Re: [PATCH v2 3/4] rust: alloc: refactor `Vec::truncate` using `dec_len`

[Tamir Duberstein]

On Tue, Mar 18, 2025 at 4:14 PM Tamir Duberstein <tamird@xxxxxxxxx> 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@xxxxxxxxx>
> ---
>  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
>