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>
---
 rust/kernel/alloc/kbox.rs | 38 ++++++++++++++++++++------------------
 rust/kernel/miscdevice.rs | 10 +++++-----
 rust/kernel/pci.rs        |  2 +-
 rust/kernel/platform.rs   |  2 +-
 rust/kernel/sync/arc.rs   | 21 ++++++++++++---------
 rust/kernel/types.rs      | 46 +++++++++++++++++++++++++++++++---------------
 6 files changed, 70 insertions(+), 49 deletions(-)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index b77d32f3a58b..6aa88b01e84d 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -360,68 +360,70 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
     }
 }
 
-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
diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
index fa9ecc42602a..b4c5f74de23d 100644
--- a/rust/kernel/miscdevice.rs
+++ b/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:
diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
index c97d6d470b28..3aeb1250c27f 100644
--- a/rust/kernel/pci.rs
+++ b/rust/kernel/pci.rs
@@ -89,7 +89,7 @@ extern "C" fn probe_callback(
     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
diff --git a/rust/kernel/platform.rs b/rust/kernel/platform.rs
index 4917cb34e2fe..fd4a494f30e8 100644
--- a/rust/kernel/platform.rs
+++ b/rust/kernel/platform.rs
@@ -80,7 +80,7 @@ extern "C" fn probe_callback(pdev: *mut bindings::platform_device) -> kernel::ff
 
     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
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 8484c814609a..a42c164e577a 100644
--- a/rust/kernel/sync/arc.rs
+++ b/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,
 }
@@ -371,18 +372,20 @@ pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<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
@@ -390,17 +393,17 @@ unsafe fn from_foreign(ptr: *mut crate::ffi::c_void) -> Self {
         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) }
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 9d0471afc964..86562e738eac 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -18,7 +18,19 @@
 ///
 /// 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 @@ unsafe fn try_from_foreign(ptr: *mut crate::ffi::c_void) -> Option<Self> {
     ///
     /// [`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 @@ unsafe fn try_from_foreign(ptr: *mut crate::ffi::c_void) -> Option<Self> {
     /// [`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.

-- 
2.49.0

On Wed, 23 Apr 2025 09:54:37 -0400
Tamir Duberstein <tamird@gmail.com> wrote:

> 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>
> ---
>  rust/kernel/alloc/kbox.rs | 38 ++++++++++++++++++++------------------
>  rust/kernel/miscdevice.rs | 10 +++++-----
>  rust/kernel/pci.rs        |  2 +-
>  rust/kernel/platform.rs   |  2 +-
>  rust/kernel/sync/arc.rs   | 21 ++++++++++++---------
>  rust/kernel/types.rs      | 46 +++++++++++++++++++++++++++++++---------------
>  6 files changed, 70 insertions(+), 49 deletions(-)
> 
> diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
> index b77d32f3a58b..6aa88b01e84d 100644
> --- a/rust/kernel/alloc/kbox.rs
> +++ b/rust/kernel/alloc/kbox.rs
> @@ -360,68 +360,70 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
>      }
>  }
>  
> -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;

I don't think this is the correct solution for this. The returned
pointer is supposed to opaque, and exposing this type may encourage
this is to be wrongly used.

IIUC, the only reason for this to be exposed is for XArray to be able
to check alignment. However `align_of::<PointedTo>()` is not the
minimum guaranteed alignment.

For example, if the type is allocated via kernel allocator then it can
always guarantee to be at least usize-aligned. ZST can be arbitrarily
aligned so ForeignOwnable` implementation could return a
validly-aligned pointer for XArray. Actually, I think all current
ForeignOwnable implementation can be modified to always give 4-byte
aligned pointers.

Having a const associated item indicating the minimum guaranteed
alignment for *that specific container* is the correct way IMO, not to
reason about the pointee type!

Best,
Gary

>      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 }
>      }
>  }

On Wed, Apr 30, 2025 at 11:31 AM Gary Guo <gary@garyguo.net> wrote:
>
> On Wed, 23 Apr 2025 09:54:37 -0400
> Tamir Duberstein <tamird@gmail.com> wrote:
>
> > 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>
> > ---
> >  rust/kernel/alloc/kbox.rs | 38 ++++++++++++++++++++------------------
> >  rust/kernel/miscdevice.rs | 10 +++++-----
> >  rust/kernel/pci.rs        |  2 +-
> >  rust/kernel/platform.rs   |  2 +-
> >  rust/kernel/sync/arc.rs   | 21 ++++++++++++---------
> >  rust/kernel/types.rs      | 46 +++++++++++++++++++++++++++++++---------------
> >  6 files changed, 70 insertions(+), 49 deletions(-)
> >
> > diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
> > index b77d32f3a58b..6aa88b01e84d 100644
> > --- a/rust/kernel/alloc/kbox.rs
> > +++ b/rust/kernel/alloc/kbox.rs
> > @@ -360,68 +360,70 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
> >      }
> >  }
> >
> > -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;
>
> I don't think this is the correct solution for this. The returned
> pointer is supposed to opaque, and exposing this type may encourage
> this is to be wrongly used.

