rust-kernel

# rust-kernel

A tiny hobby x86_64 kernel written in Rust, following the 2nd edition of
Philipp Oppermann's [Writing an OS in Rust](https://os.phil-opp.com/).
Right now it boots in QEMU, prints to the VGA text screen and the serial port
(COM1), handles CPU exceptions (breakpoint, page fault, double fault on its own
stack), runs the timer interrupt (uptime clock top-right), echoes what you
type on the PS/2 keyboard, walks and edits its own page tables, and has a
kernel heap (`Box`, `Vec`, `Rc` work) backed by a fixed-size-block allocator.
It multitasks two ways: an **async/await executor** (cooperative tasks; the
keyboard is one of them) and **preemptive kernel threads** switched by the timer
interrupt (threads A/B/C count on the top line while you type).
It runs **user programs in ring 3** (small Rust ELF binaries) as processes
with their own address spaces, which talk to the kernel via `syscall`
(`read`, `write`, `open`, `close`, `getpid`, `sched_yield`, `exit`) and read
files from an in-memory filesystem behind a Linux-style **VFS**.
It has a `cargo test` suite that runs inside QEMU.

Most code follows the blog_os sources (MIT / Apache-2.0) closely, with extra
comments; additions are the serial mirroring, uptime clock, demo features,
scripts, and everything past chapter 12: preemptive threads, user mode,
syscalls, ELF loading and the VFS.

![boot screenshot](boot.png)

## What's installed (on this box)

| Tool | Version | Why |
|---|---|---|
| rustup | 1.29.2 | toolchain manager (in `~/.cargo/bin`, ahead of Debian's `/usr/bin/cargo`) |
| Rust nightly | rustc 1.101.0-nightly (2026-10-29) | custom JSON targets + `build-std` are nightly-only |
| `rust-src` component | – | source of `core`, recompiled for our bare-metal target |
| `llvm-tools-preview` component | – | `llvm-objcopy` etc., used by `bootimage` |
| `bootimage` (cargo subcommand) | 0.10.5 | links kernel + bootloader into a bootable disk image |
| QEMU (`qemu-system-x86`, `qemu-system-gui`) | 10.0.13 | the emulator we boot in (`-gui` adds the GTK/SDL window) |

`rust-toolchain.toml` pins the project to nightly with the two components, so
`cargo` inside this directory picks them up automatically. It also installs
the built-in `x86_64-unknown-none` target, used to compile the user programs.

Crates: `bootloader` 0.9.35 (BIOS bootloader used by the 2nd edition),
`volatile` 0.2, `spin` 0.5, `x86_64` 0.15, `uart_16550` 0.3, `lazy_static` 1.5,
`pic8259` 0.11 (interrupt controller), `pc-keyboard` 0.7 (scancode decoding),
`linked_list_allocator` 0.9 (fallback heap allocator).

Fresh setup on another machine:

```sh
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh   # rustup
rustup toolchain install nightly -c rust-src -c llvm-tools-preview -t x86_64-unknown-none
cargo install bootimage
sudo apt install qemu-system-x86 qemu-system-gui
```

## Build and run

```sh
cargo bootimage                     # build -> target/x86_64-rust_kernel/debug/bootimage-rust_kernel.bin
cargo run                           # build + boot in a QEMU window; serial output also prints in the terminal
```

**Trying the keyboard:** with `cargo run`, click into the QEMU window and type;
characters appear on the bottom line of the screen *and* in your terminal (via
serial). Press Enter and the `echo` user program reads the line with the
`read` syscall and prints `[echo] you typed: ...` (type `exit` to stop it). Shift works; Enter starts a new line; arrows/F-keys show as e.g.
`<ArrowUp>`; Backspace erases the previous character (on the current line). To get the mouse/keyboard back from QEMU press Ctrl+Alt+G; close the
window or press Ctrl+C in the terminal to quit. (Over SSH without X, use
`cargo run -- -display curses` to see the VGA screen right in your terminal;
quit from another terminal with `pkill qemu-system`.)

Headless (no window, good for SSH / scripts):

```sh
./scripts/run-headless.sh                 # boot, breakpoint, demos, ~5 s of threads + timer ticks, then QEMU exits (script exit 0)
./scripts/run-headless.sh stack-overflow  # deliberately overflow the stack; passes if the double-fault handler catches it
./scripts/run-headless.sh page-fault      # write to unmapped 0xdeadbeaf; passes if the page-fault handler catches it
cargo run --features headless -- -display none                 # same as the first, via cargo
cargo run --features headless,stack-overflow -- -display none  # same as the second
                                          # (QEMU's exit status is 33 = success; cargo just reports it as non-zero)
cargo run -- -display none                # headless but kernel keeps running; Ctrl+C to stop
./scripts/screenshot.sh [out.png] [text] [expected]
                                          # boot, "type" text via QEMU monitor `sendkey` ("<" = Backspace,
                                          # "|" = Enter; default "Hello, kernex<l!|"), check serial shows the
                                          # expected result ("[echo] you typed: Hello, kernel!"), save the
                                          # VGA screen to boot.png; exits 1 if the text is missing
```

Cargo features (in `Cargo.toml`):

| Feature | Effect |
|---|---|
| `headless` | print a `.` per timer tick to serial and exit QEMU after 90 ticks (~5 s); panics exit with failure |
| `stack-overflow` | after boot, recurse forever to test the double-fault handler (with `headless`: exit success when caught) |
| `page-fault` | after reading CR3, write to unmapped address `0xdeadbeaf` (with `headless`: exit success when caught) |

