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

[Danilo Krummrich <dakr@kernel.org>]

On Mon, Mar 17, 2025 at 10:39:05AM -0400, Tamir Duberstein wrote:
> On Mon, Mar 17, 2025 at 10:34 AM Benno Lossin <benno.lossin@proton.me> 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@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..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.

Yes, that was the justification to use unsafe calls instead.

(This may also justify keeping dec_len() unsafe, since otherwise it would
introduce an additional boundary check for pop().)

> 
> 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.

Sounds reasonable.