rust: types: add ForeignOwnable::PointedTo

Allow implementors to specify the foreign pointer type; this exposes
information about the pointed-to type such as its alignment.

This requires the trait to be `unsafe` since it is now possible for
implementors to break soundness by returning a misaligned pointer.

Encoding the pointer type in the trait (and avoiding pointer casts)
allows the compiler to check that implementors return the correct
pointer type. This is preferable to directly encoding the alignment in
the trait using a constant as the compiler would be unable to check it.

Acked-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250423-rust-xarray-bindings-v19-1-83cdcf11c114@gmail.com
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

Author: tamird

rust/kernel/alloc/kbox.rs

@@ -360,68 +360,70 @@ where
     }
 }
 
-impl<T: 'static, A> ForeignOwnable for Box<T, A>
+// SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
+unsafe impl<T: 'static, A> ForeignOwnable for Box<T, A>
 where
     A: Allocator,
 {
+    type PointedTo = T;
     type Borrowed<'a> = &'a T;
     type BorrowedMut<'a> = &'a mut T;
 
-    fn into_foreign(self) -> *mut crate::ffi::c_void {
-        Box::into_raw(self).cast()
+    fn into_foreign(self) -> *mut Self::PointedTo {
+        Box::into_raw(self)
     }
 
-    unsafe fn from_foreign(ptr: *mut crate::ffi::c_void) -> Self {
+    unsafe fn from_foreign(ptr: *mut Self::PointedTo) -> Self {
         // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
         // call to `Self::into_foreign`.
-        unsafe { Box::from_raw(ptr.cast()) }
+        unsafe { Box::from_raw(ptr) }
     }
 
-    unsafe fn borrow<'a>(ptr: *mut crate::ffi::c_void) -> &'a T {
+    unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> &'a T {
         // SAFETY: The safety requirements of this method ensure that the object remains alive and
         // immutable for the duration of 'a.
-        unsafe { &*ptr.cast() }
+        unsafe { &*ptr }
     }
 
-    unsafe fn borrow_mut<'a>(ptr: *mut crate::ffi::c_void) -> &'a mut T {
-        let ptr = ptr.cast();
+    unsafe fn borrow_mut<'a>(ptr: *mut Self::PointedTo) -> &'a mut T {
         // SAFETY: The safety requirements of this method ensure that the pointer is valid and that
         // nothing else will access the value for the duration of 'a.
         unsafe { &mut *ptr }
     }
 }
 
-impl<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
+// SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
+unsafe impl<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
 where
     A: Allocator,
 {
+    type PointedTo = T;
     type Borrowed<'a> = Pin<&'a T>;
     type BorrowedMut<'a> = Pin<&'a mut T>;
 
-    fn into_foreign(self) -> *mut crate::ffi::c_void {
+    fn into_foreign(self) -> *mut Self::PointedTo {
         // SAFETY: We are still treating the box as pinned.
-        Box::into_raw(unsafe { Pin::into_inner_unchecked(self) }).cast()
+        Box::into_raw(unsafe { Pin::into_inner_unchecked(self) })
     }
 
-    unsafe fn from_foreign(ptr: *mut crate::ffi::c_void) -> Self {
+    unsafe fn from_foreign(ptr: *mut Self::PointedTo) -> Self {
         // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
         // call to `Self::into_foreign`.
-        unsafe { Pin::new_unchecked(Box::from_raw(ptr.cast())) }
+        unsafe { Pin::new_unchecked(Box::from_raw(ptr)) }
     }
 
-    unsafe fn borrow<'a>(ptr: *mut crate::ffi::c_void) -> Pin<&'a T> {
+    unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> Pin<&'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.
-        let r = unsafe { &*ptr.cast() };
+        let r = unsafe { &*ptr };
 
         // SAFETY: This pointer originates from a `Pin<Box<T>>`.
         unsafe { Pin::new_unchecked(r) }
     }
 
