Re: [PATCH 2/5] rust: types: introduce `ForeignOwnable`

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Jan 19, 2023 at 02:40:33PM -0300, Wedson Almeida Filho wrote:
> It was originally called `PointerWrapper`. It is used to convert
> a Rust object to a pointer representation (void *) that can be
> stored on the C side, used, and eventually returned to Rust.
> 
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> ---
>  rust/kernel/lib.rs   |  1 +
>  rust/kernel/types.rs | 54 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 55 insertions(+)
> 
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index e0b0e953907d..223564f9f0cc 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -16,6 +16,7 @@
>  #![feature(coerce_unsized)]
>  #![feature(core_ffi_c)]
>  #![feature(dispatch_from_dyn)]
> +#![feature(generic_associated_types)]
>  #![feature(receiver_trait)]
>  #![feature(unsize)]
>  
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index f0ad4472292d..5475f6163002 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -9,6 +9,60 @@ use core::{
>      ops::{Deref, DerefMut},
>  };
>  
> +/// Used to transfer ownership to and from foreign (non-Rust) languages.
> +///
> +/// Ownership is transferred from Rust to a foreign language by calling [`Self::into_foreign`] and
> +/// later may be transferred back to Rust by calling [`Self::from_foreign`].
> +///
> +/// This trait is meant to be used in cases when Rust objects are stored in C objects and
> +/// eventually "freed" back to Rust.
> +pub trait ForeignOwnable {
> +    /// Type of values borrowed between calls to [`ForeignOwnable::into_foreign`] and
> +    /// [`ForeignOwnable::from_foreign`].
> +    type Borrowed<'a>;
> +
> +    /// Converts a Rust-owned object to a foreign-owned one.
> +    ///
> +    /// The foreign representation is a pointer to void.
> +    fn into_foreign(self) -> *const core::ffi::c_void;
> +
> +    /// Borrows a foreign-owned object.
> +    ///
> +    /// # Safety
> +    ///
> +    /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
> +    /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
> +    /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow_mut`]
> +    /// for this object must have been dropped.
> +    unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> Self::Borrowed<'a>;
> +
> +    /// Mutably borrows a foreign-owned object.
> +    ///
> +    /// # Safety
> +    ///
> +    /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
> +    /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
> +    /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] and
> +    /// [`ForeignOwnable::borrow_mut`] for this object must have been dropped.
> +    unsafe fn borrow_mut<T: ForeignOwnable>(ptr: *const core::ffi::c_void) -> ScopeGuard<T, fn(T)> {
> +        // SAFETY: The safety requirements ensure that `ptr` came from a previous call to
> +        // `into_foreign`.
> +        ScopeGuard::new_with_data(unsafe { T::from_foreign(ptr) }, |d| {
> +            d.into_foreign();
> +        })

I kinda want to suggest borrow_mut() to be implemented as:

    pub trait ForeignOwnable {
        ...
        unsafe fn borrow_mut(ptr: *const core::ffi::c_void) -> ScopeGuard<Self, fn(Self)> {
            // SAFETY: The safety requirements ensure that `ptr` came from a previous call to
            // `into_foreign`.
            ScopeGuard::new_with_data(unsafe { Self::from_foreign(ptr) }, |d| {
                d.into_foreign();
            })
    }

to avoid funny code as follow:

	let a = Box::new(0).into_foreign();
	// Using an irrelevant `impl ForeignOwnable` to `borrow_mut`
	let borrowed_a: ScopeGuard<Box<i32>, ...> = unsafe { Arc::<u64>::borrow_mut(a) };

but that requires `Self: Sized`. Is it too restrictive?

Regards,
Boqun

> +    }
> +
> +    /// Converts a foreign-owned object back to a Rust-owned one.
> +    ///
> +    /// # Safety
> +    ///
> +    /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
> +    /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
> +    /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] and
> +    /// [`ForeignOwnable::borrow_mut`] for this object must have been dropped.
> +    unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self;
> +}
> +
>  /// Runs a cleanup function/closure when dropped.
>  ///
>  /// The [`ScopeGuard::dismiss`] function prevents the cleanup function from running.
> -- 
> 2.34.1
>