Expected serial output of `./scripts/run-headless.sh` (stack frame dump trimmed):

```
Hello from Tyler's Rust kernel!
EXCEPTION: BREAKPOINT
InterruptStackFrame { instruction_pointer: ..., ... }
Survived the breakpoint, still running.
[paging] CR3 (level 4 table) at phys 0x1000
... (paging + memory lines as before)
[paging] mapped page 0xdeadbeaf000 -> VGA frame, wrote "New!" through it
.[heap] 256 KiB heap at 0x444444440000 (fixed-size-block allocator)
[heap] Box::new(41) = 41 at 0x444444440000
[heap] Vec of 500 numbers at 0x444444441000, sum = 124750
[heap] Rc count after clone = 2
[heap] Rc count after drop  = 1
[thread] spawned threads A, B, C (status on the top line)
.[thread A] count 1
.[thread B] count 1
..[vfs] mounted ramfs at /
[vfs] mounted ramfs at /tmp
[process] started pid 1 (hello), entry 0x8000400180
[process] started pid 2 (evil), entry 0x80004000f0
[process] started pid 3 (spin), entry 0x80004000e0
[process] started pid 4 (echo), entry 0x8000400180
Type a line + Enter: the echo program (ring 3) reads it
[async] async number: 42
[async] example task step 1 (after yield)
[async] example task step 2 (after yield)
[async] example task step 3 (after yield)
....Hello from user mode! I'm pid 1.
[hello] /hello.txt says: Hello from the ramfs! This is /hello.txt.
[hello] wrote and read back /tmp/note.txt
[hello] all syscall checks passed, exiting with 0
[process] pid 1 (hello) exited with code 0
[evil] pid 2: reading kernel memory at 0x200000...
[kernel] user page fault: address Ok(VirtAddr(0x200000)), error PageFaultErrorCode(PROTECTION_VIOLATION | USER_MODE)
[kernel] page fault in user mode at rip 0x800040017e
[process] pid 2 (evil) killed: page fault
[spin] pid 3: spinning without syscalls...
.[echo] pid 4: type a line and press Enter
..[thread A] count 2
.[thread B] count 2
.....[thread A] count 3
. 18 ticks
.....[thread A] count 4
.[thread B] count 3
.[thread C] busy loop round 1 (count = 2000000)
.[spin] round 1 done (count = 30000000)
...[thread A] count 5
....... 36 ticks
[thread B] count 4
..[spin] round 2 done (count = 60000000)
..........[thread B] count 5
.[thread C] busy loop round 2 (count = 4000000)
.[spin] round 3 done (count = 90000000)
[process] pid 3 (spin) exited with code 0
.... 54 ticks
.................. 72 ticks
..[thread C] busy loop round 3 (count = 6000000)
...............[thread C] busy loop round 4 (count = 8000000)
. 90 ticks
[ok] timer works, exiting
```

Each `.` is a timer tick. Threads A and B only print their first 5 counts to
serial (so the keyboard echo in your terminal stays readable); after that they
keep counting on the VGA top line. (Exact interleaving and dot counts vary a
little from run to run -- that's preemption at work.) Spin's rounds interleave
with threads A/B/C: that's preemption of ring-3 code by the timer.

## Tests

```sh
cargo test                        # everything (lib unit tests + tests/*.rs), each booted in QEMU headless
cargo test --lib                  # just the unit tests inside src/
cargo test --test heap_allocation # one integration test
```

Each test binary is its own little kernel: `bootimage runner` boots it with the
`test-args` from `Cargo.toml` (no display, serial to stdout, isa-debug-exit) and
treats QEMU exit code 33 as a pass. Output looks like:

```
Running 5 tests
heap_allocation::large_allocation_uses_fallback...  [ok]
heap_allocation::large_vec...                       [ok]
heap_allocation::many_boxes...                      [ok]
heap_allocation::many_boxes_long_lived...           [ok]
heap_allocation::simple_allocation...               [ok]
```

| Test | What it checks |
|---|---|
| lib (`src/*.rs` `#[test_case]`s) | trivial assertion, breakpoint handler returns, VGA `println!` writes the right cells |
| `tests/basic_boot.rs` | printing works straight after boot, before any `init()` |
| `tests/heap_allocation.rs` | Box, big Vec, 100 K short-lived boxes (memory reuse), same with one long-lived box, >2 KiB fallback allocation |
| `tests/should_panic.rs` | a failing `assert_eq!` reaches the panic handler (`harness = false`) |
| `tests/stack_overflow.rs` | stack overflow lands in the double-fault handler on the IST stack (`harness = false`) |
| `tests/executor.rs` | `SimpleExecutor` runs an async fn to completion; the waker-based `Executor` interleaves two tasks that `yield_now().await` (a1 b1 a2 b2 a3 b3) |
| `tests/user_mode.rs` | `hello` exits 0 (all its syscall checks pass) and its `/tmp/note.txt` is visible to the kernel; `evil` is `Killed("page fault")`; `spin` finishes while the test thread keeps running (preemption of ring 3); the ELF loader rejects garbage/truncated files; user pointers to kernel/unmapped/overflowing ranges give `EFAULT` |
| `tests/vfs.rs` | read `/hello.txt` to EOF, path normalisation, `ENOENT`/`ENOTDIR`/`EINVAL`/`EISDIR`/`EEXIST`, `/tmp` is a separate ramfs, a third mount wins by longest prefix, sparse writes zero-fill, directory listing |
| `tests/threads.rs` | two busy-looping threads that never yield both make progress while the main thread busy-waits too (only possible with preemption); a thread can exit and the others keep running |

How it works: `#![feature(custom_test_frameworks)]` collects every `#[test_case]`
function and passes them to `rust_kernel::test_runner` (in `src/lib.rs`), which
prints each name to serial, runs it, prints `[ok]`, then exits QEMU with success.
A panic goes to `test_panic_handler`, which prints `[failed]` and exits with failure.

