[PATCH v5 0/6] rust: add improved version of `ForeignOwnable::borrow_mut`

[PATCH v5 0/6] rust: add improved version of `ForeignOwnable::borrow_mut`

[Tamir Duberstein]

This is a re-submission of Alice's patch[0]. The leading commits are
intended to improve the consistency and ergonomics of `ForeignOwnable`,
and to split out the code movement originally included in the patch.

`ForeignOwnable::borrow_mut` is a dependency of the memory backing
feature of `rnull`, the Rust null block driver.

Link: https://lore.kernel.org/all/20230710074642.683831-1-aliceryhl@google.com/T/#u [0]

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
Changes in v5:
- Rebase on d072acda4862 ("rust: use custom FFI integer types") [0]
- Link: https://lore.kernel.org/all/20240913213041.395655-4-gary@garyguo.net/ [0]
- Link to v4: https://lore.kernel.org/r/20241110-borrow-mut-v4-0-053976068215@gmail.com

Changes in v4:
- Remove (another) superfluous `cast_const()`. (Alice Ryhl)
- Link to v3: https://lore.kernel.org/r/20241108-borrow-mut-v3-0-b7144945714e@gmail.com

Changes in v3:
- Remove superfluous `cast_const()`. (Alice Ryhl)
- Correct SoB placement. (Alice Ryhl)
- Mention `as` cast removal. (Miguel Ojeda)
- Link to v2: https://lore.kernel.org/r/20241104-borrow-mut-v2-0-de650678648d@gmail.com

Changes in v2:
- Call out ordering inconsistency in commit message.
- Restore pointer type ascriptionin Arc. (Andreas Hindborg)
- Remove most reduction of unsafe blocks. (Andreas Hindborg)
- Lift splitting of unsafe block into separate patch.
- Link to v1: https://lore.kernel.org/r/20241030-borrow-mut-v1-0-8f0ceaf78eaf@gmail.com

---
Alice Ryhl (1):
      rust: add improved version of `ForeignOwnable::borrow_mut`

Tamir Duberstein (5):
      rust: arc: use `NonNull::new_unchecked`
      rust: types: avoid `as` casts
      rust: arc: split unsafe block, add missing comment
      rust: change `ForeignOwnable` pointer to mut
      rust: reorder `ForeignOwnable` items

 rust/kernel/alloc/kbox.rs | 41 ++++++++++++++++-----
 rust/kernel/sync/arc.rs   | 42 ++++++++++++++-------
 rust/kernel/types.rs      | 93 +++++++++++++++++++++++++++++++++++------------
 3 files changed, 129 insertions(+), 47 deletions(-)
---
base-commit: b2603f8ac8217bc59f5c7f248ac248423b9b99cb
change-id: 20241030-borrow-mut-75f181feef4c

Best regards,
-- 
Tamir Duberstein <tamird@gmail.com>

[PATCH v5 1/6] rust: arc: use `NonNull::new_unchecked`

[Tamir Duberstein]

There is no need to check (and panic on violations of) the safety
requirements on `ForeignOwnable` functions. Avoiding the check is
consistent with the implementation of `ForeignOwnable` for `Box`.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
 rust/kernel/sync/arc.rs | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index fa4509406ee909ca0677b78d5ece966089ce6366..b4e492dd712137c7c39e3de3d39c0c833944828c 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -337,9 +337,9 @@ fn into_foreign(self) -> *const crate::ffi::c_void {
     }
 
     unsafe fn borrow<'a>(ptr: *const crate::ffi::c_void) -> ArcBorrow<'a, T> {
-        // By the safety requirement of this function, we know that `ptr` came from
-        // a previous call to `Arc::into_foreign`.
-        let inner = NonNull::new(ptr as *mut ArcInner<T>).unwrap();
+        // 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 as *mut ArcInner<T>) };
 
         // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
         // for the lifetime of the returned value.
@@ -347,10 +347,14 @@ unsafe fn borrow<'a>(ptr: *const crate::ffi::c_void) -> ArcBorrow<'a, T> {
     }
 
     unsafe fn from_foreign(ptr: *const crate::ffi::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 as *mut 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
         // holds a reference count increment that is transferrable to us.
-        unsafe { Self::from_inner(NonNull::new(ptr as _).unwrap()) }
+        unsafe { Self::from_inner(inner) }
     }
 }
 

-- 
2.47.0

[PATCH v5 2/6] rust: types: avoid `as` casts

[Tamir Duberstein]

Replace `as` casts with `cast{,_mut}` calls which are a bit safer.

