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Message-ID: <CAJ-ks9n597+YPap+xMGbMwJ_AUGNRNjB7npaE5dee8SG5W=6iQ@mail.gmail.com>
Date: Tue, 18 Mar 2025 16:16:44 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Danilo Krummrich <dakr@...nel.org>, Andrew Ballance <andrewjballance@...il.com>,
Alice Ryhl <aliceryhl@...gle.com>, Miguel Ojeda <ojeda@...nel.org>,
Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>,
Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>,
Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>,
Trevor Gross <tmgross@...ch.edu>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/4] rust: alloc: refactor `Vec::truncate` using `dec_len`
On Tue, Mar 18, 2025 at 4:14 PM Tamir Duberstein <tamird@...il.com> wrote:
>
> Use `checked_sub` to satisfy the safety requirements of `dec_len` and
> replace nearly the whole body of `truncate` with a call to `dec_len`.
>
> Signed-off-by: Tamir Duberstein <tamird@...il.com>
> ---
> rust/kernel/alloc/kvec.rs | 29 +++++++++++------------------
> 1 file changed, 11 insertions(+), 18 deletions(-)
>
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index 97cc5ab11e2a..6f4dc89ef7f8 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -489,25 +489,18 @@ pub fn reserve(&mut self, additional: usize, flags: Flags) -> Result<(), AllocEr
> /// # Ok::<(), Error>(())
> /// ```
> pub fn truncate(&mut self, len: usize) {
> - if len >= self.len() {
> - return;
> + match self.len().checked_sub(len) {
> + None => {}
> + Some(count) => {
> + // SAFETY: `count` is `self.len() - len` so it is guaranteed to be less than or
> + // equal to `self.len()`.
> + let tail = unsafe { self.dec_len(count) };
> +
> + // SAFETY: the contract of `dec_len` guarantees that the elements in `tail` are
> + // valid elements whose ownership has been transferred to the caller.
> + unsafe { ptr::drop_in_place(ptr) };
Whoops, this should be s/ptr/tail/. Will fix on respin if necessary.
> + }
> }
> -
> - let drop_range = len..self.len();
> -
> - // SAFETY: `drop_range` is a subrange of `[0, len)` by the bounds check above.
> - let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };
> -
> - // SAFETY:
> - // - this will always shrink the vector because of the above bounds check
> - // - [`new_len`, `self.len`) will be dropped through the call to `drop_in_place` below
> - unsafe { self.set_len(len) };
> -
> - // SAFETY:
> - // - the dropped values are valid `T`s by the type invariant
> - // - we are allowed to invalidate [`new_len`, `old_len`) because we just changed the
> - // len, therefore we have exclusive access to [`new_len`, `old_len`)
> - unsafe { ptr::drop_in_place(ptr) };
> }
> }
>
>
> --
> 2.48.1
>
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