Re: [PATCH 1/2] rust: sync: add `ArcBorrow::from_raw`

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Feb 20, 2024 at 6:36 AM Boqun Feng <boqun.feng@gmail.com> wrote:
>
> On Mon, Feb 19, 2024 at 02:54:11PM +0000, Alice Ryhl wrote:
> > Allows access to a value in an `Arc` that is currently held as a raw
> > pointer due to use of `Arc::into_raw`, without destroying or otherwise
> > consuming that raw pointer.
> >
> > This is a dependency of the linked list that Rust Binder uses. The
> > linked list uses this method when iterating over the linked list.
> >
> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> > ---
> >  rust/kernel/sync/arc.rs | 72 ++++++++++++++++++++++++++++++++++++-------------
> >  1 file changed, 54 insertions(+), 18 deletions(-)
> >
> > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> > index 7d4c4bf58388..a5314df409e7 100644
> > --- a/rust/kernel/sync/arc.rs
> > +++ b/rust/kernel/sync/arc.rs
> > @@ -232,27 +232,13 @@ pub fn into_raw(self) -> *const T {
> >      /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
> >      /// must not be called more than once for each previous call to [`Arc::into_raw`].
> >      pub unsafe fn from_raw(ptr: *const T) -> Self {
> > -        let refcount_layout = Layout::new::<bindings::refcount_t>();
> > -        // SAFETY: The caller guarantees that the pointer is valid.
> > -        let val_layout = Layout::for_value(unsafe { &*ptr });
> > -        // SAFETY: We're computing the layout of a real struct that existed when compiling this
> > -        // binary, so its layout is not so large that it can trigger arithmetic overflow.
> > -        let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
> > -
> > -        // Pointer casts leave the metadata unchanged. This is okay because the metadata of `T` and
> > -        // `ArcInner<T>` is the same since `ArcInner` is a struct with `T` as its last field.
> > -        //
> > -        // This is documented at:
> > -        // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
> > -        let ptr = ptr as *const ArcInner<T>;
> > -
> > -        // SAFETY: The pointer is in-bounds of an allocation both before and after offsetting the
> > -        // pointer, since it originates from a previous call to `Arc::into_raw` and is still valid.
> > -        let ptr = unsafe { ptr.byte_sub(val_offset) };
> > +        // SAFETY: The pointer returned by `into_raw` points at the `data` field of an
> > +        // `ArcInner<T>`, as promised by the caller.
> > +        let ptr = unsafe { raw_to_inner_ptr(ptr) };
> >
> >          // SAFETY: By the safety requirements we know that `ptr` came from `Arc::into_raw`, so the
> >          // reference count held then will be owned by the new `Arc` object.
> > -        unsafe { Self::from_inner(NonNull::new_unchecked(ptr.cast_mut())) }
> > +        unsafe { Self::from_inner(ptr) }
> >      }
> >
> >      /// Returns an [`ArcBorrow`] from the given [`Arc`].
> > @@ -273,6 +259,35 @@ pub fn ptr_eq(this: &Self, other: &Self) -> bool {
> >      }
> >  }
> >
> > +/// Converts a pointer to the contents of an [`Arc`] into a pointer to the [`ArcInner`].
> > +///
> > +/// # Safety
> > +///
> > +/// The provided pointer must point the `data` field of an `ArcInner<T>` value.
> > +unsafe fn raw_to_inner_ptr<T: ?Sized>(ptr: *const T) -> NonNull<ArcInner<T>> {
>
> Nit: put this into an `impl<T:?Sized> ArcInner<T>` block maybe?
>
> > +    let refcount_layout = Layout::new::<bindings::refcount_t>();
> > +    // SAFETY: The caller guarantees that the pointer is valid.
> > +    let val_layout = Layout::for_value(unsafe { &*ptr });
> > +    // SAFETY: We're computing the layout of a real struct that existed when compiling this
> > +    // binary, so its layout is not so large that it can trigger arithmetic overflow.
> > +    let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
> > +
> > +    // Pointer casts leave the metadata unchanged. This is okay because the metadata of `T` and
> > +    // `ArcInner<T>` is the same since `ArcInner` is a struct with `T` as its last field.
> > +    //
> > +    // This is documented at:
> > +    // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
> > +    let ptr = ptr as *const ArcInner<T>;
> > +
> > +    // SAFETY: The pointer is in-bounds of an allocation both before and after offsetting the
> > +    // pointer, since it originates from a previous call to `Arc::into_raw` and is still valid.
>
> "since it originate from a previous call to `Arc::into_raw`" is the
> safety requirement of `Arc::from_raw`, since the safety requirement of
> `raw_to_inner_ptr` is different, so I think we should say "since the
> function safety requirement guarantees `ptr` points to `data` field,
> which is exactly `val_offset` away from the beginning of `ArcInner<T>`".
> Thoughts?
>
> BTW, in fat pointer cases, by "must point the `data` field of an
> `ArcInner<T>` value", it means both the address and the metadata should
> be the same as the original object in `ArcInner<T>`, right? In other
> words, the following code should not be safe, i.e. the
> raw_to_inner_ptr() safety requirement is not satisfied.
>
>         let x: Arc<[u8]> // assume x.len() == 4
>
>         let y = &(x[0..1]) as *const [u8] // y has the same address of
>                                           // the `data` field of `x`.
>
>         let inner = unsafe { raw_to_inner_ptr(y) };
>         // ^^^ the safety requirement is not satisfied???
>
> This may not be important since the users of `raw_to_inner_ptr` all have
> stronger safey guarantees ("`ptr` must come from `Arc::into_raw()`"),
> and `raw_to_inner_ptr` is not a pub function, but I just wonder whether
> we need to improve the current safety requirements, or "point" means
> both address and metadata for fat pointers?

I mean, really, the same problem arises for Sized pointers where the
pointer points at the first field of a struct.

We may have to take inspiration from the std Arc::from_raw. It says
that the pointee must have the same size and alignment as what was
used when you called Arc::into_raw, and then it further says that if
the pointer type is not exactly the same, then you are effectively
performing a transmute.

Alice