rust: types: add FOREIGN_ALIGN to ForeignOwnable

The current implementation of `ForeignOwnable` is leaking the type of the
opaque pointer to consumers of the API. This allows consumers of the opaque
pointer to rely on the information that can be extracted from the pointer
type.

To prevent this, change the API to the version suggested by Maira
Canal (link below): Remove `ForeignOwnable::PointedTo` in favor of a
constant, which specifies the alignment of the pointers returned by
`into_foreign`.

With this change, `ArcInner` no longer needs `pub` visibility, so change it
to private.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Suggested-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20240309235927.168915-3-mcanal@igalia.com
Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250612-pointed-to-v3-1-b009006d86a1@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

7 files changed, 70 insertions, 63 deletions

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index c386ff771d50..bffe72f44cb3 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -15,6 +15,7 @@ use core::pin::Pin;
 use core::ptr::NonNull;
 use core::result::Result;
 
+use crate::ffi::c_void;
 use crate::init::InPlaceInit;
 use crate::types::ForeignOwnable;
 use pin_init::{InPlaceWrite, Init, PinInit, ZeroableOption};
@@ -398,70 +399,74 @@ where
     }
 }
 
-// SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
+// SAFETY: The pointer returned by `into_foreign` comes from a well aligned
+// pointer to `T`.
 unsafe impl<T: 'static, A> ForeignOwnable for Box<T, A>
 where
     A: Allocator,
 {
-    type PointedTo = T;
+    const FOREIGN_ALIGN: usize = core::mem::align_of::<T>();
     type Borrowed<'a> = &'a T;
     type BorrowedMut<'a> = &'a mut T;
 
-    fn into_foreign(self) -> *mut Self::PointedTo {
-        Box::into_raw(self)
+    fn into_foreign(self) -> *mut c_void {
+        Box::into_raw(self).cast()
     }
 
-    unsafe fn from_foreign(ptr: *mut Self::PointedTo) -> Self {
+    unsafe fn from_foreign(ptr: *mut c_void) -> 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) }
+        unsafe { Box::from_raw(ptr.cast()) }
     }
 
-    unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> &'a T {
+    unsafe fn borrow<'a>(ptr: *mut c_void) -> &'a T {
         // SAFETY: The safety requirements of this method ensure that the object remains alive and
         // immutable for the duration of 'a.
-        unsafe { &*ptr }
+        unsafe { &*ptr.cast() }
     }
 
-    unsafe fn borrow_mut<'a>(ptr: *mut Self::PointedTo) -> &'a mut T {
+    unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> &'a mut T {
+        let ptr = ptr.cast();
         // 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 }
     }
 }
 
-// SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
+// SAFETY: The pointer returned by `into_foreign` comes from a well aligned
+// pointer to `T`.
 unsafe impl<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
 where
     A: Allocator,
 {
-    type PointedTo = T;
+    const FOREIGN_ALIGN: usize = core::mem::align_of::<T>();
     type Borrowed<'a> = Pin<&'a T>;
     type BorrowedMut<'a> = Pin<&'a mut T>;
 
-    fn into_foreign(self) -> *mut Self::PointedTo {
+    fn into_foreign(self) -> *mut c_void {
         // SAFETY: We are still treating the box as pinned.
-        Box::into_raw(unsafe { Pin::into_inner_unchecked(self) })
+        Box::into_raw(unsafe { Pin::into_inner_unchecked(self) }).cast()
     }
 
-    unsafe fn from_foreign(ptr: *mut Self::PointedTo) -> Self {
+    unsafe fn from_foreign(ptr: *mut c_void) -> 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)) }
+        unsafe { Pin::new_unchecked(Box::from_raw(ptr.cast())) }
     }
 
-    unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> Pin<&'a T> {
+    unsafe fn borrow<'a>(ptr: *mut c_void) -> 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 };
+        let r = unsafe { &*ptr.cast() };
 
         // SAFETY: This pointer originates from a `Pin<Box<T>>`.
         unsafe { Pin::new_unchecked(r) }
     }
 
-    unsafe fn borrow_mut<'a>(ptr: *mut Self::PointedTo) -> Pin<&'a mut T> {
+    unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> Pin<&'a mut T> {
+        let ptr = ptr.cast();
         // 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
diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
index 288f40e79906..ad51ffc549b8 100644
--- a/rust/kernel/miscdevice.rs
+++ b/rust/kernel/miscdevice.rs
@@ -217,7 +217,7 @@ 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().cast() };
+        unsafe { (*raw_file).private_data = ptr.into_foreign() };
 
         0
     }
@@ -228,7 +228,7 @@ 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 }.cast();
+        let private = unsafe { (*file).private_data };
         // SAFETY: The release call of a file owns the private data.
         let ptr = unsafe { <T::Ptr as ForeignOwnable>::from_foreign(private) };
 
@@ -272,7 +272,7 @@ 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 }.cast();
+        let private = unsafe { (*file).private_data };
         // SAFETY: Ioctl calls can borrow the private data of the file.
         let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };
 
