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

[Benno Lossin]

On Mon Mar 17, 2025 at 6:09 PM CET, Danilo Krummrich wrote:
> 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@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.
>
> 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().)

If we use saturating_sub then we don't need a bounds check (at least on
non-debug builds), right? 

>> 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.

+1