This patch is basically a proof of concept intendend to gather feedback about how to do this properly. Normally I would want to use the crate from crates.io[1], but that's not an option in the kernel. We could also vendor the entire source code of arrayvec. I'm not sure if people will be happy with that. [1] https://crates.io/crates/arrayvec Signed-off-by: Remo Senekowitsch <remo@xxxxxxxxxxx> --- rust/kernel/arrayvec.rs | 81 +++++++++++++++++++++++++++++++++++++++++ rust/kernel/lib.rs | 1 + 2 files changed, 82 insertions(+) create mode 100644 rust/kernel/arrayvec.rs diff --git a/rust/kernel/arrayvec.rs b/rust/kernel/arrayvec.rs new file mode 100644 index 000000000..041e7dcce --- /dev/null +++ b/rust/kernel/arrayvec.rs @@ -0,0 +1,81 @@ +// SPDX-License-Identifier: GPL-2.0 + +//! Provides [ArrayVec], a stack-allocated vector with static capacity. + +use core::mem::MaybeUninit; + +/// A stack-allocated vector with statically fixed capacity. +/// +/// This can be useful to avoid heap allocation and still ensure safety where a +/// small but dynamic number of elements is needed. +/// +/// For example, consider a function that returns a variable number of values, +/// but no more than 8. In C, one might achieve this by passing a pointer to +/// a stack-allocated array as an out-parameter and making the function return +/// the actual number of elements. This is not safe, because nothing prevents +/// the caller from reading elements from the array that weren't actually +/// initialized by the function. `ArrayVec` solves this problem, users are +/// prevented from accessing uninitialized elements. +/// +/// This basically exists already (in a much more mature form) on crates.io: +/// <https://crates.io/crates/arrayvec> +#[derive(Debug)] +pub struct ArrayVec<const N: usize, T> { + array: [core::mem::MaybeUninit<T>; N], + len: usize, +} + +impl<const N: usize, T> ArrayVec<N, T> { + /// Adds a new element to the end of the vector. + /// + /// # Panics + /// + /// Panics if the vector is already full. + pub fn push(&mut self, elem: T) { + if self.len == N { + panic!("OOM") + } + self.array[self.len] = MaybeUninit::new(elem); + self.len += 1; + } + + /// Returns the length of the vector. + pub fn len(&self) -> usize { + self.len + } +} + +impl<const N: usize, T> Default for ArrayVec<N, T> { + fn default() -> Self { + Self { + array: [const { MaybeUninit::uninit() }; N], + len: 0, + } + } +} + +impl<const N: usize, T> AsRef<[T]> for ArrayVec<N, T> { + fn as_ref(&self) -> &[T] { + // SAFETY: As per the type invariant, all elements at index < self.len + // are initialized. + unsafe { core::mem::transmute(&self.array[..self.len]) } + } +} + +impl<const N: usize, T> AsMut<[T]> for ArrayVec<N, T> { + fn as_mut(&mut self) -> &mut [T] { + // SAFETY: As per the type invariant, all elements at index < self.len + // are initialized. + unsafe { core::mem::transmute(&mut self.array[..self.len]) } + } +} + +impl<const N: usize, T> Drop for ArrayVec<N, T> { + fn drop(&mut self) { + unsafe { + let slice: &mut [T] = + core::slice::from_raw_parts_mut(self.array.as_mut_ptr().cast(), self.len); + core::ptr::drop_in_place(slice); + } + } +} diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs index ca233fd20..0777f7a42 100644 --- a/rust/kernel/lib.rs +++ b/rust/kernel/lib.rs @@ -36,6 +36,7 @@ pub use ffi; pub mod alloc; +pub mod arrayvec; #[cfg(CONFIG_BLOCK)] pub mod block; #[doc(hidden)] -- 2.49.0
