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

[Tamir Duberstein]

On Mon, Mar 17, 2025 at 10:34 AM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
>
> On Mon Mar 17, 2025 at 12:42 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").
> >
> > Signed-off-by: Tamir Duberstein <tamird@xxxxxxxxx>
> > ---
> >  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..d2bc3d02179e 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) };
> > +        // The call to `reserve` was successful so the spare capacity is at least 1.
> > +        self.spare_capacity_mut()[0].write(v);
>
> I think the code uses unsafe to avoid a bounds check, but I'm not 100%
> sure. Danilo might remember more info.

We could use `slice::get_unchecked_mut` here to retain the same
guarantee of no bounds check. That would still be one fewer unsafe
blocks.