Extra args after `--` in `cargo run -- ...` are appended to the QEMU command
line, e.g. `cargo run -- -d int -no-reboot` to log every interrupt and stop on
triple faults, or `cargo run -- -s -S` to wait for `gdb` on port 1234.

## Project layout

```
.cargo/config.toml        build-std (core, alloc) + custom target + scripts/runner.sh as the cargo runner
rust-toolchain.toml       pin nightly + rust-src + llvm-tools-preview
x86_64-rust_kernel.json   custom target: no OS, no red zone, soft-float, abort on panic, rust-lld
Cargo.toml                deps, features, test settings, QEMU run/test args
src/lib.rs                the kernel library: module list, init(), kprintln!, test framework, exit_qemu
src/main.rs               the bootable binary: entry_point!(kernel_main), paging + heap demos, panic handler
src/gdt.rs                GDT (kernel + user segments) + TSS (double-fault IST stack, rsp0 per thread)
src/interrupts.rs         IDT; breakpoint / page fault / GPF / invalid opcode / double fault handlers
                          (faults from ring 3 kill the process); 8259 PIC;
                          timer handler (uptime, ticks, calls the thread scheduler),
                          keyboard handler (just queues the scancode)
src/memory.rs             OffsetPageTable setup from CR3, example mapping, BootInfoFrameAllocator,
                          global frame allocator, per-process address spaces, user pointer checks
src/allocator.rs          heap location/size, init_heap (maps heap pages), #[global_allocator], Locked<A>
src/allocator/bump.rs     bump allocator (not active, for learning)
src/allocator/fixed_size_block.rs  fixed-size-block allocator (ACTIVE), falls back to linked_list_allocator
src/task/mod.rs           async: Task (pinned boxed future + id), yield_now()
src/task/simple_executor.rs  first executor: polls every task in a loop (no wakers)
src/task/executor.rs      real executor: task queue + wakers, sleeps with hlt when idle
src/task/keyboard.rs      lock-free scancode queue, ScancodeStream, async keyboard echo task
src/thread.rs             preemptive threads: spawn, round-robin schedule(), asm context switch,
                          per-thread kernel stack (TSS.rsp0) + CR3 switching
src/syscall.rs            syscall/sysret MSR setup, asm entry stub, dispatch, read/write/open/close/...
src/process.rs            Process (pid, fd table, exit status), spawn from ELF, iretq into ring 3, kill
src/elf.rs                minimal ELF64 loader (PT_LOAD segments -> user pages)
src/errno.rs              Linux errno numbers
src/tty.rs                keyboard line buffer + Console (the File behind fds 0/1/2)
src/fs/mod.rs             VFS: Inode + FileSystem traits, mount table, path lookup, init (boot files)
src/fs/file.rs            File trait (what an fd points to), InodeFile (inode + offset)
src/fs/ramfs.rs           in-memory filesystem
build.rs                  compiles user/*.rs to ELF files in OUT_DIR
user/ulib.rs              tiny user-space "libc": syscall wrappers, println!, _start, panic handler
user/link.ld              linker script for user programs (base 0x80_0040_0000)
user/{hello,evil,spin,echo}.rs  the user programs
src/vga_buffer.rs         VGA text-mode writer, print!/println!, print_at (status line), backspace
src/serial.rs             COM1 UART writer + serial_print!/serial_println!
tests/                    integration tests, one QEMU-booted kernel each
scripts/runner.sh         `cargo run`/`cargo test` runner: `bootimage runner` + stale-image fix
scripts/run-headless.sh   headless boot with timeout + exit-code check
scripts/screenshot.sh     headless boot + typed keys + PNG screenshot via QEMU monitor
boot.png                  screenshot of the VGA screen
```

## What the interrupt code does (chapters 5-7)

- **CPU exceptions / IDT** (`interrupts.rs`): the IDT maps vector numbers to handler
  functions written with `extern "x86-interrupt"` (nightly `abi_x86_interrupt`), so
  the compiler saves all registers and returns with `iretq`. `_start` fires an
  `int3` on purpose; the breakpoint handler prints the stack frame and execution
  continues. A page-fault handler prints the faulting address from CR2.
- **Double faults** (`gdt.rs`): a stack overflow hits the guard page -> page fault ->
  the CPU can't push the exception frame on the broken stack -> double fault ->
  same again -> *triple fault* = machine reset. We load our own GDT containing a
  TSS whose Interrupt Stack Table entry 0 points at a separate 20 KiB static stack,
  and mark the double-fault IDT entry to use it. Try `./scripts/run-headless.sh stack-overflow`.
- **Hardware interrupts**: the two 8259 PICs are remapped to vectors 32-47 (0-31 are
  CPU exceptions) and then `sti` enables interrupts. The timer (IRQ 0, ~18.2 Hz)
  bumps an atomic tick counter and redraws the uptime clock; the keyboard (IRQ 1)
  reads the scancode from port 0x60 and queues it; an async task decodes it with
  `pc-keyboard` (US layout, scancode set 1) and echoes it (see Multitasking below). Each handler must send End-Of-Interrupt to the
  PIC or no further interrupts arrive. `print!`/`serial_print!` disable interrupts
  while holding their lock, otherwise an interrupt handler printing while
  `_start` holds the lock would deadlock. The idle loop uses `hlt`, so QEMU
  doesn't burn a host CPU core.

