+++
title = "Catching Exceptions"
weight = 1
path = "catching-exceptions"
aliases = ["catching-exceptions.html"]
date  = 2016-05-28
template = "first-edition/page.html"
[extra]
updated = "2016-06-25"
+++

In this post, we start exploring exceptions. We set up an interrupt descriptor table and add handler functions. At the end of this post, our kernel will be able to catch divide-by-zero faults.

<!-- more -->

As always, the complete source code is on [GitHub]. Please file [issues] for any problems, questions, or improvement suggestions. There is also a comment section at the end of this page.

[GitHub]: https://github.com/phil-opp/blog_os/tree/catching_exceptions
[issues]: https://github.com/phil-opp/blog_os/issues

> **Note**: This post describes how to handle exceptions using naked functions (see [“Handling Exceptions with Naked Functions”] for an overview). Our new way of handling exceptions can be found in the [“Handling Exceptions”] post.

[“Handling Exceptions with Naked Functions”]: @/first-edition/extra/naked-exceptions/_index.md
[“Handling Exceptions”]: @/first-edition/posts/09-handling-exceptions/index.md

## Exceptions
An exception signals that something is wrong with the current instruction. For example, the CPU issues an exception if the current instruction tries to divide by 0. When an exception occurs, the CPU interrupts its current work and immediately calls a specific exception handler function, depending on the exception type.

We've already seen several types of exceptions in our kernel:

- **Invalid Opcode**: This exception occurs when the current instruction is invalid. For example, this exception occurred when we tried to use SSE instructions before enabling SSE. Without SSE, the CPU didn't know the `movups` and `movaps` instructions, so it throws an exception when it stumbles over them.
- **Page Fault**: A page fault occurs on illegal memory accesses. For example, if the current instruction tries to read from an unmapped page or tries to write to a read-only page.
- **Double Fault**: When an exception occurs, the CPU tries to call the corresponding handler function. If another exception exception occurs _while calling the exception handler_, the CPU raises a double fault exception. This exception also occurs when there is no handler function registered for an exception.
- **Triple Fault**: If an exception occurs while the CPU tries to call the double fault handler function, it issues a fatal _triple fault_. We can't catch or handle a triple fault. Most processors react by resetting themselves and rebooting the operating system. This causes the bootloops we experienced in the previous posts.

For the full list of exceptions check out the [OSDev wiki][exceptions].

[exceptions]: http://wiki.osdev.org/Exceptions

### The Interrupt Descriptor Table
In order to catch and handle exceptions, we have to set up a so-called _Interrupt Descriptor Table_ (IDT). In this table we can specify a handler function for each CPU exception. The hardware uses this table directly, so we need to follow a predefined format. Each entry must have the following 16-byte structure:

Type| Name                     | Description
----|--------------------------|-----------------------------------
u16 | Function Pointer [0:15]  | The lower bits of the pointer to the handler function.
u16 | GDT selector             | Selector of a code segment in the GDT.
u16 | Options                  | (see below)
u16 | Function Pointer [16:31] | The middle bits of the pointer to the handler function.
u32 | Function Pointer [32:63] | The remaining bits of the pointer to the handler function.
u32 | Reserved                 |

The options field has the following format:

Bits  | Name                              | Description
------|-----------------------------------|-----------------------------------
0-2   | Interrupt Stack Table Index       | 0: Don't switch stacks, 1-7: Switch to the n-th stack in the Interrupt Stack Table when this handler is called.
3-7   | Reserved              |
8     | 0: Interrupt Gate, 1: Trap Gate   | If this bit is 0, interrupts are disabled when this handler is called.
9-11  | must be one                       |
12    | must be zero                      |
13‑14 | Descriptor Privilege Level (DPL)  | The minimal privilege level required for calling this handler.
15    | Present                           |

Each exception has a predefined IDT index. For example the invalid opcode exception has table index 6 and the page fault exception has table index 14. Thus, the hardware can automatically load the corresponding IDT entry for each exception. The [Exception Table][exceptions] in the OSDev wiki shows the IDT indexes of all exceptions in the “Vector nr.” column.