@@ -297,7 +297,7 @@ 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 }.cast();
+        let private = unsafe { (*file).private_data };
         // SAFETY: Ioctl calls can borrow the private data of the file.
         let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };
 
@@ -318,7 +318,7 @@ 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 }.cast();
+        let private = unsafe { (*file).private_data };
         // SAFETY: Ioctl calls can borrow the private data of the file.
         let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };
         // SAFETY:
diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
index 6b94fd7a3ce9..5ce07999168e 100644
--- a/rust/kernel/pci.rs
+++ b/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) }.cast();
+        let ptr = unsafe { bindings::pci_get_drvdata(pdev) };
 
         // 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
diff --git a/rust/kernel/platform.rs b/rust/kernel/platform.rs
index 0a6a6be732b2..e894790c510c 100644
--- a/rust/kernel/platform.rs
+++ b/rust/kernel/platform.rs
@@ -81,7 +81,7 @@ 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) }.cast();
+        let ptr = unsafe { bindings::platform_get_drvdata(pdev) };
 
         // 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
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 499175f637a7..63a66761d0c7 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -19,6 +19,7 @@
 use crate::{
     alloc::{AllocError, Flags, KBox},
     bindings,
+    ffi::c_void,
     init::InPlaceInit,
     try_init,
     types::{ForeignOwnable, Opaque},
@@ -141,10 +142,9 @@ pub struct Arc<T: ?Sized> {
     _p: PhantomData<ArcInner<T>>,
 }
 
-#[doc(hidden)]
 #[pin_data]
 #[repr(C)]
-pub struct ArcInner<T: ?Sized> {
+struct ArcInner<T: ?Sized> {
     refcount: Opaque<bindings::refcount_t>,
     data: T,
 }
@@ -373,20 +373,22 @@ impl<T: ?Sized> Arc<T> {
     }
 }
 
-// SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
+// SAFETY: The pointer returned by `into_foreign` comes from a well aligned
+// pointer to `ArcInner<T>`.
 unsafe impl<T: 'static> ForeignOwnable for Arc<T> {
-    type PointedTo = ArcInner<T>;
+    const FOREIGN_ALIGN: usize = core::mem::align_of::<ArcInner<T>>();
+
     type Borrowed<'a> = ArcBorrow<'a, T>;
     type BorrowedMut<'a> = Self::Borrowed<'a>;
 
-    fn into_foreign(self) -> *mut Self::PointedTo {
-        ManuallyDrop::new(self).ptr.as_ptr()
+    fn into_foreign(self) -> *mut c_void {
+        ManuallyDrop::new(self).ptr.as_ptr().cast()
     }
 
-    unsafe fn from_foreign(ptr: *mut Self::PointedTo) -> Self {
+    unsafe fn from_foreign(ptr: *mut c_void) -> 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) };
+        let inner = unsafe { NonNull::new_unchecked(ptr.cast::<ArcInner<T>>()) };
 
         // 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
@@ -394,17 +396,17 @@ unsafe impl<T: 'static> ForeignOwnable for Arc<T> {
         unsafe { Self::from_inner(inner) }
     }
 
-    unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> ArcBorrow<'a, T> {
+    unsafe fn borrow<'a>(ptr: *mut c_void) -> 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) };
+        let inner = unsafe { NonNull::new_unchecked(ptr.cast::<ArcInner<T>>()) };
 
         // 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 Self::PointedTo) -> ArcBorrow<'a, T> {
+    unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> ArcBorrow<'a, T> {
         // SAFETY: The safety requirements for `borrow_mut` are a superset of the safety
         // requirements for `borrow`.
         unsafe { <Self as ForeignOwnable>::borrow(ptr) }
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 22985b6f6982..c156808a78d3 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -2,6 +2,7 @@
 
 //! Kernel types.
 
+use crate::ffi::c_void;
 use core::{
     cell::UnsafeCell,
     marker::{PhantomData, PhantomPinned},
@@ -21,15 +22,10 @@ use pin_init::{PinInit, Zeroable};
 ///
 /// # 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
+/// - Implementations must satisfy the guarantees of [`Self::into_foreign`].
 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;
+    /// The alignment of pointers returned by `into_foreign`.
+    const FOREIGN_ALIGN: usize;
 
     /// Type used to immutably borrow a value that is currently foreign-owned.
     type Borrowed<'a>;
@@ -39,18 +35,20 @@ pub unsafe trait ForeignOwnable: Sized {
 
     /// Converts a Rust-owned object to a foreign-owned one.
     ///
+    /// The foreign representation is a pointer to void. Aside from the guarantees listed below,
+    /// there are no other 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.
+    /// - Minimum alignment of returned pointer is [`Self::FOREIGN_ALIGN`].
     ///
     /// [`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 Self::PointedTo;
+    fn into_foreign(self) -> *mut c_void;
 
