Re: [PATCH v3 09/25] rust: alloc: implement kernel `Box`

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Aug 1, 2024 at 2:07 AM Danilo Krummrich <dakr@kernel.org> wrote:
>
> `Box` provides the simplest way to allocate memory for a generic type
> with one of the kernel's allocators, e.g. `Kmalloc`, `Vmalloc` or
> `KVmalloc`.
>
> In contrast to Rust's `Box` type, the kernel `Box` type considers the
> kernel's GFP flags for all appropriate functions, always reports
> allocation failures through `Result<_, AllocError>` and remains
> independent from unstable features.
>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
>
> [...]
>
> +    /// Constructs a `Box<T, A>` from a raw pointer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// `raw` must point to valid memory, previously allocated with `A`, and at least the size of
> +    /// type `T`.
> +    #[inline]
> +    pub const unsafe fn from_raw_alloc(raw: *mut T, alloc: PhantomData<A>) -> Self {
> +        // SAFETY: Safe by the requirements of this function.
> +        Box(unsafe { Unique::new_unchecked(raw) }, alloc)
> +    }

I don't think it makes sense to take the PhantomData as a parameter.
You can always create a PhantomData value out of thin air.

Box(unsafe { Unique::new_unchecked(raw) }, PhantomData)

> +    /// Consumes the `Box<T, A>`, returning a wrapped raw pointer and `PhantomData` of the allocator
> +    /// it was allocated with.
> +    pub fn into_raw_alloc(b: Self) -> (*mut T, PhantomData<A>) {
> +        let b = ManuallyDrop::new(b);
> +        let alloc = unsafe { ptr::read(&b.1) };
> +        (b.0.as_ptr(), alloc)
> +    }

I don't think there's any need to have this function. The caller can
always create the PhantomData themselves. I would just keep into_raw
only.

> +    /// Converts a `Box<T>` into a `Pin<Box<T>>`.
> +    #[inline]
> +    pub fn into_pin(b: Self) -> Pin<Self>
> +    where
> +        A: 'static,
> +    {
> +        // SAFETY: It's not possible to move or replace the insides of a `Pin<Box<T>>` when
> +        // `T: !Unpin`, so it's safe to pin it directly without any additional requirements.
> +        unsafe { Pin::new_unchecked(b) }
> +    }

In the standard library, this functionality is provided using the From
trait rather than an inherent method. I think it makes sense to match
std here.

> +impl<T, A> Drop for Box<T, A>
> +where
> +    T: ?Sized,
> +    A: Allocator,
> +{
> +    fn drop(&mut self) {
> +        let ptr = self.0.as_ptr();
> +
> +        // SAFETY: We need to drop `self.0` in place, before we free the backing memory.
> +        unsafe { core::ptr::drop_in_place(ptr) };
> +
> +        // SAFETY: `ptr` is always properly aligned, dereferenceable and points to an initialized
> +        // instance of `T`.
> +        if unsafe { core::mem::size_of_val(&*ptr) } != 0 {
> +            // SAFETY: `ptr` was previously allocated with `A`.
> +            unsafe { A::free(self.0.as_non_null().cast()) };
> +        }

You just destroyed the value by calling `drop_in_place`, so `ptr` no
longer points at an initialized instance of `T`. Please compute
whether the allocation has non-zero size before you call
`drop_in_place`.

Also, in normal Rust this code would leak the allocation on panic in
the destructor. We may not care, but it's worth taking into account if
anybody else copies this code to a different project with a different
panic configuration.

> +impl<T: 'static, A> ForeignOwnable for crate::alloc::Box<T, A>
> +where
> +    A: Allocator,
> +{
> +    type Borrowed<'a> = &'a T;
> +
> +    fn into_foreign(self) -> *const core::ffi::c_void {
> +        crate::alloc::Box::into_raw(self) as _
> +    }
> +
> +    unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> &'a T {
> +        // SAFETY: The safety requirements for this function ensure that the object is still alive,
> +        // so it is safe to dereference the raw pointer.
> +        // The safety requirements of `from_foreign` also ensure that the object remains alive for
> +        // the lifetime of the returned value.
> +        unsafe { &*ptr.cast() }
> +    }
> +
> +    unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
> +        // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
> +        // call to `Self::into_foreign`.
> +        unsafe { crate::alloc::Box::from_raw(ptr as _) }
> +    }
> +}

You may want to also implement ForeignOwnable for Pin<Box<T>>. See:
https://lore.kernel.org/all/20240730-foreign-ownable-pin-box-v1-1-b1d70cdae541@google.com/

Alice