When an exception occurs, the CPU roughly does the following:

1. Read the corresponding entry from the Interrupt Descriptor Table (IDT). For example, the CPU reads the 14-th entry when a page fault occurs.
2. Check if the entry is present. Raise a double fault if not.
3. Push some registers on the stack, including the instruction pointer and the [EFLAGS] register. (We will use these values in a future post.)
4. Disable interrupts if the entry is an interrupt gate (bit 40 not set).
5. Load the specified GDT selector into the CS segment.
6. Jump to the specified handler function.

[EFLAGS]: https://en.wikipedia.org/wiki/FLAGS_register

## Handling Exceptions
Let's try to catch and handle CPU exceptions. We start by creating a new `interrupts` module with an `idt` submodule:

``` rust
// in src/lib.rs
...
mod interrupts;
...
```
``` rust
// src/interrupts/mod.rs

mod idt;
```

Now we create types for the IDT and its entries:

```rust
// src/interrupts/idt.rs

use x86_64::instructions::segmentation;
use x86_64::structures::gdt::SegmentSelector;
use x86_64::PrivilegeLevel;

pub struct Idt([Entry; 16]);

#[derive(Debug, Clone, Copy)]
#[repr(C, packed)]
pub struct Entry {
    pointer_low: u16,
    gdt_selector: SegmentSelector,
    options: EntryOptions,
    pointer_middle: u16,
    pointer_high: u32,
    reserved: u32,
}
```

The IDT is variable sized and can have up to 256 entries. We only need the first 16 entries in this post, so we define the table as `[Entry; 16]`. The remaining 240 handlers are treated as non-present by the CPU.

The `Entry` type is the translation of the above table to Rust. The `repr(C, packed)` attribute ensures that the compiler keeps the field ordering and does not add any padding between them. Instead of describing the `gdt_selector` as a plain `u16`, we use the `SegmentSelector` type of the `x86` crate. We also merge bits 32 to 47 into an `option` field, because Rust has no `u3` or `u1` type. The `EntryOptions` type is described below:

### Entry Options
The `EntryOptions` type has the following skeleton:

``` rust
#[derive(Debug, Clone, Copy)]
pub struct EntryOptions(u16);

impl EntryOptions {
    fn new() -> Self {...}

    pub fn set_present(&mut self, present: bool) {...}

    pub fn disable_interrupts(&mut self, disable: bool) {...}

    pub fn set_privilege_level(&mut self, dpl: u16) {...}

    pub fn set_stack_index(&mut self, index: u16) {...}
}
```

The implementations of these methods need to modify the correct bits of the `u16` without touching the other bits. For example, we would need the following bit-fiddling to set the stack index:

``` rust
self.0 = (self.0 & 0xfff8) | stack_index;
```

Or alternatively:

``` rust
self.0 = (self.0 & (!0b111)) | stack_index;
```

Or:

``` rust
self.0 = ((self.0 >> 3) << 3) | stack_index;
```

Well, none of these variants is really _readable_ and it's very easy to make mistakes somewhere. Therefore I created a `BitField` trait that provides the following [Range]-based API:

[Range]: https://doc.rust-lang.org/nightly/core/ops/struct.Range.html

``` rust
self.0.set_bits(0..3, stack_index);
```

I think it is much more readable, since we abstracted away all bit-masking details. The `BitField` trait is contained in the [bit_field] crate. (It's pretty new, so it might still contain bugs.) To add it as dependency, we run `cargo add bit_field` and add `extern crate bit_field;` to our `src/lib.rs`.

