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

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Message-ID: <CAH5fLgh=MCDw74XEYPh4_T9fryrdA=aBLioewe+=biJc8C2PzA@mail.gmail.com>
Date: Wed, 23 Apr 2025 10:33:26 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Danilo Krummrich <dakr@...nel.org>, Matthew Maurer <mmaurer@...gle.com>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 6/7] rust: alloc: add Vec::remove

On Wed, Apr 23, 2025 at 12:24 AM Boqun Feng <boqun.feng@...il.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@...gle.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