blog_os/blog/post/exceptions.md

+++ title = "Catching Exceptions" date = "2016-05-10" +++

TODO We will catch page faults,

Interrupts

Whenever a device (e.g. the keyboard contoller) needs

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.
• Triple Fault: If another exception occurs when 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.

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 required 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 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.

Handling Exceptions

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

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

// src/interrupts/mod.rs

mod idt;

Now we create types for the IDT and its entries:

// src/interrupts/idt.rs

use x86::segmentation::SegmentSelector;

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:

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

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

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

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

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

    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:

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

Or alternatively:

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

Or:

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 type with the following API:

self.0.set_range(0..3, stack_index);

I think it is much more readable, since we abstracted away all bit-masking details. The BitField type 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.

Now we can use the crate to implement the methods of EntryOptions:

// in src/interrupts/idt.rs

use bit_field::BitField;

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

impl EntryOptions {
    fn minimal() -> Self {
        let mut options = BitField::new(0);
        options.set_range(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
    }

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

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

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

    fn set_stack_index(&mut self, index: u16) -> &mut Self {
        self.0.set_range(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:

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:

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.

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:

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 diverge¹ .

IDT methods

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

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

impl Entry {
    fn missing() -> Self {
        Entry {
            gdt_selector: 0,
            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:

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::cs² function of the x86 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.

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.

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 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:

impl Idt {
    pub fn load(&self) {
        use x86::dtables::{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. We convert this reference to an u64 and calculate the table size using mem::size_of. The additional -1 is needed because the limit field has to be the maximum addressable byte.

Then we pass a pointer to our ptr structure to the lidt function, which calls the lidt assembly instruction in order to reload the IDT register. We need an unsafe block here, because the lidt assumes that the specified handler addresses are valid.

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:

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. It declares an array of integers, which overwrite the entries of our loaded IDT. 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. Let's formulate our requirement:

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:

pub fn load(&'static self) {...}

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. We will most likely only need a single IDT for the foreseeable future, so let's try the static approach:

// in src/interrupts/mod.rs

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

    idt.set_handler(14, page_fault_handler);

    idt
};

extern "C" fn page_fault_handler() -> ! {
  println!("EXCEPTION: PAGE FAULT");
  loop {}
}

Well… this doesn't work:

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]
...

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 evaluates it when it's referenced the first time. Thus, we can do almost everything in it and are even able to read runtime values (e.g. the number of cores).

With lazy_static, we can define our IDT without problems:

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

    idt.set_handler(14, page_fault_handler);

    idt
  };
}

Now we're ready to load it! We add a interrupts::init function, which takes care of it:

// in src/interrupts/mod.rs

pub fn init() {
    assert_has_not_been_called!();

    IDT.load();
}

We're using our assert_has_not_been_called macro to ensure that the init function is called only once. It doesn't really matter in this case since we would just load the table again. However, calling an initialization function twice is a sign of some bug, so we leave it in.

Testing it

TODO page fault, some other fault to trigger double fault, kernel stack overflow

Switching stacks

The Interrupt Stack Table

The Task State Segment

The Global Descriptor Table (again)

Putting it together

What's next?

---

1. 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. ↩︎

2. The segmentation::cs function was added in version 0.7.0, so you might need to update your x86 version in your Cargo.toml. ↩︎