[bit_field]: https://crates.io/crates/bit_field

Now we can use the trait to implement the methods of `EntryOptions`:

```rust
// in src/interrupts/idt.rs

use bit_field::BitField;

#[derive(Debug, Clone, Copy)]
pub struct EntryOptions(u16);

impl EntryOptions {
    fn minimal() -> Self {
        let mut options = 0;
        options.set_bits(9..12, 0b111); // 'must-be-one' bits
        EntryOptions(options)
    }

    fn new() -> Self {
        let mut options = Self::minimal();
        options.set_present(true).disable_interrupts(true);
        options
    }

    pub fn set_present(&mut self, present: bool) -> &mut Self {
        self.0.set_bit(15, present);
        self
    }

    pub fn disable_interrupts(&mut self, disable: bool) -> &mut Self {
        self.0.set_bit(8, !disable);
        self
    }

    pub fn set_privilege_level(&mut self, dpl: u16) -> &mut Self {
        self.0.set_bits(13..15, dpl);
        self
    }

    pub fn set_stack_index(&mut self, index: u16) -> &mut Self {
        self.0.set_bits(0..3, index);
        self
    }
}
```
Note that the ranges are _exclusive_ the upper bound. The `minimal` function creates an `EntryOptions` type with only the “must-be-one” bits set. The `new` function, on the other hand, chooses reasonable defaults: It sets the present bit (why would you want to create a non-present entry?) and disables interrupts (normally we don't want that our exception handlers can be interrupted). By returning the self pointer from the `set_*` methods, we allow easy method chaining such as `options.set_present(true).disable_interrupts(true)`.

### Creating IDT Entries
Now we can add a function to create new IDT entries:

```rust
impl Entry {
    fn new(gdt_selector: SegmentSelector, handler: HandlerFunc) -> Self {
        let pointer = handler as u64;
        Entry {
            gdt_selector: gdt_selector,
            pointer_low: pointer as u16,
            pointer_middle: (pointer >> 16) as u16,
            pointer_high: (pointer >> 32) as u32,
            options: EntryOptions::new(),
            reserved: 0,
        }
    }
}
```
We take a GDT selector and a handler function as arguments and create a new IDT entry for it. The `HandlerFunc` type is described below. It is a function pointer that can be converted to an `u64`. We choose the lower 16 bits for `pointer_low`, the next 16 bits for `pointer_middle` and the remaining 32 bits for `pointer_high`. For the options field we choose our default options, i.e. present and disabled interrupts.

### The Handler Function Type

The `HandlerFunc` type is a type alias for a function type:

``` rust
pub type HandlerFunc = extern "C" fn() -> !;
```
It needs to be a function with a defined [calling convention], as it called directly by the hardware. The C calling convention is the de facto standard in OS development, so we're using it, too. The function takes no arguments, since the hardware doesn't supply any arguments when jumping to the handler function.

[calling convention]: https://en.wikipedia.org/wiki/Calling_convention

It is important that the function is [diverging], i.e. it must never return. The reason is that the hardware doesn't _call_ the handler functions, it just _jumps_ to them after pushing some values to the stack. So our stack might look different:

[diverging]: https://doc.rust-lang.org/book/functions.html#diverging-functions

![normal function return vs interrupt function return](normal-vs-interrupt-function-return.svg)

If our handler function returned normally, it would try to pop the return address from the stack. But it might get some completely different value then. For example, the CPU pushes an error code for some exceptions. Bad things would happen if we interpreted this error code as return address and jumped to it. Therefore interrupt handler functions must diverge[^fn-must-diverge].

[^fn-must-diverge]: Another reason is that we overwrite the current register values by executing the handler function. Thus, the interrupted function looses its state and can't proceed anyway.

### IDT methods
Let's add a function to create new interrupt descriptor tables:

```rust
impl Idt {
    pub fn new() -> Idt {
        Idt([Entry::missing(); 16])
    }
}

impl Entry {
    fn missing() -> Self {
        Entry {
            gdt_selector: SegmentSelector::new(0, PrivilegeLevel::Ring0),
            pointer_low: 0,
            pointer_middle: 0,
            pointer_high: 0,
            options: EntryOptions::minimal(),
            reserved: 0,
        }
    }
}
```
The `missing` function creates a non-present Entry. We could choose any values for the pointer and GDT selector fields as long as the present bit is not set.