-    unsafe fn borrow_mut<'a>(ptr: *mut crate::ffi::c_void) -> Pin<&'a mut T> {
-        let ptr = ptr.cast();
+    unsafe fn borrow_mut<'a>(ptr: *mut Self::PointedTo) -> Pin<&'a mut 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

rust/kernel/miscdevice.rs

@@ -200,7 +200,7 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
         // type.
         //
         // SAFETY: The open call of a file can access the private data.
-        unsafe { (*raw_file).private_data = ptr.into_foreign() };
+        unsafe { (*raw_file).private_data = ptr.into_foreign().cast() };
 
         0
     }
@@ -211,7 +211,7 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
     /// must be associated with a `MiscDeviceRegistration<T>`.
     unsafe extern "C" fn release(_inode: *mut bindings::inode, file: *mut bindings::file) -> c_int {
         // SAFETY: The release call of a file owns the private data.
-        let private = unsafe { (*file).private_data };
+        let private = unsafe { (*file).private_data }.cast();
         // SAFETY: The release call of a file owns the private data.
         let ptr = unsafe { <T::Ptr as ForeignOwnable>::from_foreign(private) };
 
@@ -228,7 +228,7 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
     /// `file` must be a valid file that is associated with a `MiscDeviceRegistration<T>`.
     unsafe extern "C" fn ioctl(file: *mut bindings::file, cmd: c_uint, arg: c_ulong) -> c_long {
         // SAFETY: The ioctl call of a file can access the private data.
-        let private = unsafe { (*file).private_data };
+        let private = unsafe { (*file).private_data }.cast();
         // SAFETY: Ioctl calls can borrow the private data of the file.
         let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };
 
@@ -253,7 +253,7 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
         arg: c_ulong,
     ) -> c_long {
         // SAFETY: The compat ioctl call of a file can access the private data.
-        let private = unsafe { (*file).private_data };
+        let private = unsafe { (*file).private_data }.cast();
         // SAFETY: Ioctl calls can borrow the private data of the file.
         let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };
 
@@ -274,7 +274,7 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
     /// - `seq_file` must be a valid `struct seq_file` that we can write to.
     unsafe extern "C" fn show_fdinfo(seq_file: *mut bindings::seq_file, file: *mut bindings::file) {
         // SAFETY: The release call of a file owns the private data.
-        let private = unsafe { (*file).private_data };
+        let private = unsafe { (*file).private_data }.cast();
         // SAFETY: Ioctl calls can borrow the private data of the file.
         let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };
         // SAFETY:

rust/kernel/pci.rs

@@ -89,7 +89,7 @@ impl<T: Driver + 'static> Adapter<T> {
     extern "C" fn remove_callback(pdev: *mut bindings::pci_dev) {
         // SAFETY: The PCI bus only ever calls the remove callback with a valid pointer to a
         // `struct pci_dev`.
-        let ptr = unsafe { bindings::pci_get_drvdata(pdev) };
+        let ptr = unsafe { bindings::pci_get_drvdata(pdev) }.cast();
 
         // SAFETY: `remove_callback` is only ever called after a successful call to
         // `probe_callback`, hence it's guaranteed that `ptr` points to a valid and initialized

rust/kernel/platform.rs

@@ -80,7 +80,7 @@ impl<T: Driver + 'static> Adapter<T> {
 
     extern "C" fn remove_callback(pdev: *mut bindings::platform_device) {
         // SAFETY: `pdev` is a valid pointer to a `struct platform_device`.
-        let ptr = unsafe { bindings::platform_get_drvdata(pdev) };
+        let ptr = unsafe { bindings::platform_get_drvdata(pdev) }.cast();
 
         // SAFETY: `remove_callback` is only ever called after a successful call to
         // `probe_callback`, hence it's guaranteed that `ptr` points to a valid and initialized

rust/kernel/sync/arc.rs

@@ -140,9 +140,10 @@ pub struct Arc<T: ?Sized> {
     _p: PhantomData<ArcInner<T>>,
 }
 