## Memory (chapters 8-11)

**Paging.** Every address the kernel uses is *virtual*. The CPU translates it with
a 4-level page table tree (level 4 -> 3 -> 2 -> 1 -> 4 KiB page); 9 bits of the
address index each level, the low 12 bits are the offset in the page. CR3 holds the
*physical* address of the level-4 table (here `0x1000`). The `page-fault` feature
writes to `0xdeadbeaf`, which no table maps, and the page-fault handler reports
the address (from CR2) and the reason (`CAUSED_BY_WRITE`, not `PROTECTION_VIOLATION`
-> the page simply isn't present).

**Reaching page tables.** Page tables contain physical addresses, but we can only
dereference virtual ones. With the bootloader's `map_physical_memory` feature, all of
physical RAM is also mapped at `physical_memory_offset` (`0x18000000000` here), so
`virt = offset + phys`. `memory::init` wraps that in the x86_64 crate's
`OffsetPageTable`, which can `translate_addr` (walk the tables) and `map_to` (add entries).

**Virtual memory layout** (as set up by bootloader 0.9 in this build):

| Virtual address | What |
|---|---|
| `0xb8000` | VGA text buffer (identity-mapped: virt = phys) |
| `0x200000`... | kernel code + data (the ELF; maps to phys around `0x400000`) |
| `0x0100_0000_0000`... | kernel stack (80 pages, guard page below it) |
| `0x0180_0000_0000`... | all physical memory (`physical_memory_offset`) |
| `0x4444_4444_0000` | kernel heap, 256 KiB (also holds thread stacks) (we map it in `allocator::init_heap`) |
| `0xdead_beaf_000` | demo page we map to the VGA frame (writes "New!" on screen) |

The "New!" demo writes to screen row 20 through the new mapping; by the time you look,
later lines have scrolled it up, so in `boot.png` it overwrote the start of the first line.

**Physical frames.** `BootInfoFrameAllocator` hands out 4 KiB frames from the regions
the BIOS memory map marks `Usable` (~121 MiB of QEMU's default 128 MiB). It never frees;
fine for now.

**Heap.** `allocator::init_heap` maps the 25 pages of `HEAP_START..+HEAP_SIZE` to fresh
frames, then gives the range to the `#[global_allocator]`. From then on the `alloc`
crate (`Box`, `Vec`, `Rc`, `String`, `BTreeMap`...) works. `alloc` is compiled for our
target by `build-std` in `.cargo/config.toml`.

**Allocator designs** (`src/allocator/`): the *bump* allocator just moves a pointer
forward and can only reclaim memory when every allocation is freed (it fails the
`many_boxes_long_lived` test). The `linked_list_allocator` crate keeps a list of free
regions (first fit, slower, but reuses any freed memory). The active
*fixed-size-block* allocator rounds each request up to 8/16/.../2048 bytes and keeps
one free list per size, so alloc/free are O(1) pops/pushes; bigger requests and empty
lists fall back to the linked-list allocator. Switch allocators by editing one line in
`src/allocator.rs`.

## Multitasking (async/await + threads)

### Cooperative: async/await (`src/task/`, phil-opp chapter 12)

An `async fn` compiles into a state machine (a `Future`). An *executor* calls
`poll` on it; the future runs until it has to wait, then returns `Pending` and
remembers a `Waker`. Whoever makes progress possible later (here: the keyboard
interrupt) calls `waker.wake()`, which puts the task's id back into the
executor's queue. When the queue is empty, the executor `hlt`s -- carefully:
it disables interrupts, checks the queue, and then uses `sti; hlt` as one step
(`enable_and_hlt`), so a wakeup arriving in between can't be missed.

The keyboard interrupt handler now does almost nothing: read the scancode from
port 0x60 and push it into a fixed-size lock-free queue (`crossbeam_queue::ArrayQueue`),
then wake the keyboard task (`futures_util::task::AtomicWaker`). No locks in the
interrupt handler means no deadlock if the interrupt arrives while the task
holds one. The task (`print_keypresses`) is an `async` loop over a
`ScancodeStream` that decodes and echoes characters.

"Cooperative" means a task only gives up the CPU at an `.await`. A task that
loops forever without awaiting freezes every other task. Upside: switching is
cheap (just return from `poll`), and all tasks share one stack.

### Preemptive: kernel threads (`src/thread.rs`, beyond the blog series)

Each thread has its own 16 KiB stack (a `Vec<u8>` on the heap). On every timer
interrupt (~18 times a second) the handler sends the EOI to the PIC and then
calls `thread::schedule()`, which picks the next thread round-robin and calls
`switch_context(&mut old.rsp, new.rsp)` (a dozen lines of `global_asm!`):

```
push rbp, rbx, r12, r13, r14, r15   ; callee-saved regs of the OLD thread, onto its stack
mov  [rdi], rsp                     ; remember where the old stack ends
mov  rsp, rsi                       ; ---- now on the NEW thread's stack ----
pop  r15, r14, r13, r12, rbx, rbp   ; its callee-saved regs
ret                                 ; return to wherever IT called switch_context
```

Why only those six registers? `switch_context` is called like a normal
function, so the compiler already saved the caller-saved registers, and the
`x86-interrupt` handler around it saved everything the interrupted code was
using (plus the CPU pushed RIP/RSP/RFLAGS). So a paused thread's complete
state sits on its own stack; the scheduler only stores its `rsp`. The new
thread "returns" from `schedule()`, from its timer handler, and `iretq`s back to
where it was interrupted.