     /// Converts a foreign-owned object back to a Rust-owned one.
     ///
@@ -60,7 +58,7 @@ pub unsafe trait ForeignOwnable: Sized {
     /// must not be passed to `from_foreign` more than once.
     ///
     /// [`into_foreign`]: Self::into_foreign
-    unsafe fn from_foreign(ptr: *mut Self::PointedTo) -> Self;
+    unsafe fn from_foreign(ptr: *mut c_void) -> Self;
 
     /// Tries to convert a foreign-owned object back to a Rust-owned one.
     ///
@@ -72,7 +70,7 @@ pub unsafe 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 Self::PointedTo) -> Option<Self> {
+    unsafe fn try_from_foreign(ptr: *mut c_void) -> Option<Self> {
         if ptr.is_null() {
             None
         } else {
@@ -95,7 +93,7 @@ pub unsafe trait ForeignOwnable: Sized {
     ///
     /// [`into_foreign`]: Self::into_foreign
     /// [`from_foreign`]: Self::from_foreign
-    unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> Self::Borrowed<'a>;
+    unsafe fn borrow<'a>(ptr: *mut c_void) -> Self::Borrowed<'a>;
 
     /// Borrows a foreign-owned object mutably.
     ///
@@ -123,23 +121,24 @@ pub unsafe trait ForeignOwnable: Sized {
     /// [`from_foreign`]: Self::from_foreign
     /// [`borrow`]: Self::borrow
     /// [`Arc`]: crate::sync::Arc
-    unsafe fn borrow_mut<'a>(ptr: *mut Self::PointedTo) -> Self::BorrowedMut<'a>;
+    unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> Self::BorrowedMut<'a>;
 }
 
-// SAFETY: The `into_foreign` function returns a pointer that is dangling, but well-aligned.
+// SAFETY: The pointer returned by `into_foreign` comes from a well aligned
+// pointer to `()`.
 unsafe impl ForeignOwnable for () {
-    type PointedTo = ();
+    const FOREIGN_ALIGN: usize = core::mem::align_of::<()>();
     type Borrowed<'a> = ();
     type BorrowedMut<'a> = ();
 
-    fn into_foreign(self) -> *mut Self::PointedTo {
+    fn into_foreign(self) -> *mut c_void {
         core::ptr::NonNull::dangling().as_ptr()
     }
 
-    unsafe fn from_foreign(_: *mut Self::PointedTo) -> Self {}
+    unsafe fn from_foreign(_: *mut c_void) -> Self {}
 
-    unsafe fn borrow<'a>(_: *mut Self::PointedTo) -> Self::Borrowed<'a> {}
-    unsafe fn borrow_mut<'a>(_: *mut Self::PointedTo) -> Self::BorrowedMut<'a> {}
+    unsafe fn borrow<'a>(_: *mut c_void) -> Self::Borrowed<'a> {}
+    unsafe fn borrow_mut<'a>(_: *mut c_void) -> Self::BorrowedMut<'a> {}
 }
 
 /// Runs a cleanup function/closure when dropped.
diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs
index 75719e7bb491..a49d6db28845 100644
--- a/rust/kernel/xarray.rs
+++ b/rust/kernel/xarray.rs
@@ -7,9 +7,10 @@
 use crate::{
     alloc, bindings, build_assert,
     error::{Error, Result},
+    ffi::c_void,
     types::{ForeignOwnable, NotThreadSafe, Opaque},
 };
-use core::{iter, marker::PhantomData, mem, pin::Pin, ptr::NonNull};
+use core::{iter, marker::PhantomData, pin::Pin, ptr::NonNull};
 use pin_init::{pin_data, pin_init, pinned_drop, PinInit};
 
 /// An array which efficiently maps sparse integer indices to owned objects.
@@ -101,7 +102,7 @@ impl<T: ForeignOwnable> XArray<T> {
         })
     }
 
-    fn iter(&self) -> impl Iterator<Item = NonNull<T::PointedTo>> + '_ {
+    fn iter(&self) -> impl Iterator<Item = NonNull<c_void>> + '_ {
         let mut index = 0;
 
         // SAFETY: `self.xa` is always valid by the type invariant.
@@ -179,7 +180,7 @@ impl<T> From<StoreError<T>> for Error {
 impl<'a, T: ForeignOwnable> Guard<'a, T> {
     fn load<F, U>(&self, index: usize, f: F) -> Option<U>
     where
-        F: FnOnce(NonNull<T::PointedTo>) -> U,
+        F: FnOnce(NonNull<c_void>) -> U,
     {
         // SAFETY: `self.xa.xa` is always valid by the type invariant.
         let ptr = unsafe { bindings::xa_load(self.xa.xa.get(), index) };
@@ -230,7 +231,7 @@ impl<'a, T: ForeignOwnable> Guard<'a, T> {
         gfp: alloc::Flags,
     ) -> Result<Option<T>, StoreError<T>> {
         build_assert!(
-            mem::align_of::<T::PointedTo>() >= 4,
+            T::FOREIGN_ALIGN >= 4,
             "pointers stored in XArray must be 4-byte aligned"
         );
         let new = value.into_foreign();