However, a table with non-present entries is not very useful. So we create a `set_handler` method to add new handler functions:

```rust
impl Idt {
    pub fn set_handler(&mut self, entry: u8, handler: HandlerFunc)
        -> &mut EntryOptions
    {
        self.0[entry as usize] = Entry::new(segmentation::cs(), handler);
        &mut self.0[entry as usize].options
    }
}
```
The method overwrites the specified entry with the given handler function. We use the `segmentation::cs` function of the [x86_64 crate] to get the current code segment descriptor. There's no need for different kernel code segments in long mode, so the current `cs` value should be always the right choice.

[x86_64 crate]: https://docs.rs/x86_64

By returning a mutual reference to the entry's options, we allow the caller to override the default settings. For example, the caller could add a non-present entry by executing: `idt.set_handler(11, handler_fn).set_present(false)`.

### Loading the IDT
Now we're able to create new interrupt descriptor tables with registered handler functions. We just need a way to load an IDT, so that the CPU uses it. The x86 architecture uses a special register to store the active IDT and its length. In order to load a new IDT we need to update this register through the [lidt] instruction.

[lidt]: https://www.felixcloutier.com/x86/lgdt:lidt

The `lidt` instruction expects a pointer to a special data structure, which specifies the start address of the IDT and its length:


Type    | Name    | Description
--------|---------|-----------------------------------
u16     | Limit   | The maximum addressable byte in the table. Equal to the table size in bytes minus 1.
u64     | Offset  | Virtual start address of the table.

This structure is already contained [in the x86_64 crate], so we don't need to create it ourselves. The same is true for the [lidt function]. So we just need to put the pieces together to create a `load`  method:

[in the x86_64 crate]: http://docs.rs/x86_64/0.1.0/x86_64/instructions/tables/struct.DescriptorTablePointer.html
[lidt function]: http://docs.rs/x86_64/0.1.0/x86_64/instructions/tables/fn.lidt.html

```rust
impl Idt {
    pub fn load(&self) {
        use x86_64::instructions::tables::{DescriptorTablePointer, lidt};
        use core::mem::size_of;

        let ptr = DescriptorTablePointer {
            base: self as *const _ as u64,
            limit: (size_of::<Self>() - 1) as u16,
        };

        unsafe { lidt(&ptr) };
    }
}
```
The method does not need to modify the IDT, so it takes `self` by immutable reference. First, we create a `DescriptorTablePointer` and then we pass it to `lidt`. The `lidt` function expects that the `base` field has the type `u64`, therefore we need to cast the `self` pointer. For calculating the `limit` we use [mem::size_of]. The additional `-1` is needed because the limit field has to be the maximum addressable byte (inclusive bound). We need an unsafe block around `lidt`, because the function assumes that the specified handler addresses are valid.

[mem::size_of]: https://doc.rust-lang.org/nightly/core/mem/fn.size_of.html

#### Safety
But can we really guarantee that handler addresses are always valid? Let's see:

- The `Idt::new` function creates a new table populated with non-present entries. There's no way to set these entries to present from outside of this module, so this function is fine.
- The `set_handler` method allows us to overwrite a specified entry and point it to some handler function. Rust's type system guarantees that function pointers are always valid (as long as no `unsafe` is involved), so this function is fine, too.

There are no other public functions in the `idt` module (except `load`), so it should be safe… right?

Wrong! Imagine the following scenario:

```rust
pub fn init() {
    load_idt();
    cause_page_fault();
}

fn load_idt() {
    let mut idt = idt::Idt::new();
    idt.set_handler(14, page_fault_handler);
    idt.load();
}

fn cause_page_fault() {
    let x = [1,2,3,4,5,6,7,8,9];
    unsafe{ *(0xdeadbeaf as *mut u64) = x[4] };
}
```
This won't work. If we're lucky, we get a triple fault and a boot loop. If we're unlucky, our kernel does strange things and fails at some completely unrelated place. So what's the problem here?

