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

[Tamir Duberstein]

On Wed, Mar 19, 2025 at 5:53 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>
> On Tue, Mar 18, 2025 at 04:13:55PM -0400, Tamir Duberstein 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) => {
>
> This could be simplified as:
> if let Some(count) = self.len().checked_sub(len) {
>     // logic here
> }
>
> or
> let Some(count) = self.len().checked_sub(len) else {
>     return;
> }
> // logic here

👍

>
> > +                // 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) };
>
> We have a mutable reference to these elements until after the
> `drop_in_place` call, but the elements are invalidated by that call.
> This means that we have a mutable reference to invalid values, which
> violates the invariants for mutable references.
>
> Consider converting to a raw pointer when creating `tail` instead to
> avoid that:
>
> let tail: *mut [T] = unsafe { self.dec_len(count) };
> unsafe { ptr::drop_in_place(ptr) };

👍