config.toml`
+```toml
+[build]
+target = "i686-unknown-bare.json" # Use the custom target
+
+[unstable]
+build-std-features = ["compiler-builtins-mem"] # To use a custom target, I need to manually compile the standard library.
+build-std = ["core", "compiler_builtins"]
+```
+
+## Code
+Just to test the kernel I put this in `kernel/src/main.rs`
+```rust
+#![no_std] // Don't compile with std
+#![no_main] // Don't compile with a main function
+
+#[no_mangle]
+fn kernel_main() {
+    let vga_buffer = 0xb8000 as *mut u8; // Make a pointer to address 0xB8000
+
+    let string: &[u8] = b":3"; // Define a string to print
+    for (i, &byte) in string.iter().enumerate() { // For every character in the string
+        unsafe {
+            *vga_buffer.offset(i as isize * 2) = byte; // Print the character
+            *vga_buffer.offset(i as isize * 2 + 1) = 0xf; // Set the color to white
+        }
+    }
+
+
+    loop {} // End of program
+}
+ 
+#[panic_handler]
+fn panic(_info: &core::panic::PanicInfo) -> ! {
+    loop {}
+}
+```
+
+# Building the kernel
+To build the kernel I just run `cargo build` in the `kernel` directory. This will produce a binary called `kernel/target/i686-bare-metal/debug/kernel.elf`. I now have a multiboot enabled kernel :)
+
+# File structure
+```
+geos/
+| opt/
+| | gcc and binutils binaries and lib.
+| kernel/
+| | rust project
+| gcc/
+| | gcc source code
+| binutils
+| | binutils source code
+```
+

shell.nix

@@ -1,8 +1,6 @@
 { pkgs ? import <nixpkgs> {} }:
 
-let
-  crossPkgs = pkgs.pkgsCross.aarch64-multiplatform;
-in pkgs.mkShell rec {
+pkgs.mkShell rec {
 	buildInputs = with pkgs; [
       clang
       llvmPackages_17.bintools
@@ -18,23 +16,16 @@ in pkgs.mkShell rec {
       texinfo
       isl
     ];
-    RUSTC_VERSION = "nightly";
-    hardeningDisable = [ "all" ];
-    # https://github.com/rust-lang/rust-bindgen#environment-variables
+    RUSTC_VERSION = "nightly"; # Required for some experimental cargo features
+    hardeningDisable = [ "all" ]; # Required to compile gcc
     LIBCLANG_PATH = pkgs.lib.makeLibraryPath [ pkgs.llvmPackages_latest.libclang.lib ];
 	shellHook = ''
-      rustup default nightly
-	  rustup component add rust-analyzer
-      rustup component add rust-src
-	  rustup target add x86_64-unknown-none
+      rustup component add rust-analyzer
+      rustup component add rust-src # Needed for compiling to a custom target
       export PATH=$PATH:''${CARGO_HOME:-~/.cargo}/bin
       export PATH=$PATH:''${RUSTUP_HOME:-~/.rustup}/toolchains/nightly-x86_64-unknown-linux-gnu/bin/
-      export PATH=$PATH:$PWD/opt/bin
+      export PATH="$PATH:$PWD/opt/bin"
       '';
-    # Add precompiled library to rustc search path
-    RUSTFLAGS = (builtins.map (a: ''-L ${a}/lib'') [
-      # add libraries here (e.g. pkgs.libvmi)
-    ]);
     # Add glibc, clang, glib, and other headers to bindgen search path
     BINDGEN_EXTRA_CLANG_ARGS = 
     # Includes normal include path