+#[doc(hidden)]
 #[pin_data]
 #[repr(C)]
-struct ArcInner<T: ?Sized> {
+pub struct ArcInner<T: ?Sized> {
     refcount: Opaque<bindings::refcount_t>,
     data: T,
 }
@@ -371,36 +372,38 @@ impl<T: ?Sized> Arc<T> {
     }
 }
 
-impl<T: 'static> ForeignOwnable for Arc<T> {
+// SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
+unsafe impl<T: 'static> ForeignOwnable for Arc<T> {
+    type PointedTo = ArcInner<T>;
     type Borrowed<'a> = ArcBorrow<'a, T>;
     type BorrowedMut<'a> = Self::Borrowed<'a>;
 
-    fn into_foreign(self) -> *mut crate::ffi::c_void {
-        ManuallyDrop::new(self).ptr.as_ptr().cast()
+    fn into_foreign(self) -> *mut Self::PointedTo {
+        ManuallyDrop::new(self).ptr.as_ptr()
     }
 
-    unsafe fn from_foreign(ptr: *mut crate::ffi::c_void) -> Self {
+    unsafe fn from_foreign(ptr: *mut Self::PointedTo) -> Self {
         // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
         // call to `Self::into_foreign`.
-        let inner = unsafe { NonNull::new_unchecked(ptr.cast::<ArcInner<T>>()) };
+        let inner = unsafe { NonNull::new_unchecked(ptr) };
 
         // SAFETY: By the safety requirement of this function, we know that `ptr` came from
         // a previous call to `Arc::into_foreign`, which guarantees that `ptr` is valid and
         // holds a reference count increment that is transferrable to us.
         unsafe { Self::from_inner(inner) }
     }
 
-    unsafe fn borrow<'a>(ptr: *mut crate::ffi::c_void) -> ArcBorrow<'a, T> {
+    unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> ArcBorrow<'a, T> {
         // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
         // call to `Self::into_foreign`.
-        let inner = unsafe { NonNull::new_unchecked(ptr.cast::<ArcInner<T>>()) };
+        let inner = unsafe { NonNull::new_unchecked(ptr) };
 
         // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
         // for the lifetime of the returned value.
         unsafe { ArcBorrow::new(inner) }
     }
 
-    unsafe fn borrow_mut<'a>(ptr: *mut crate::ffi::c_void) -> ArcBorrow<'a, T> {
+    unsafe fn borrow_mut<'a>(ptr: *mut Self::PointedTo) -> ArcBorrow<'a, T> {
         // SAFETY: The safety requirements for `borrow_mut` are a superset of the safety
         // requirements for `borrow`.
         unsafe { Self::borrow(ptr) }

rust/kernel/types.rs

@@ -18,7 +18,19 @@ use pin_init::{PinInit, Zeroable};
 ///
 /// 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: Sized {
+///
+/// # Safety
+///
+/// Implementers must ensure that [`into_foreign`] returns a pointer which meets the alignment
+/// requirements of [`PointedTo`].
+///
+/// [`into_foreign`]: Self::into_foreign
+/// [`PointedTo`]: Self::PointedTo
+pub unsafe trait ForeignOwnable: Sized {
+    /// Type used when the value is foreign-owned. In practical terms only defines the alignment of
+    /// the pointer.
+    type PointedTo;
+
     /// Type used to immutably borrow a value that is currently foreign-owned.
     type Borrowed<'a>;
 