A brand-new thread was never interrupted, so `spawn` hand-builds a stack that
looks as if `switch_context` had saved it: six register slots (r12 = the
thread's function) and a return address pointing to `thread_trampoline`, which
moves r12 into rdi and calls `thread_start(entry)`. That re-enables interrupts
(we arrive from inside the timer interrupt, where they're off), runs the
function, and marks the thread finished when it returns. The code that booted
the kernel becomes thread 0, "main", which runs the async executor.

Preemption means even `loop {}` can't hog the CPU (thread C in the demo is a
pure busy loop). The price: any code can be interrupted anywhere, so shared
data needs locks, and a lock that the timer handler might also want must be
taken with interrupts disabled. Here: the print macros and `print_at` already
hold their spinlocks with interrupts off (`without_interrupts`), so a thread
can't be switched away while holding them; the scheduler lock is only taken
with interrupts off and is released *before* switching (the next thread would
otherwise wait forever for a lock held by a paused thread). The heap allocator's
lock is never taken in an interrupt handler, so a thread preempted while
holding it only makes others spin until it runs again -- slow, not a deadlock.

Known simplifications (good exercises): sleeping threads (`sleep_ticks`) just
`hlt` and still get scheduled -- a real scheduler has a *blocked* state and a
sleep queue; finished threads' stacks are never freed; no guard page under
thread stacks (an overflow silently corrupts the heap); no priorities; a single
CPU, so `spin::Mutex` is enough.

## User mode, syscalls and processes

Four tiny Rust programs in `user/` run in **ring 3** as separate processes, each
with its own address space and file-descriptor table:

| Program | What it does |
|---|---|
| `hello` | prints "Hello from user mode!", `getpid`, reads `/hello.txt` from the ramfs, creates and reads back `/tmp/note.txt`, checks error cases (`ENOENT`, `EBADF`, `EFAULT`, `ENOSYS`), exits with the number of failed checks (0) |
| `evil` | reads kernel memory at `0x200000` -> page fault in ring 3 -> the kernel kills it and keeps going |
| `spin` | busy-loops ~2 s with no syscalls; it still gets preempted (threads A/B/C keep printing) |
| `echo` | `read(0)` a line from the keyboard, prints `[echo] you typed: ...`; type `exit` to end it |

### Rings in one paragraph

The CPU's *current privilege level* (CPL) is the low 2 bits of the CS register:
0 = kernel, 3 = user. In ring 3 the CPU refuses privileged instructions (`hlt`,
`cli`, writing CR3, `in`/`out`...) and refuses to touch any page whose page-table
entries lack the `USER_ACCESSIBLE` bit. User code can only get back into ring 0
through doors the kernel set up: interrupts/exceptions (via the IDT) and the
`syscall` instruction (via the LSTAR MSR). Every such door switches to a kernel
stack, so a user program can't corrupt kernel state through its own stack pointer.

### GDT and TSS (`src/gdt.rs`)

```
0x00 null
0x08 kernel code (ring 0)       0x10 kernel data (ring 0)
0x18 user data   (ring 3) |3    0x20 user code   (ring 3) |3
0x28 TSS (16 bytes: two slots)
```

The odd order (user *data* before user *code*) is forced by `sysret`: it loads
SS = STAR[63:48] + 8 and CS = STAR[63:48] + 16, so the two user segments must
be adjacent in that order. The TSS has two jobs: IST[0] is the double-fault
stack (stage 2), and **`rsp0`** is the stack the CPU switches to when an
interrupt arrives while in ring 3. Each thread has its own kernel stack, so the
scheduler writes the next thread's stack top into `TSS.rsp0` on every switch
(`gdt::set_kernel_stack`).

### The syscall path (`src/syscall.rs`)

`syscall` was chosen over `int 0x80`: it's what Linux uses on x86_64, it's
faster, and the setup is only four MSRs. The catch is that, unlike an
interrupt, `syscall` does **not** switch stacks -- the entry code must.

Setup (`syscall::init`): `EFER.SCE = 1` (enable), `STAR` = the selectors above,
`LSTAR` = address of `syscall_entry`, `SFMASK = IF|DF|TF` (so we enter with
interrupts off until we're on a kernel stack).

One call, step by step:

1. User code puts the number in `rax`, args in `rdi, rsi, rdx` (Linux ABI; we
   also save `r10, r8, r9`) and executes `syscall`.
2. CPU: `rcx` = user RIP, `r11` = user RFLAGS, CS/SS = kernel, RFLAGS &= ~SFMASK,
   RIP = LSTAR. RSP is still the *user* stack.
3. `syscall_entry` (asm): stash user RSP in a global, load this thread's kernel
   stack top (set by the scheduler), push user RSP, `rcx`, `r11` and the args.
4. Shuffle into the C ABI and call `dispatch(nr, a1..a5)` in Rust with
   interrupts re-enabled (so a long syscall, e.g. a blocking `read`, can be preempted).
5. `dispatch` validates user pointers (`user_slice`: range must be inside the
   user half *and* mapped with `USER_ACCESSIBLE` in the current page table),
   runs the call, and returns a value or `-errno` in `rax`.
6. `cli`, pop everything back, restore user RSP, `sysretq` (RIP = rcx,
   RFLAGS = r11, CPL = 3).