In one instance, remove an unnecessary `as` cast without replacement.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
 rust/kernel/alloc/kbox.rs | 8 ++++----
 rust/kernel/sync/arc.rs   | 9 +++++----
 rust/kernel/types.rs      | 2 +-
 3 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index 9ce414361c2c6dd8eea09b11041f6c307cbc7864..3f0b04609bd487e3f50247f9f1abd5394b749c7e 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -356,13 +356,13 @@ impl<T: 'static, A> ForeignOwnable for Box<T, A>
     type Borrowed<'a> = &'a T;
 
     fn into_foreign(self) -> *const crate::ffi::c_void {
-        Box::into_raw(self) as _
+        Box::into_raw(self).cast()
     }
 
     unsafe fn from_foreign(ptr: *const crate::ffi::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 as _) }
+        unsafe { Box::from_raw(ptr.cast_mut().cast()) }
     }
 
     unsafe fn borrow<'a>(ptr: *const crate::ffi::c_void) -> &'a T {
@@ -380,13 +380,13 @@ impl<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
 
     fn into_foreign(self) -> *const crate::ffi::c_void {
         // SAFETY: We are still treating the box as pinned.
-        Box::into_raw(unsafe { Pin::into_inner_unchecked(self) }) as _
+        Box::into_raw(unsafe { Pin::into_inner_unchecked(self) }).cast()
     }
 
     unsafe fn from_foreign(ptr: *const crate::ffi::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 as _)) }
+        unsafe { Pin::new_unchecked(Box::from_raw(ptr.cast_mut().cast())) }
     }
 
     unsafe fn borrow<'a>(ptr: *const crate::ffi::c_void) -> Pin<&'a T> {
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index b4e492dd712137c7c39e3de3d39c0c833944828c..50645660a9c33cb121ee1b2469003b325000d840 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -201,10 +201,11 @@ pub fn new(contents: T, flags: Flags) -> Result<Self, AllocError> {
         };
 
         let inner = KBox::new(value, flags)?;
+        let inner = KBox::leak(inner).into();
 
         // SAFETY: We just created `inner` with a reference count of 1, which is owned by the new
         // `Arc` object.
-        Ok(unsafe { Self::from_inner(KBox::leak(inner).into()) })
+        Ok(unsafe { Self::from_inner(inner) })
     }
 }
 
@@ -333,13 +334,13 @@ impl<T: 'static> ForeignOwnable for Arc<T> {
     type Borrowed<'a> = ArcBorrow<'a, T>;
 
     fn into_foreign(self) -> *const crate::ffi::c_void {
-        ManuallyDrop::new(self).ptr.as_ptr() as _
+        ManuallyDrop::new(self).ptr.as_ptr().cast()
     }
 
     unsafe fn borrow<'a>(ptr: *const crate::ffi::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 as *mut ArcInner<T>) };
+        let inner = unsafe { NonNull::new_unchecked(ptr.cast_mut().cast::<ArcInner<T>>()) };
 
         // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
         // for the lifetime of the returned value.
@@ -349,7 +350,7 @@ unsafe fn borrow<'a>(ptr: *const crate::ffi::c_void) -> ArcBorrow<'a, T> {
     unsafe fn from_foreign(ptr: *const crate::ffi::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 as *mut ArcInner<T>) };
+        let inner = unsafe { NonNull::new_unchecked(ptr.cast_mut().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
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index a7eaa29f08a40a0f46d616c5b6aab8f9897a0f62..f51d965c54ced5d6653b7d3f8f29fb206a5ad2b6 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -418,7 +418,7 @@ pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
     /// }
     ///
     /// let mut data = Empty {};
-    /// let ptr = NonNull::<Empty>::new(&mut data as *mut _).unwrap();
+    /// let ptr = NonNull::<Empty>::new(&mut data).unwrap();
     /// # // SAFETY: TODO.
     /// let data_ref: ARef<Empty> = unsafe { ARef::from_raw(ptr) };
     /// let raw_ptr: NonNull<Empty> = ARef::into_raw(data_ref);

-- 
2.47.0

[PATCH v5 3/6] rust: arc: split unsafe block, add missing comment

[Tamir Duberstein]

The new SAFETY comment style is taken from existing comments in `deref`
and `drop.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
 rust/kernel/sync/arc.rs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 50645660a9c33cb121ee1b2469003b325000d840..a11f267ce5d40b987f1f3c459271e5317ea0bae8 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -377,10 +377,14 @@ fn as_ref(&self) -> &T {
 
 impl<T: ?Sized> Clone for Arc<T> {
     fn clone(&self) -> Self {
+        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
+        // safe to dereference it.
+        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
+
         // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero.
         // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
         // safe to increment the refcount.
-        unsafe { bindings::refcount_inc(self.ptr.as_ref().refcount.get()) };
+        unsafe { bindings::refcount_inc(refcount) };
 
         // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
         unsafe { Self::from_inner(self.ptr) }

-- 
2.47.0

[PATCH v5 4/6] rust: change `ForeignOwnable` pointer to mut

[Tamir Duberstein]

It is slightly more convenient to operate on mut pointers, and this also
properly conveys the desired ownership semantics of the trait.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
 rust/kernel/alloc/kbox.rs | 16 ++++++++--------
 rust/kernel/sync/arc.rs   | 10 +++++-----
 rust/kernel/types.rs      | 14 +++++++-------
 3 files changed, 20 insertions(+), 20 deletions(-)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index 3f0b04609bd487e3f50247f9f1abd5394b749c7e..e00c14053efbfb08d053e0f0b11247fa25d9d516 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -355,17 +355,17 @@ impl<T: 'static, A> ForeignOwnable for Box<T, A>
 {
     type Borrowed<'a> = &'a T;
 
-    fn into_foreign(self) -> *const crate::ffi::c_void {
+    fn into_foreign(self) -> *mut crate::ffi::c_void {
         Box::into_raw(self).cast()
     }
 
-    unsafe fn from_foreign(ptr: *const crate::ffi::c_void) -> Self {
+    unsafe fn from_foreign(ptr: *mut crate::ffi::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.cast_mut().cast()) }
+        unsafe { Box::from_raw(ptr.cast()) }
     }
 
-    unsafe fn borrow<'a>(ptr: *const crate::ffi::c_void) -> &'a T {
+    unsafe fn borrow<'a>(ptr: *mut crate::ffi::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.cast() }
@@ -378,18 +378,18 @@ impl<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
 {
     type Borrowed<'a> = Pin<&'a T>;
 
-    fn into_foreign(self) -> *const crate::ffi::c_void {
+    fn into_foreign(self) -> *mut crate::ffi::c_void {
         // SAFETY: We are still treating the box as pinned.
         Box::into_raw(unsafe { Pin::into_inner_unchecked(self) }).cast()
     }
 
-    unsafe fn from_foreign(ptr: *const crate::ffi::c_void) -> Self {
+    unsafe fn from_foreign(ptr: *mut crate::ffi::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.cast_mut().cast())) }
+        unsafe { Pin::new_unchecked(Box::from_raw(ptr.cast())) }
     }
 
