Re: [PATCH v2] rust: alloc: use `spare_capacity_mut` to reduce unsafe

[Benno Lossin <benno.lossin@proton.me>]

On Mon Mar 17, 2025 at 6:23 PM CET, Tamir Duberstein wrote:
> Use `spare_capacity_mut` in the implementation of `push` to reduce the
> use of `unsafe`. Both methods were added in commit 2aac4cd7dae3 ("rust:
> alloc: implement kernel `Vec` type").
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Tamir Duberstein <tamird@gmail.com>

One nit below, with or without that fixed:

Reviewed-by: Benno Lossin <benno.lossin@proton.me>

> ---
> Changes in v2:
> - Use `slice::get_unchecked_mut` to ensure we avoid a bounds check.
>   (Benno Lossin)
> - Link to v1: https://lore.kernel.org/r/20250317-vec-push-use-spare-v1-1-7e025ef4ae14@gmail.com
> ---
>  rust/kernel/alloc/kvec.rs | 11 ++---------
>  1 file changed, 2 insertions(+), 9 deletions(-)
>
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index ae9d072741ce..b91b287e0d22 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -285,15 +285,8 @@ pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<T>] {
>      pub fn push(&mut self, v: T, flags: Flags) -> Result<(), AllocError> {
>          self.reserve(1, flags)?;
>  
> -        // SAFETY:
> -        // - `self.len` is smaller than `self.capacity` and hence, the resulting pointer is
> -        //   guaranteed to be part of the same allocated object.
> -        // - `self.len` can not overflow `isize`.
> -        let ptr = unsafe { self.as_mut_ptr().add(self.len) };
> -
> -        // SAFETY:
> -        // - `ptr` is properly aligned and valid for writes.
> -        unsafe { core::ptr::write(ptr, v) };
> +        // SAFETY: The call to `reserve` was successful so the spare capacity is at least 1.
> +        unsafe { self.spare_capacity_mut().get_unchecked_mut(0) }.write(v);

IMO it's difficult to tell which operation is the `unsafe` one here, so
how about you factor the `spare_capacity_mut` call out.

---
Cheers,
Benno

>  
>          // SAFETY: We just initialised the first spare entry, so it is safe to increase the length
>          // by 1. We also know that the new length is <= capacity because of the previous call to
>
> ---
> base-commit: cf25bc61f8aecad9b0c45fe32697e35ea4b13378
> change-id: 20250317-vec-push-use-spare-27484fd016a9
>
> Best regards,