Date:   Sun, 22 Jan 2023 03:34:40 -0300
From:   Wedson Almeida Filho <wedsonaf@...il.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] rust: types: introduce `ForeignOwnable`

On Fri, 20 Jan 2023 at 16:59, Boqun Feng <boqun.feng@...il.com> wrote:
>
> 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@...il.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();
>             })
>     }

Oh, eliminate the generic type `T` and just use `Self`. Yes, I think
that's a good simplification.

>
> 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?

It isn't. Since we want the raw pointer to fit in a single pointer
(from C's perspective), we already require `Self: Sized`.

I'll make this change in v2.

>
> 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
> >

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