Can you give an example?

> IIUC, the only reason for this to be exposed is for XArray to be able
> to check alignment. However `align_of::<PointedTo>()` is not the
> minimum guaranteed alignment.
>
> For example, if the type is allocated via kernel allocator then it can
> always guarantee to be at least usize-aligned. ZST can be arbitrarily
> aligned so ForeignOwnable` implementation could return a
> validly-aligned pointer for XArray. Actually, I think all current
> ForeignOwnable implementation can be modified to always give 4-byte
> aligned pointers.

Is your concern that this is *too restrictive*? It might be that the
minimum alignment is greater than `align_of::<PointedTo>()`, but not
less.

> Having a const associated item indicating the minimum guaranteed
> alignment for *that specific container* is the correct way IMO, not to
> reason about the pointee type!

How do you determine the value? You're at quite some distance from the
allocator implementation.

On Wed, Apr 30, 2025 at 8:57 PM Tamir Duberstein <tamird@gmail.com> wrote:
>
> On Wed, Apr 30, 2025 at 11:31 AM Gary Guo <gary@garyguo.net> wrote:
> >
> > On Wed, 23 Apr 2025 09:54:37 -0400
> > Tamir Duberstein <tamird@gmail.com> wrote:
> > > -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;
> >
> > I don't think this is the correct solution for this. The returned
> > pointer is supposed to opaque, and exposing this type may encourage
> > this is to be wrongly used.
>
> Can you give an example?

This came up when we discussed this patch in the meeting yesterday:
https://lore.kernel.org/all/20250227-configfs-v5-1-c40e8dc3b9cd@kernel.org/

This is incorrect use of the trait. The pointer is supposed to be
opaque, and you can't dereference it. See my reply to that patch as
well:
https://lore.kernel.org/all/CAH5fLggDwPBzMO2Z48oMjDm4qgoNM0NQs_63TxmVEGy+gtMpOA@mail.gmail.com/

Alice

"Alice Ryhl" <aliceryhl@google.com> writes:

> On Wed, Apr 30, 2025 at 8:57 PM Tamir Duberstein <tamird@gmail.com> wrote:
>>
>> On Wed, Apr 30, 2025 at 11:31 AM Gary Guo <gary@garyguo.net> wrote:
>> >
>> > On Wed, 23 Apr 2025 09:54:37 -0400
>> > Tamir Duberstein <tamird@gmail.com> wrote:
>> > > -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;
>> >
>> > I don't think this is the correct solution for this. The returned
>> > pointer is supposed to opaque, and exposing this type may encourage
>> > this is to be wrongly used.
>>
>> Can you give an example?
>
> This came up when we discussed this patch in the meeting yesterday:
> https://lore.kernel.org/all/20250227-configfs-v5-1-c40e8dc3b9cd@kernel.org/
>
> This is incorrect use of the trait. The pointer is supposed to be
> opaque, and you can't dereference it. See my reply to that patch as
> well:
> https://lore.kernel.org/all/CAH5fLggDwPBzMO2Z48oMjDm4qgoNM0NQs_63TxmVEGy+gtMpOA@mail.gmail.com/


For reference, the outcome of the discussion yesterday:

 - The use of `ForeignOwnable` in the configfs series is not correct. The pointer
   must be opaque. I will drop the use of `ForeignOwnable` and adapt
   `Arc` methods `into_raw`/`from_raw` instead. I had a plan to make the
   code generic over the pointer type with a bound on `ForeignOwnable`.
   A new trait is required for that now.

 - There may be a use case for a trait that allows passing ownership of
   an object to C, similar to `ForeignOwnable` but with a non-opaque
   pointer. Trait methods would be `into_raw`, `from_raw`, `borrow`.

 - The solution for alignment adopted in this (xarray) series is not
   ideal. However, given the timeline we will proceed merging the series
   as is, and then change the solution to the one outlined by Gary in
   the next cycle.

@Gary you mentioned an implementation of the solution you outlined is
already posted to the list. I can't seem to find it, can you point to
it?

Best regards,
Andreas Hindborg