Re: [PATCH v3 6/7] rust: alloc: add Vec::remove

Boqun Feng
2025-04-22T22:24:32Z

On Tue, Apr 22, 2025 at 09:52:21AM +0000, Alice Ryhl wrote:
> This is needed by Rust Binder in the range allocator, and by upcoming
> GPU drivers during firmware initialization.
>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
> rust/kernel/alloc/kvec.rs | 31 +++++++++++++++++++++++++++++++
> 1 file changed, 31 insertions(+)
>
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index 2f894eac02212d15d902fe6702d6155f3128997c..2f28fda793e13841b59e83f34681e71ac815aff2 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -386,6 +386,37 @@ pub fn pop(&mut self) -> Option<T> {
> Some(unsafe { removed.read() })
> }
>
> + /// Removes the element at the given index.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// let mut v = kernel::kvec![1, 2, 3]?;
> + /// assert_eq!(v.remove(1), 2);
> + /// assert_eq!(v, [1, 3]);
> + /// # Ok::<(), Error>(())
> + /// ```
> + pub fn remove(&mut self, i: usize) -> T {
> + // INVARIANT: This breaks the invariants by invalidating the value at index `i`, but we
> + // restore the invariants below.
> + // SAFETY: Since `&self[i]` did not result in a panic, the value at index `i` is valid.

So a out-of-bound `i` would result into a panic? Then I think we need a
"# Panics" section?

> + let value = unsafe { ptr::read(&self[i]) };
> +
> + // SAFETY: Since the above access did not panic, the length is at least one.
> + unsafe { self.dec_len(1) };
> +

I think you need to move this line after the `ptr::copy()`, right?
Otherwise, you're using the *new* length to calculate how many elements
you are copying. (For example, in your above example, self.len is 2
after self.dec_len(), and the the following copy would be copy(p.add(1),
p, 2 - 1 - 1), which copies zero data, but it would be wrong.)

Regards,
Boqun

> + // SAFETY: We checked that `i` is in-bounds.
> + let p = unsafe { self.as_mut_ptr().add(i) };
> +
> + // INVARIANT: This restores the Vec invariants by moving the valid values into the region
> + // that is required to hold valid values.
> + // SAFETY: `p.add(1).add(self.len - i - 1)` is `i+1+len-i-1 == len` elements after the
> + // beginning of the vector, so this is in-bounds of the vector.
> + unsafe { ptr::copy(p.add(1), p, self.len - i - 1) };
> +
> + value
> + }
> +
> /// Creates a new [`Vec`] instance with at least the given capacity.
> ///
> /// # Examples
>
> --
> 2.49.0.805.g082f7c87e0-goog
>
>