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// Copyright (c) Microsoft Corporation.
// Licensed under the MIT License.
//! This module implements the `HeaderVec` type for constructing dynamically
//! sized values that have a fixed size header and a variable sized element
//! type. This is a common pattern in IOCTL input buffers.
// UNSAFETY: Implementing a custom data structure that requires manual memory
// management and pointer manipulation.
#![allow(unsafe_code)]
#![allow(clippy::undocumented_unsafe_blocks)]
use std::alloc::Layout;
use std::alloc::{self};
use std::cmp;
use std::mem::MaybeUninit;
use std::ops::Deref;
use std::ops::DerefMut;
use std::ops::Index;
use std::ops::IndexMut;
use std::ptr::NonNull;
use std::slice::Iter;
use std::slice::IterMut;
use std::slice::SliceIndex;
/// Trait implemented by fixed-sized arrays that can be used as the element type
/// for HeaderVec. Once Rust supports const generics, this can be removed.
///
/// # Safety
///
/// Must only be implemented on fixed size arrays (i.e: `[T; N]`)
pub unsafe trait FixedArray {
type Element: Copy;
const COUNT: usize;
}
// SAFETY: Only implementing for fixed size arrays.
unsafe impl<T: Copy, const N: usize> FixedArray for [T; N] {
type Element = T;
const COUNT: usize = N;
}
#[repr(C)]
#[derive(Debug)]
struct Combined<T, U> {
head: T,
tail: MaybeUninit<U>,
}
#[derive(Debug)]
enum Data<T, U: FixedArray> {
Fixed(Combined<T, U>),
Alloc(NonNull<Combined<T, U>>, usize),
}
// SAFETY: Data essentially has non-thread-specific ownership of (T, [U]), so it
// is Send + Sync if T and U are Send + Sync.
unsafe impl<T, U: FixedArray> Send for Data<T, U>
where
T: Send,
U: Send,
{
}
// SAFETY: See above comment
unsafe impl<T, U: FixedArray> Sync for Data<T, U>
where
T: Sync,
U: Sync,
{
}
impl<T, U: FixedArray> Data<T, U> {
fn head(&self) -> &T {
match self {
Data::Fixed(Combined { head, .. }) => head,
Data::Alloc(p, _) => unsafe { &p.as_ref().head },
}
}
fn head_mut(&mut self) -> &mut T {
match self {
Data::Fixed(Combined { head, .. }) => head,
Data::Alloc(p, _) => unsafe { &mut p.as_mut().head },
}
}
/// SAFETY: the caller must ensure that the first `len` elements have been
/// initialized.
unsafe fn tail(&self, len: usize) -> &[U::Element] {
match self {
Data::Fixed(Combined { tail, .. }) => {
assert!(len <= U::COUNT || size_of::<U::Element>() == 0);
unsafe { std::slice::from_raw_parts(tail.as_ptr().cast::<U::Element>(), len) }
}
Data::Alloc(p, cap) => {
assert!(len <= *cap);
unsafe {
std::slice::from_raw_parts(p.as_ref().tail.as_ptr().cast::<U::Element>(), len)
}
}
}
}
/// SAFETY: the caller must ensure that the first `len` elements have been
/// initialized.
unsafe fn tail_mut(&mut self, len: usize) -> &mut [U::Element] {
match self {
Data::Fixed(Combined { tail, .. }) => {
assert!(len <= U::COUNT || size_of::<U::Element>() == 0);
unsafe { std::slice::from_raw_parts_mut(tail.as_ptr() as *mut U::Element, len) }
}
Data::Alloc(p, cap) => {
assert!(len <= *cap);
unsafe {
std::slice::from_raw_parts_mut(p.as_ref().tail.as_ptr() as *mut U::Element, len)
}
}
}
}
fn capacity(&self) -> usize {
match self {
Data::Fixed(_) => U::COUNT,
Data::Alloc(_, cap) => *cap,
}
}
fn tail_mut_uninit(&mut self) -> &mut [MaybeUninit<U::Element>] {
match self {
Data::Fixed(Combined { tail, .. }) => unsafe {
std::slice::from_raw_parts_mut(
tail.as_mut_ptr().cast::<MaybeUninit<U::Element>>(),
U::COUNT,
)
},
Data::Alloc(p, cap) => unsafe {
std::slice::from_raw_parts_mut(
p.as_mut()
.tail
.as_mut_ptr()
.cast::<MaybeUninit<U::Element>>(),
*cap,
)
},
}
}
/// Compute the allocation layout for `cap` elements.
