Re: [PATCH 1/3] rust: alloc: add Vec::truncate method

[Andrew Ballance <andrewjballance@gmail.com>]

On Sat, Mar 15, 2025 at 10:09:26AM +0000, Benno Lossin wrote:
> On Sat Mar 15, 2025 at 3:42 AM CET, Andrew Ballance wrote:
> > implements the equivalent to the std's Vec::truncate
> > on the kernel's Vec type.
> >
> > Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
> > ---
> >  rust/kernel/alloc/kvec.rs | 36 ++++++++++++++++++++++++++++++++++++
> >  1 file changed, 36 insertions(+)
> >
> > diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> > index ae9d072741ce..75e9feebb81f 100644
> > --- a/rust/kernel/alloc/kvec.rs
> > +++ b/rust/kernel/alloc/kvec.rs
> > @@ -452,6 +452,42 @@ pub fn reserve(&mut self, additional: usize, flags: Flags) -> Result<(), AllocEr
> >  
> >          Ok(())
> >      }
> > +
> > +    /// Shortens the vector, setting the length to `len` and drops the removed values.
> > +    /// If `len` is greater than or equal to the current length, this does nothing.
> > +    ///
> > +    /// This has no effect on the capacity and will not allocate.
> > +    /// # Examples
> > +    /// ```
> > +    /// let mut v = kernel::kvec![1, 2, 3]?;
> > +    /// v.truncate(1);
> > +    /// assert_eq!(v.len(), 1);
> > +    /// assert_eq!(&v, &[1]);
> > +    ///
> > +    /// # Ok::<(), Error>(())
> > +    /// ```
> > +    pub fn truncate(&mut self, len: usize) {
> > +        if len >= self.len() {
> > +            return;
> > +        }
> > +
> > +        // [new_len, len) is guaranteed to be valid because [0, len) is guaranteed to be valid
> > +        let drop_range = len..self.len();
> > +
> > +        // SAFETY:
> > +        // we can safely ignore the bounds check because we already did our own check
> > +        let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };
> 
> What's this `get_unchecked_mut` method, I don't see it in `rust-next` or
> `alloc-next`.

Vec derefs into a slice which implements get_uncheked_mut
https://rust.docs.kernel.org/next/kernel/alloc/kvec/struct.Vec.html#method.get_unchecked_mut

> > +
> > +        // SAFETY:
> > +        // it is safe to shrink the length because the new length is
> > +        // guaranteed to be less than the old length
> 
> Please take a look at the documentation of `set_len`, in the safety
> section you'll find what you need to justify here.
> 
> > +        unsafe { self.set_len(len) };
> > +
> > +        // SAFETY:
> 
> A couple points missing:
> - why is the pointer valid?
> 
> > +        // - the dropped values are valid `T`s
> > +        // - we are allowed to invalidate [new_len, old_len) because we just changed the len
> 
> This should justify why the value will never be accessed again.
> 

I will fixup the safety comments for the v2. Thanks.

> ---
> Cheers,
> Benno
> 
> > +        unsafe { ptr::drop_in_place(ptr) };
> > +    }
> >  }
> >  
> >  impl<T: Clone, A: Allocator> Vec<T, A> {
> 
>