-    unsafe fn borrow<'a>(ptr: *const crate::ffi::c_void) -> Pin<&'a T> {
+    unsafe fn borrow<'a>(ptr: *mut crate::ffi::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
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index a11f267ce5d40b987f1f3c459271e5317ea0bae8..01d85da799d77127fc99a9b270b8a7b1ef435b6f 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -333,24 +333,24 @@ pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
 impl<T: 'static> ForeignOwnable for Arc<T> {
     type Borrowed<'a> = ArcBorrow<'a, T>;
 
-    fn into_foreign(self) -> *const crate::ffi::c_void {
+    fn into_foreign(self) -> *mut crate::ffi::c_void {
         ManuallyDrop::new(self).ptr.as_ptr().cast()
     }
 
-    unsafe fn borrow<'a>(ptr: *const crate::ffi::c_void) -> ArcBorrow<'a, T> {
+    unsafe fn borrow<'a>(ptr: *mut crate::ffi::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.cast_mut().cast::<ArcInner<T>>()) };
+        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 from_foreign(ptr: *const crate::ffi::c_void) -> Self {
+    unsafe fn from_foreign(ptr: *mut crate::ffi::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.cast_mut().cast::<ArcInner<T>>()) };
+        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
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index f51d965c54ced5d6653b7d3f8f29fb206a5ad2b6..fd7685921c7c9866f963adef122dfe45a7afd87b 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -29,7 +29,7 @@ pub trait ForeignOwnable: Sized {
     /// For example, it might be invalid, dangling or pointing to uninitialized memory. Using it in
     /// any way except for [`ForeignOwnable::from_foreign`], [`ForeignOwnable::borrow`],
     /// [`ForeignOwnable::try_from_foreign`] can result in undefined behavior.
-    fn into_foreign(self) -> *const crate::ffi::c_void;
+    fn into_foreign(self) -> *mut crate::ffi::c_void;
 
     /// Borrows a foreign-owned object.
     ///
@@ -37,7 +37,7 @@ pub trait ForeignOwnable: Sized {
     ///
     /// `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.
-    unsafe fn borrow<'a>(ptr: *const crate::ffi::c_void) -> Self::Borrowed<'a>;
+    unsafe fn borrow<'a>(ptr: *mut crate::ffi::c_void) -> Self::Borrowed<'a>;
 
     /// Converts a foreign-owned object back to a Rust-owned one.
     ///
@@ -47,7 +47,7 @@ pub trait ForeignOwnable: Sized {
     /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
     /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] for
     /// this object must have been dropped.
-    unsafe fn from_foreign(ptr: *const crate::ffi::c_void) -> Self;
+    unsafe fn from_foreign(ptr: *mut crate::ffi::c_void) -> Self;
 
     /// Tries to convert a foreign-owned object back to a Rust-owned one.
     ///
@@ -58,7 +58,7 @@ pub trait ForeignOwnable: Sized {
     ///
     /// `ptr` must either be null or satisfy the safety requirements for
     /// [`ForeignOwnable::from_foreign`].
-    unsafe fn try_from_foreign(ptr: *const crate::ffi::c_void) -> Option<Self> {
+    unsafe fn try_from_foreign(ptr: *mut crate::ffi::c_void) -> Option<Self> {
         if ptr.is_null() {
             None
         } else {
@@ -72,13 +72,13 @@ unsafe fn try_from_foreign(ptr: *const crate::ffi::c_void) -> Option<Self> {
 impl ForeignOwnable for () {
     type Borrowed<'a> = ();
 
-    fn into_foreign(self) -> *const crate::ffi::c_void {
+    fn into_foreign(self) -> *mut crate::ffi::c_void {
         core::ptr::NonNull::dangling().as_ptr()
     }
 
-    unsafe fn borrow<'a>(_: *const crate::ffi::c_void) -> Self::Borrowed<'a> {}
+    unsafe fn borrow<'a>(_: *mut crate::ffi::c_void) -> Self::Borrowed<'a> {}
 
-    unsafe fn from_foreign(_: *const crate::ffi::c_void) -> Self {}
+    unsafe fn from_foreign(_: *mut crate::ffi::c_void) -> Self {}
 }
 
 /// Runs a cleanup function/closure when dropped.