fn layout(cap: usize) -> Layout {
assert!(size_of::<U::Element>() > 0);
assert!(cap > U::COUNT);
let base_layout = Layout::new::<Combined<T, [U::Element; 0]>>();
Layout::from_size_align(
base_layout
.size()
.checked_add(size_of::<U::Element>().checked_mul(cap).unwrap())
.unwrap(),
base_layout.align(),
)
.unwrap()
}
/// Returns a pointer to the start of the [Combined] data that may
/// either be inline or dynamically allocated.
fn data_start_ptr(&self) -> *const Combined<T, U> {
match self {
Data::Fixed(combined_ref) => combined_ref,
Data::Alloc(p, _) => p.as_ptr(),
}
}
}
impl<T, U: FixedArray> Drop for Data<T, U> {
fn drop(&mut self) {
match self {
Data::Fixed(_) => (),
Data::Alloc(p, cap) => unsafe {
alloc::dealloc(p.as_ptr().cast::<u8>(), Self::layout(*cap))
},
}
}
}
/// Implements a `Vec`-like type for building structures with a fixed-sized
/// prefix before a dynamic number of elements.
///
/// To avoid allocations in common cases, the header and elements are stored
/// internally without allocating until the element count would exceed the
/// statically determined capacity.
///
/// Only a small portion of the `Vec` interface is supported. Additional methods
/// can be added as needed.
///
/// The data managed by this type must be `Copy`. This simplifies the resource
/// management and should be sufficient for most use cases.
///
/// # Example
/// ```
/// # use pal::HeaderVec;
/// #[derive(Copy, Clone)]
/// struct Header { x: u32 }
/// let mut v = HeaderVec::<Header, [u8; 10]>::new(Header{ x: 1234 });
/// v.push(5);
/// v.push(6);
/// assert_eq!(v.x, 1234);
/// assert_eq!(&v[..], &[5, 6]);
/// ```
#[derive(Debug)]
pub struct HeaderVec<T, U: FixedArray> {
data: Data<T, U>,
len: usize,
}
impl<T: Copy + Default, U: FixedArray> Default for HeaderVec<T, U> {
fn default() -> Self {
Self::new(Default::default())
}
}
impl<T: Copy, U: FixedArray> HeaderVec<T, U> {
/// Constructs a new `HeaderVec` with a header of `head` and no tail
/// elements.
pub fn new(head: T) -> Self {
Self {
data: Data::Fixed(Combined {
head,
tail: MaybeUninit::uninit(),
}),
len: 0,
}
}
/// Constructs a new `HeaderVec` with a header of `head` and no tail
/// elements, but with a dynamically allocated capacity for `cap` elements.
pub fn with_capacity(head: T, cap: usize) -> Self {
let mut vec = Self::new(head);
if cap > U::COUNT {
vec.realloc(cap);
}
vec
}
fn realloc(&mut self, cap: usize) {
assert!(cap > self.len);
let layout = Data::<T, U>::layout(cap);
unsafe {
let alloc = alloc::alloc(layout).cast::<Combined<T, U>>();
if let Some(alloc) = NonNull::new(alloc) {
// Copy the old header and elements.
alloc.as_ptr().cast::<u8>().copy_from(
self.data.data_start_ptr().cast::<u8>(),
self.total_byte_len(),
);
self.data = Data::Alloc(alloc, cap);
} else {
alloc::handle_alloc_error(layout);
}
}
}
fn extend_tail(&mut self, n: usize) -> &mut [MaybeUninit<U::Element>] {
let cap = self.capacity();
if cap - self.len < n {
// Double the current capacity to ensure a geometric progression
// (avoiding O(n^2) allocations).
let new_cap = cmp::max(
cmp::max(8, cap.checked_mul(2).unwrap()),
self.len.checked_add(n).unwrap(),
);
self.realloc(new_cap);
}
&mut self.data.tail_mut_uninit()[self.len..self.len + n]
}
pub fn reserve(&mut self, n: usize) {
self.extend_tail(n);
}
/// Returns the remaining spare capacity of the tail as a slice of
/// `MaybeUninit<U::Element>`.
///
/// The returned slice can be used to fill the tail with data before marking
/// the data as initialized using [`Self::set_len].
pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<U::Element>] {
&mut self.data.tail_mut_uninit()[self.len..]