Well, we construct an IDT _on the stack_ and load it. It is perfectly valid until the end of the `load_idt` function. But as soon as the function returns, its stack frame can be reused by other functions. Thus, the IDT gets overwritten by the stack frame of the `cause_page_fault` function. So when the page fault occurs and the CPU tries to read the entry, it only sees some garbage values and issues a double fault, which escalates to a triple fault and a CPU reset.

Now imagine that the `cause_page_fault` function declared an array of pointers instead. If the present was coincidentally set, the CPU would jump to some random pointer and interpret random memory as code. This would be a clear violation of memory safety.

#### Fixing the load method
So how do we fix it? We could make the load function itself `unsafe` and push the unsafety to the caller. However, there is a much better solution in this case. In order to see it, we formulate the requirement for the `load` method:

> The referenced IDT must be valid until a new IDT is loaded.

We can't know when the next IDT will be loaded. Maybe never. So in the worst case:

> The referenced IDT must be valid as long as our kernel runs.

This is exactly the definition of a [static lifetime]. So we can easily ensure that the IDT lives long enough by adding a `'static` requirement to the signature of the `load` function:

[static lifetime]: https://doc.rust-lang.org/rust-by-example/scope/lifetime/static_lifetime.html

```rust
pub fn load(&'static self) {...}
//           ^^^^^^^ ensure that the IDT reference has the 'static lifetime
```

That's it! Now the Rust compiler ensures that the above error can't happen anymore:

```
error: `idt` does not live long enough
  --> src/interrupts/mod.rs:78:5
78 |>     idt.load();
   |>     ^^^
note: reference must be valid for the static lifetime...
note: ...but borrowed value is only valid for the block suffix following
          statement 0 at 75:34
  --> src/interrupts/mod.rs:75:35
75 |>     let mut idt = idt::Idt::new();
   |>                                   ^
```

### A static IDT
So a valid IDT needs to have the `'static` lifetime. We can either create a `static` IDT or [deliberately leak a Box][into_raw]. We will most likely only need a single IDT for the foreseeable future, so let's try the `static` approach:

[into_raw]: https://doc.rust-lang.org/nightly/alloc/boxed/struct.Box.html#method.into_raw

```rust
// in src/interrupts/mod.rs

static IDT: idt::Idt = {
    let mut idt = idt::Idt::new();

    idt.set_handler(0, divide_by_zero_handler);

    idt
};

extern "C" fn divide_by_zero_handler() -> ! {
    println!("EXCEPTION: DIVIDE BY ZERO");
    loop {}
}
```
We register a single handler function for a [divide by zero error] \(index 0). Like the name says, this exception occurs when dividing a number by 0. Thus we have an easy way to test our new exception handler.

[divide by zero error]: http://wiki.osdev.org/Exceptions#Divide-by-zero_Error

However, it doesn't work this way:

```
error: calls in statics are limited to constant functions, struct and enum
       constructors [E0015]
...
error: blocks in statics are limited to items and tail expressions [E0016]
...
error: references in statics may only refer to immutable values [E0017]
...
```
The reason is that the Rust compiler is not able to evaluate the value of the `static` at compile time. Maybe it will work someday when `const` functions become more powerful. But until then, we have to find another solution.

#### Lazy Statics to the Rescue
Fortunately the `lazy_static` macro exists. Instead of evaluating a `static` at compile time, the macro performs the initialization when the `static` is referenced the first time. Thus, we can do almost everything in the initialization block and are even able to read runtime values.

Let's add the `lazy_static` crate to our project:

```rust
// in src/lib.rs

#[macro_use]
extern crate lazy_static;
```

```toml
# in Cargo.toml

[dependencies.lazy_static]
version = "0.2.1"
features = ["spin_no_std"]
```
We need the `spin_no_std` feature, since we don't link the standard library.