-- 
2.47.0

[PATCH v5 5/6] rust: reorder `ForeignOwnable` items

[Tamir Duberstein]

`{into,from}_foreign` before `borrow` is slightly more logical.

This removes an inconsistency with `kbox.rs` which already uses this
ordering.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
 rust/kernel/sync/arc.rs | 18 +++++++++---------
 rust/kernel/types.rs    | 20 ++++++++++----------
 2 files changed, 19 insertions(+), 19 deletions(-)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 01d85da799d77127fc99a9b270b8a7b1ef435b6f..1d26c309d21db53f1fc769562c2afb4e881c3b5b 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -337,25 +337,25 @@ fn into_foreign(self) -> *mut crate::ffi::c_void {
         ManuallyDrop::new(self).ptr.as_ptr().cast()
     }
 
-    unsafe fn borrow<'a>(ptr: *mut crate::ffi::c_void) -> ArcBorrow<'a, T> {
+    unsafe fn from_foreign(ptr: *mut crate::ffi::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.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) }
+        // 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 from_foreign(ptr: *mut crate::ffi::c_void) -> Self {
+    unsafe fn borrow<'a>(ptr: *mut crate::ffi::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.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
-        // holds a reference count increment that is transferrable to us.
-        unsafe { Self::from_inner(inner) }
+        // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
+        // for the lifetime of the returned value.
+        unsafe { ArcBorrow::new(inner) }
     }
 }
 
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index fd7685921c7c9866f963adef122dfe45a7afd87b..c2ac1626f8ddff7397ec00f502abcfe78829b532 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -31,14 +31,6 @@ pub trait ForeignOwnable: Sized {
     /// [`ForeignOwnable::try_from_foreign`] can result in undefined behavior.
     fn into_foreign(self) -> *mut crate::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.
-    unsafe fn borrow<'a>(ptr: *mut crate::ffi::c_void) -> Self::Borrowed<'a>;
-
     /// Converts a foreign-owned object back to a Rust-owned one.
     ///
     /// # Safety
@@ -67,6 +59,14 @@ unsafe fn try_from_foreign(ptr: *mut crate::ffi::c_void) -> Option<Self> {
             unsafe { Some(Self::from_foreign(ptr)) }
         }
     }
+
+    /// 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.
+    unsafe fn borrow<'a>(ptr: *mut crate::ffi::c_void) -> Self::Borrowed<'a>;
 }
 
 impl ForeignOwnable for () {
@@ -76,9 +76,9 @@ fn into_foreign(self) -> *mut crate::ffi::c_void {
         core::ptr::NonNull::dangling().as_ptr()
     }
 
-    unsafe fn borrow<'a>(_: *mut crate::ffi::c_void) -> Self::Borrowed<'a> {}
-
     unsafe fn from_foreign(_: *mut crate::ffi::c_void) -> Self {}
+
+    unsafe fn borrow<'a>(_: *mut crate::ffi::c_void) -> Self::Borrowed<'a> {}
 }
 
 /// Runs a cleanup function/closure when dropped.

-- 
2.47.0

[PATCH v5 6/6] rust: add improved version of `ForeignOwnable::borrow_mut`

[Tamir Duberstein]

From: Alice Ryhl <aliceryhl@google.com>

Previously, the `ForeignOwnable` trait had a method called `borrow_mut`
that was intended to provide mutable access to the inner value. However,
the method accidentally made it possible to change the address of the
object being modified, which usually isn't what we want. (And when we
want that, it can be done by calling `from_foreign` and `into_foreign`,
like how the old `borrow_mut` was implemented.)

In this patch, we introduce an alternate definition of `borrow_mut` that
solves the previous problem. Conceptually, given a pointer type `P` that
implements `ForeignOwnable`, the `borrow_mut` method gives you the same
kind of access as an `&mut P` would, except that it does not let you
change the pointer `P` itself.

This is analogous to how the existing `borrow` method provides the same
kind of access to the inner value as an `&P`.