}
/// Pushes a tail element, reallocating if necessary.
pub fn push(&mut self, val: U::Element) {
// For zero-sized types (unlikely to be useful but hard to prohibit),
// just increment len.
if size_of_val(&val) > 0 {
unsafe {
self.extend_tail(1)[0].as_mut_ptr().write(val);
}
}
self.len += 1;
}
/// Extends the tail elements from the given slice.
pub fn extend_from_slice(&mut self, other: &[U::Element]) {
if size_of::<U::Element>() > 0 && !other.is_empty() {
unsafe {
std::ptr::copy(
other.as_ptr(),
self.extend_tail(other.len())[0].as_mut_ptr(),
other.len(),
);
}
}
self.len += other.len();
}
/// Retrieves a pointer to the head. The tail is guaranteed to immediately
/// after the head (with appropriate padding).
pub fn as_ptr(&self) -> *const T {
self.data.head()
}
/// Retrieves a mutable pointer to the head. The tail is guaranteed to
/// immediately after the head (with appropriate padding).
pub fn as_mut_ptr(&mut self) -> *mut T {
self.data.head_mut()
}
/// Returns a slice of the tail elements.
pub fn as_slice(&self) -> &[U::Element] {
unsafe { self.data.tail(self.len) }
}
/// Returns a mutable slice of the tail elements.
pub fn as_mut_slice(&mut self) -> &mut [U::Element] {
unsafe { self.data.tail_mut(self.len) }
}
/// Returns the number of tail elements.
pub fn len(&self) -> usize {
self.len
}
pub fn capacity(&self) -> usize {
self.data.capacity()
}
/// Returns `true` if there are no tail elements.
pub fn is_empty(&self) -> bool {
self.len == 0
}
/// Sets the number of tail elements to 0.
pub fn clear(&mut self) {
self.len = 0;
}
/// Truncates the tail to `len` elements. Has no effect if there are already
/// fewer than `len` tail elements.
pub fn truncate(&mut self, len: usize) {
if len < self.len {
self.len = len;
}
}
/// Sets the number of tail elements.
///
/// Panics if `len` is greater than the capacity.
///
/// # Safety
///
/// The caller must ensure that all `len` elements have been initialized.
pub unsafe fn set_len(&mut self, len: usize) {
assert!(len <= self.capacity());
self.len = len;
}
/// Returns the total contiguous byte length of the structure, including
/// both the head and tail elements.
pub fn total_byte_len(&self) -> usize {
// N.B. this calculation cannot overflow unless len is corrupted.
size_of::<Combined<T, [U::Element; 0]>>() + size_of::<U::Element>() * self.len
}
/// Returns the total contiguous byte length of the structure, including
/// both the head and tail elements, including the tail's capacity.
pub fn total_byte_capacity(&self) -> usize {
// N.B. this calculation cannot overflow unless len is corrupted.
size_of::<Combined<T, [U::Element; 0]>>() + size_of::<U::Element>() * self.capacity()
}
/// Returns an iterator of the tail elements.
pub fn iter(&self) -> Iter<'_, U::Element> {
self.as_slice().iter()
}
/// Returns a mutable iterator of the tail elements.
pub fn iter_mut(&mut self) -> IterMut<'_, U::Element> {
self.as_mut_slice().iter_mut()
}
}
impl<T: Copy, U: FixedArray, I: SliceIndex<[U::Element]>> Index<I> for HeaderVec<T, U> {
type Output = I::Output;
fn index(&self, index: I) -> &Self::Output {
self.as_slice().index(index)
}
}
impl<T: Copy, U: FixedArray, I: SliceIndex<[U::Element]>> IndexMut<I> for HeaderVec<T, U> {
fn index_mut(&mut self, index: I) -> &mut Self::Output {
self.as_mut_slice().index_mut(index)
}
}
impl<T, U: FixedArray> Deref for HeaderVec<T, U> {
type Target = T;
fn deref(&self) -> &Self::Target {
self.data.head()
}
}
impl<T, U: FixedArray> DerefMut for HeaderVec<T, U> {
fn deref_mut(&mut self) -> &mut Self::Target {
self.data.head_mut()
}
}
impl<T: Copy, U: FixedArray> Extend<U::Element> for HeaderVec<T, U> {
fn extend<I: IntoIterator<Item = U::Element>>(&mut self, iter: I) {
for item in iter {
self.push(item);
}
}
}
#[cfg(test)]
mod tests {
use super::FixedArray;
use super::HeaderVec;
use std::fmt::Debug;
fn test<T: Copy + Eq + Debug, U: FixedArray>(head: T, vals: Vec<U::Element>)
where
U::Element: Eq + Debug,
{
let mut v: HeaderVec<T, U> = HeaderVec::new(head);
for i in vals.iter() {
v.push(*i);
}
assert_eq!(*v, head);
assert_eq!(v.as_slice(), vals.as_slice());
}
#[test]
fn test_push() {
test::<u8, [u32; 3]>(0x10, (0..200).collect());
}
#[test]
fn test_zero_array() {
test::<u8, [u32; 0]>(0x10, (0..200).collect());
}
#[test]
fn test_zst_head() {
test::<(), [u32; 3]>((), (0..200).collect());
}
#[test]
fn test_zst_tail() {
test::<u8, [(); 0]>(0x10, (0..200).map(|_| ()).collect());
}
#[test]
fn test_zst_both() {
test::<(), [(); 0]>((), (0..200).map(|_| ()).collect());
}
}