Single CPU, so the "current kernel stack" and "saved user RSP" are plain
globals; SMP kernels use per-CPU data reached with `swapgs`.

| nr | name | args | notes |
|---|---|---|---|
| 0 | `read` | fd, buf, len | fd 0 = keyboard (blocks until Enter), others = files |
| 1 | `write` | fd, buf, len | fd 1/2 = console (VGA + serial) |
| 2 | `open` | path, path_len, flags | **differs from Linux**: path is ptr+len, not a C string; `O_CREAT` = 0o100 |
| 3 | `close` | fd | |
| 24 | `sched_yield` | | |
| 39 | `getpid` | | |
| 60 | `exit` | code | |

Errors are negative errno values like Linux (`-2` ENOENT, `-9` EBADF, `-14`
EFAULT, `-38` ENOSYS...). Unknown numbers return `-ENOSYS`.

### Address spaces (`src/memory.rs`, `src/process.rs`)

Each process gets a fresh level-4 page table. The kernel's L4 entries are
**copied** into it (so the kernel, heap, physical-memory map and thread stacks
are mapped in every address space and the kernel keeps working after a CR3
switch), and L4 slot 1 is the process's own:

```
0x0000_0000_0020_0000   kernel code (bootloader 0.9 puts it here)        shared, ring 0 only
0x0000_0080_0000_0000   USER_START (L4 slot 1)                           per process
0x0000_0080_0040_0000     program (.text R-X, .rodata R--, .data/.bss RW-)
0x0000_00ff_ffff_b000     user stack (16 KiB, RW-, grows down)
0x0000_0100_0000_0000   USER_END
0x0000_0100_0020_0000   boot stack (L4 slot 2)                           shared
0x0000_0180_0000_0000   all physical memory (phys_mem_offset, slot 3)    shared
0x0000_4444_4444_0000   kernel heap (thread kernel stacks live here)     shared
```

Real kernels put the kernel in the upper half (`0xffff_8000_...`) and give
user space the whole lower half. With bootloader 0.9 the kernel sits low, so
here user space is one 512 GiB slot that the kernel doesn't use. Only user
pages have `USER_ACCESSIBLE`, which is why `evil` faults: the page at
`0x200000` *is* mapped, just not for ring 3 (`PROTECTION_VIOLATION | USER_MODE`).
`.text` is not writable and everything else is `NO_EXECUTE` (when EFER.NXE is on).

A **process** = a kernel thread (`thread.rs`, it already has a kernel stack and
gets preempted) + a CR3 value + a `Process` struct (pid, name, fd table, exit
status). The scheduler, on each switch, sets `TSS.rsp0` and the syscall kernel
stack to the next thread's stack and writes CR3 if it differs. A new process's
thread starts in the kernel, then `iretq`s into ring 3 with a hand-built frame
(SS=user data, RSP=user stack, RFLAGS=IF, CS=user code, RIP=ELF entry).

When ring-3 code faults (page fault with `USER_MODE`, general protection fault
or invalid opcode with CS RPL 3), the handler calls `process::exit_current`
instead of panicking: the process is marked `Killed`, its thread exits, the
scheduler picks someone else. A fault in ring 0 still panics -- that's a kernel bug.

### ELF loading and building user programs (`src/elf.rs`, `build.rs`, `user/`)

`build.rs` compiles each `user/*.rs` with plain `rustc` for the built-in
`x86_64-unknown-none` target (no build-std needed: it ships a precompiled
`core` and uses soft-float, so no SSE state to save). Flags worth knowing:
`-C relocation-model=static` (no dynamic loader), `-C code-model=large`
(the program lives above 4 GiB; the default model assumes 32-bit addresses
and fails with "relocation overflow"), `-T user/link.ld` (one PT_LOAD
segment per permission, page-aligned). The large code model names its
sections `.ltext`/`.lrodata`/`.ldata`/`.lbss`, so the linker script lists
those too -- forgetting that put the code into a non-executable segment and
every program died at its first instruction. The ELFs (~15-20 KB) are embedded in the
kernel with `include_bytes!` (`rust_kernel::user_programs`).

`user/ulib.rs` is a tiny libc-equivalent included by each program: the
`syscall` inline asm, wrappers, `print!/println!`, `_start` (calls `main`,
then `exit` with its return value) and a panic handler (`exit(101)`).

The loader (~160 lines, no crate) checks the header (magic, 64-bit, little
endian, x86_64, `ET_EXEC`), and for every `PT_LOAD` segment: checks it's
inside the user range, maps zeroed frames with flags from the segment
(`W` -> writable, no `X` -> NX), copies `filesz` bytes through the
physical-memory map and leaves the rest (`.bss`) zero.

### VFS (`src/fs/`) and file descriptors

Designed like Linux's so real filesystems can plug in later:

- `trait Inode` (`fs/mod.rs`): a file or directory: `kind`, `size`,
  `read_at(offset, buf)`, `write_at`, `lookup(name)`, `create(name, kind)`, `list`.
  Methods have default impls returning an error, so a read-only fs implements
  only what it supports.
- `trait FileSystem`: `name()` and `root() -> Arc<dyn Inode>`. A disk filesystem
  will hold its block device and parsed superblock.
- **Mount table**: `mount("/tmp", fs)`; `lookup(path)` picks the longest mount
  point that prefixes the path, then walks the rest with `Inode::lookup`.
  Boot mounts a ramfs on `/` (`/hello.txt`, `/etc/motd`, `/tmp`) and a
  second, separate ramfs on `/tmp`.
- `trait File` (`fs/file.rs`): what a file descriptor points to: `read(buf)`,
  `write(buf)`. `InodeFile` = an inode + its own offset (two opens of the same
  file have separate offsets, like Linux's `struct file`). `tty::Console` is
  another `File`: writes go to VGA+serial, reads come from the keyboard line buffer.
- **fd table**: `Vec<Option<Arc<dyn File>>>` per process (max 16), fds 0-2 =
  Console. `open` = `fs::lookup` (or `fs::create` with `O_CREAT`) + `InodeFile::open`
  + first free slot.