@@ -27,16 +39,18 @@ pub trait ForeignOwnable: Sized {
 
     /// Converts a Rust-owned object to a foreign-owned one.
     ///
-    /// The foreign representation is a pointer to void. There are no guarantees for this pointer.
-    /// For example, it might be invalid, dangling or pointing to uninitialized memory. Using it in
-    /// any way except for [`from_foreign`], [`try_from_foreign`], [`borrow`], or [`borrow_mut`] can
-    /// result in undefined behavior.
+    /// # Guarantees
+    ///
+    /// The return value is guaranteed to be well-aligned, but there are no other guarantees for
+    /// this pointer. For example, it might be null, dangling, or point to uninitialized memory.
+    /// Using it in any way except for [`ForeignOwnable::from_foreign`], [`ForeignOwnable::borrow`],
+    /// [`ForeignOwnable::try_from_foreign`] can result in undefined behavior.
     ///
     /// [`from_foreign`]: Self::from_foreign
     /// [`try_from_foreign`]: Self::try_from_foreign
     /// [`borrow`]: Self::borrow
     /// [`borrow_mut`]: Self::borrow_mut
-    fn into_foreign(self) -> *mut crate::ffi::c_void;
+    fn into_foreign(self) -> *mut Self::PointedTo;
 
     /// Converts a foreign-owned object back to a Rust-owned one.
     ///
@@ -46,7 +60,7 @@ pub trait ForeignOwnable: Sized {
     /// must not be passed to `from_foreign` more than once.
     ///
     /// [`into_foreign`]: Self::into_foreign
-    unsafe fn from_foreign(ptr: *mut crate::ffi::c_void) -> Self;
+    unsafe fn from_foreign(ptr: *mut Self::PointedTo) -> Self;
 
     /// Tries to convert a foreign-owned object back to a Rust-owned one.
     ///
@@ -58,7 +72,7 @@ pub trait ForeignOwnable: Sized {
     /// `ptr` must either be null or satisfy the safety requirements for [`from_foreign`].
     ///
     /// [`from_foreign`]: Self::from_foreign
-    unsafe fn try_from_foreign(ptr: *mut crate::ffi::c_void) -> Option<Self> {
+    unsafe fn try_from_foreign(ptr: *mut Self::PointedTo) -> Option<Self> {
         if ptr.is_null() {
             None
         } else {
@@ -81,7 +95,7 @@ pub trait ForeignOwnable: Sized {
     ///
     /// [`into_foreign`]: Self::into_foreign
     /// [`from_foreign`]: Self::from_foreign
-    unsafe fn borrow<'a>(ptr: *mut crate::ffi::c_void) -> Self::Borrowed<'a>;
+    unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> Self::Borrowed<'a>;
 
     /// Borrows a foreign-owned object mutably.
     ///
@@ -109,21 +123,23 @@ pub trait ForeignOwnable: Sized {
     /// [`from_foreign`]: Self::from_foreign
     /// [`borrow`]: Self::borrow
     /// [`Arc`]: crate::sync::Arc
-    unsafe fn borrow_mut<'a>(ptr: *mut crate::ffi::c_void) -> Self::BorrowedMut<'a>;
+    unsafe fn borrow_mut<'a>(ptr: *mut Self::PointedTo) -> Self::BorrowedMut<'a>;
 }
 
-impl ForeignOwnable for () {
+// SAFETY: The `into_foreign` function returns a pointer that is dangling, but well-aligned.
+unsafe impl ForeignOwnable for () {
+    type PointedTo = ();
     type Borrowed<'a> = ();
     type BorrowedMut<'a> = ();
 
-    fn into_foreign(self) -> *mut crate::ffi::c_void {
+    fn into_foreign(self) -> *mut Self::PointedTo {
         core::ptr::NonNull::dangling().as_ptr()
     }
 
-    unsafe fn from_foreign(_: *mut crate::ffi::c_void) -> Self {}
+    unsafe fn from_foreign(_: *mut Self::PointedTo) -> Self {}
 
-    unsafe fn borrow<'a>(_: *mut crate::ffi::c_void) -> Self::Borrowed<'a> {}
-    unsafe fn borrow_mut<'a>(_: *mut crate::ffi::c_void) -> Self::BorrowedMut<'a> {}
+    unsafe fn borrow<'a>(_: *mut Self::PointedTo) -> Self::Borrowed<'a> {}
+    unsafe fn borrow_mut<'a>(_: *mut Self::PointedTo) -> Self::BorrowedMut<'a> {}
 }
 
 /// Runs a cleanup function/closure when dropped.