With `lazy_static`, we can define our IDT without problems:

```rust
// in src/interrupts/mod.rs

lazy_static! {
    static ref IDT: idt::Idt = {
        let mut idt = idt::Idt::new();

        idt.set_handler(0, divide_by_zero_handler);

        idt
    };
}
```

Now we're ready to load our IDT! Therefore we add a `interrupts::init` function:

```rust
// in src/interrupts/mod.rs

pub fn init() {
    IDT.load();
}
```
We don't need our `assert_has_not_been_called` macro here, since nothing bad happens when `init` is called twice. It just reloads the same IDT again.

## Testing it
Now we should be able to catch page faults! Let's try it in our `rust_main`:

```rust
// in src/lib.rs

pub extern "C" fn rust_main(...) {
    ...
    memory::init(boot_info);

    // initialize our IDT
    interrupts::init();

    // provoke a divide-by-zero fault
    42 / 0;

    println!("It did not crash!");
    loop {}
}
```
When we run it, we get a runtime panic:

```
PANIC in src/lib.rs at line 57:
    attempted to divide by zero
```

That's a not our exception handler. The reason is that Rust itself checks for a possible division by zero and panics in that case. So in order to raise a divide-by-zero error in the CPU, we need to bypass the Rust compiler somehow.

### Inline Assembly
In order to cause a divide-by-zero exception, we need to execute a [div] or [idiv] assembly instruction with operand 0. We could write a small assembly function and call it from our Rust code. An easier way is to use Rust's [inline assembly] macro.

[div]: https://www.felixcloutier.com/x86/div
[idiv]: https://www.felixcloutier.com/x86/idiv
[inline assembly]: https://doc.rust-lang.org/1.10.0/book/inline-assembly.html

Inline assembly allows us to write raw x86 assembly within a Rust function. The feature is unstable, so we need to add `#![feature(asm)]` to our `src/lib.rs`. Then we're able to write a `divide_by_zero` function:

```rust
fn divide_by_zero() {
    unsafe {
        asm!("mov dx, 0; div dx" ::: "ax", "dx" : "volatile", "intel")
    }
}
```
Let's try to decode it:

- The `asm!` macro emits raw assembly instructions, so it's `unsafe` to use it.
- We insert two assembly instructions here: `mov dx, 0` and `div dx`. The former loads a 0 into the `dx` register (a subset of `rdx`) and the latter divides the `ax` register by `dx`. (The `div` instruction always implicitly operates on the `ax` register).
- The colons are separators. After the first `:` we could specify output operands and after the second `:` we could specify input operands. We need neither, so we leave these areas empty.
- After the third colon, we specify the so-called _clobbers_. These tell the compiler that our assembly modifies the values of some registers. Otherwise, the compiler assumes that the registers preserve their value. In our case, we clobber `dx` (we load 0 to it) and `ax` (the `div` instruction places the result in it).
- The last block (after the 4th colon) specifies some options. The `volatile` option tells the compiler: “This code has side effects. Do not delete it and do not move it elsewhere”. In our case, the “side effect” is the divide-by-zero exception. Finally, the `intel` option allows us to use the Intel assembly syntax instead of the default AT&T syntax.

Let's use our new `divide_by_zero` function to raise a CPU exception:

```rust
// in src/lib.rs

pub extern "C" fn rust_main(...) {
    ...

    // provoke a divide-by-zero fault
    divide_by_zero();

    println!("It did not crash!");
    loop {}
}
```

It works! We see a `EXCEPTION: DIVIDE BY ZERO` message at the bottom of our screen:

![QEMU screenshot with `EXCEPTION: DIVIDE BY ZERO` message](qemu-divide-error-println.png)

## What's next?
We've successfully caught our first exception! However, our `EXCEPTION: DIVIDE BY ZERO` message doesn't contain much information about the cause of the exception. The next post improves the situation by printing i.a. the current stack pointer and address of the causing instruction. We will also explore other exceptions such as page faults, for which the CPU pushes an _error code_ on the stack.