Keyboard input for `read(0)` goes through a minimal tty line discipline
(`src/tty.rs`): the keyboard task feeds characters in, Backspace edits the
current line, Enter makes the line (with `\n`) available to readers.

Known simplifications (exercises): a process's page tables and frames are
never freed on exit; `read(0)` waits with `hlt` instead of blocking the
thread; no `fork`/`exec`/`wait` syscalls (the kernel spawns programs and
`process::wait` is kernel-only); no `lseek`, `stat`, `readdir` yet; user
pointer checks walk the page table but don't guard against another thread
unmapping the page mid-syscall (there's no `munmap` yet, so that's fine for now).

## Roadmap

Done: all 12 chapters of the 2nd edition, plus preemptive kernel threads,
user mode + syscalls, ELF loading and a VFS with a ramfs.

- [x] Freestanding binary — [phil-opp](https://os.phil-opp.com/freestanding-rust-binary/)
- [x] Minimal kernel booting in QEMU — [phil-opp](https://os.phil-opp.com/minimal-rust-kernel/), [OSDev: Bare Bones](https://wiki.osdev.org/Bare_Bones)
- [x] VGA text mode printing — [phil-opp](https://os.phil-opp.com/vga-text-mode/), [OSDev: Printing to Screen](https://wiki.osdev.org/Printing_To_Screen)
- [x] Serial output + QEMU exit — [phil-opp: Testing](https://os.phil-opp.com/testing/), [OSDev: Serial Ports](https://wiki.osdev.org/Serial_Ports)
- [x] CPU exceptions / IDT — breakpoint & page fault handlers. [phil-opp: CPU Exceptions](https://os.phil-opp.com/cpu-exceptions/), [OSDev: IDT](https://wiki.osdev.org/Interrupt_Descriptor_Table), [OSDev: Exceptions](https://wiki.osdev.org/Exceptions)
- [x] GDT / TSS / double faults — separate IST stack so stack overflows don't triple-fault. [phil-opp: Double Faults](https://os.phil-opp.com/double-fault-exceptions/), [OSDev: GDT](https://wiki.osdev.org/Global_Descriptor_Table), [OSDev: TSS](https://wiki.osdev.org/Task_State_Segment)
- [x] Testing — `cargo test` in QEMU, custom test framework, integration tests. [phil-opp: Testing](https://os.phil-opp.com/testing/)
- [x] Hardware interrupts — 8259 PIC, timer, PS/2 keyboard echo, `hlt` idle loop. [phil-opp: Hardware Interrupts](https://os.phil-opp.com/hardware-interrupts/), [OSDev: 8259 PIC](https://wiki.osdev.org/8259_PIC), [OSDev: PIT](https://wiki.osdev.org/Programmable_Interval_Timer), [OSDev: PS/2 Keyboard](https://wiki.osdev.org/PS/2_Keyboard) (later: [APIC](https://wiki.osdev.org/APIC))

- [x] Introduction to paging — CR3, page fault demo. [phil-opp](https://os.phil-opp.com/paging-introduction/), [OSDev: Paging](https://wiki.osdev.org/Paging)
- [x] Paging implementation — OffsetPageTable, translation, frame allocator, example mapping. [phil-opp](https://os.phil-opp.com/paging-implementation/), [OSDev: Page Frame Allocation](https://wiki.osdev.org/Page_Frame_Allocation)
- [x] Heap allocation — `alloc`, heap mapping, global allocator. [phil-opp](https://os.phil-opp.com/heap-allocation/), [OSDev: Memory Allocation](https://wiki.osdev.org/Memory_Allocation)
- [x] Allocator designs — bump, linked list (crate), fixed-size block (active). [phil-opp](https://os.phil-opp.com/allocator-designs/)

- [x] Async/await — Task, simple executor, waker-based executor, lock-free async keyboard task (with Backspace). [phil-opp: Async/Await](https://os.phil-opp.com/async-await/), [OSDev: Multitasking Systems](https://wiki.osdev.org/Multitasking_Systems)
- [x] Preemptive kernel threads — own stacks, timer-driven round-robin, asm context switch. [OSDev: Brendan's Multi-tasking Tutorial](https://wiki.osdev.org/Brendan%27s_Multi-tasking_Tutorial), [OSDev: Context Switching](https://wiki.osdev.org/Context_Switching)

- [x] User mode (ring 3) & syscalls — GDT user segments, TSS.rsp0 per thread, `syscall`/`sysret`, per-process page tables, user faults kill the process. [OSDev: Getting to Ring 3](https://wiki.osdev.org/Getting_to_Ring_3), [OSDev: System Calls](https://wiki.osdev.org/System_Calls), [OSDev: SYSENTER/SYSCALL](https://wiki.osdev.org/SYSENTER)
- [x] ELF loading — hand-written PT_LOAD loader, user programs built by `build.rs`. [OSDev: ELF](https://wiki.osdev.org/ELF), [OSDev: ELF Tutorial](https://wiki.osdev.org/ELF_Tutorial)
- [x] VFS skeleton + ramfs — Inode/FileSystem/File traits, mount table, fd table, `open/read/write/close`. [OSDev: VFS](https://wiki.osdev.org/VFS), [Linux VFS overview](https://www.kernel.org/doc/html/latest/filesystems/vfs.html)

### Filesystem roadmap (Tyler's plan)

Everything below plugs into the VFS above: a filesystem is a `FileSystem`
whose `root()` returns `Inode`s; mounting it is one `fs::mount` call.

