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

Alice Ryhl
2025-04-23T08:33:39Z

On Wed, Apr 23, 2025 at 12:24 AM Boqun Feng <boqun.feng@gmail.com> wrote:
>
> 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?

I can add a 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.)

Good catch, thanks.

Alice