1. **ramfs / tmpfs** — done (`src/fs/ramfs.rs`).
2. **Block device driver** — a `BlockDevice` trait (`read_blocks(lba, buf)`,
   `write_blocks`, `block_size`) with a first driver: **virtio-blk** (modern,
   fast, what QEMU/KVM use; needs PCI enumeration + virtqueues) or **ATA PIO**
   (dead simple port I/O, good first step). Then a small block cache.
   [OSDev: ATA PIO Mode](https://wiki.osdev.org/ATA_PIO_Mode),
   [OSDev: Virtio](https://wiki.osdev.org/Virtio), [OSDev: PCI](https://wiki.osdev.org/PCI),
   [virtio spec](https://docs.oasis-open.org/virtio/virtio/v1.2/virtio-v1.2.html)
3. **Partition tables** — MBR and GPT parsing so a disk can hold several filesystems.
   [OSDev: GPT](https://wiki.osdev.org/GPT)
4. **FAT32** — the easiest real on-disk format, using the `fatfs` crate (`no_std`
   with `alloc`) behind an adapter that implements `FileSystem`/`Inode`. Test
   images are easy to make with `mkfs.vfat` + `mtools`.
   [OSDev: FAT](https://wiki.osdev.org/FAT), [fatfs crate](https://crates.io/crates/fatfs)
5. **ext2** — written by hand: superblock, block groups, inode table, direct/indirect
   blocks, directory entries. The classic "learn how Unix filesystems work" step,
   and the base for understanding ext4 later. [OSDev: Ext2](https://wiki.osdev.org/Ext2),
   [The Second Extended File System (Dave Poirier)](https://www.nongnu.org/ext2-doc/ext2.html)
6. **End goals:**
   - **exFAT** — because it works on literally anything (cameras, phones,
     Windows, macOS, Linux), so a kernel that speaks exFAT can share USB sticks
     and SD cards with every other machine. Allocation bitmap + FAT-like cluster
     chains + directory entry sets with checksums.
     [Microsoft exFAT specification](https://learn.microsoft.com/en-us/windows/win32/fileio/exfat-specification)
   - **btrfs** (read-only first) — because most of Tyler's Linux systems will use
     btrfs going forward, so this kernel could read their disks. Much bigger:
     copy-on-write B-trees, the chunk tree (logical -> physical mapping), the
     root/fs trees, checksums, compression (zlib/lzo/zstd). Read-only means
     parsing the superblock, chunk tree and fs tree -- no transactions or
     space allocation. [btrfs on-disk format](https://btrfs.readthedocs.io/en/latest/dev/On-disk-format.html),
     [btrfs docs](https://btrfs.readthedocs.io/)
7. **NTFS** — last, maybe never.

Other next stages:

- **Shell** — a user-mode shell using `read(0)`/`write`; needs `fork`/`exec`
  (or a simpler `spawn(path)`) + `wait` syscalls and `readdir`/`stat` for `ls`/`cat`.
  [OSDev: Shell](https://wiki.osdev.org/Shell)
- **Process cleanup** — free a process's page tables, frames and kernel stack on exit.
- **Real blocking** — a blocked thread state + wait queues so `read(0)` and
  `sleep` don't spin with `hlt`.

Small exercises along the way: a *blocked* state + sleep queue for threads,
free finished threads' stacks, guard pages under thread stacks, a timer-based
async `sleep` future, speed up the PIT
([OSDev: PIT](https://wiki.osdev.org/Programmable_Interval_Timer)), add handlers for
divide error / general protection fault, make `BootInfoFrameAllocator` O(1), add a
`mem` stat (bytes allocated) to the allocator.

Other good references: [OSDev wiki main page](https://wiki.osdev.org/Expanded_Main_Page),
[Intel SDM](https://www.intel.com/content/www/us/en/developer/articles/technical/intel-sdms.html),
the [blog_os repo](https://github.com/phil-opp/blog_os) (one branch per post, e.g. `post-04`),
and for a UEFI/framebuffer setup later, the newer [`bootloader` 0.11](https://github.com/rust-osdev/bootloader) crate.

## Notes / gotchas

- The target JSON changes occasionally with new nightlies (e.g. `target-pointer-width`
  became an integer, `rustc-abi` was added). If a nightly update breaks the build,
  compare with the latest `x86_64-blog_os.json` in the blog_os repo, or pin a date in
  `rust-toolchain.toml` (`channel = "nightly-2026-10-29"`).
- The "packages contain code that will be rejected by a future version of Rust: core"
  warning comes from building `core` with build-std and is harmless.
- `bootimage` prints "`CARGO_MANIFEST_DIR` env variable not set" when run outside
  `cargo run`; also harmless.
- `cargo test` with build-std breaks ("duplicate lang item in crate `core`: `sized`")
  if `Cargo.toml` sets `panic = "abort"` in `[profile.*]`. The target JSON already
  sets `"panic-strategy": "abort"`, so the profile settings were removed.
- Stale disk image when switching cargo features: cargo re-uses a previously built
  kernel ELF with an *old* mtime, so bootimage thinks its cached image is current
  and boots the wrong build. `scripts/runner.sh` and the scripts `touch` the ELF
  first to avoid this. If you call `cargo bootimage --features ...` by hand after
  switching features, `touch target/x86_64-rust_kernel/debug/rust_kernel` first.