Note that for types like `Arc`, having an `&mut Arc<T>` only gives you
immutable access to the inner `T`. This is because mutable references
assume exclusive access, but there might be other handles to the same
reference counted value, so the access isn't exclusive. The `Arc` type
implements this by making `borrow_mut` return the same type as `borrow`.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
 rust/kernel/alloc/kbox.rs | 21 ++++++++++++++
 rust/kernel/sync/arc.rs   |  7 +++++
 rust/kernel/types.rs      | 71 ++++++++++++++++++++++++++++++++++++++---------
 3 files changed, 86 insertions(+), 13 deletions(-)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index e00c14053efbfb08d053e0f0b11247fa25d9d516..4ffc4e1b22b2b7c2ea8e8ed5b7f7a8534625249f 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -354,6 +354,7 @@ impl<T: 'static, A> ForeignOwnable for Box<T, A>
     A: Allocator,
 {
     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()
@@ -370,6 +371,13 @@ unsafe fn borrow<'a>(ptr: *mut crate::ffi::c_void) -> &'a T {
         // immutable for the duration of 'a.
         unsafe { &*ptr.cast() }
     }
+
+    unsafe fn borrow_mut<'a>(ptr: *mut core::ffi::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 }
+    }
 }
 
 impl<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
@@ -377,6 +385,7 @@ impl<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
     A: Allocator,
 {
     type Borrowed<'a> = Pin<&'a T>;
+    type BorrowedMut<'a> = Pin<&'a mut T>;
 
     fn into_foreign(self) -> *mut crate::ffi::c_void {
         // SAFETY: We are still treating the box as pinned.
@@ -399,6 +408,18 @@ unsafe fn borrow<'a>(ptr: *mut crate::ffi::c_void) -> Pin<&'a T> {
         // SAFETY: This pointer originates from a `Pin<Box<T>>`.
         unsafe { Pin::new_unchecked(r) }
     }
+
+    unsafe fn borrow_mut<'a>(ptr: *mut core::ffi::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
+        // the lifetime of the returned value.
+        let r = unsafe { &mut *ptr };
+
+        // SAFETY: This pointer originates from a `Pin<Box<T>>`.
+        unsafe { Pin::new_unchecked(r) }
+    }
 }
 
 impl<T, A> Deref for Box<T, A>
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 1d26c309d21db53f1fc769562c2afb4e881c3b5b..eb5cd8b360a3507a527978aaf96dbc3a80d4ae2c 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -332,6 +332,7 @@ pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
 
 impl<T: 'static> ForeignOwnable for Arc<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()
@@ -357,6 +358,12 @@ unsafe fn borrow<'a>(ptr: *mut crate::ffi::c_void) -> ArcBorrow<'a, T> {
         // for the lifetime of the returned value.
         unsafe { ArcBorrow::new(inner) }
     }
+
+    unsafe fn borrow_mut<'a>(ptr: *mut core::ffi::c_void) -> ArcBorrow<'a, T> {
+        // SAFETY: The safety requirements for `borrow_mut` are a superset of the safety
+        // requirements for `borrow`.
+        unsafe { Self::borrow(ptr) }
+    }
 }
 
 impl<T: ?Sized> Deref for Arc<T> {
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index c2ac1626f8ddff7397ec00f502abcfe78829b532..c150cf509d52fd5006834cf1c6745e4ea3504a55 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -19,26 +19,33 @@
 /// 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 {
-    /// Type of values borrowed between calls to [`ForeignOwnable::into_foreign`] and
-    /// [`ForeignOwnable::from_foreign`].
+    /// Type used to immutably borrow a value that is currently foreign-owned.
     type Borrowed<'a>;
 
+    /// Type used to mutably borrow a value that is currently foreign-owned.
+    type BorrowedMut<'a>;
+
     /// 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 [`ForeignOwnable::from_foreign`], [`ForeignOwnable::borrow`],
-    /// [`ForeignOwnable::try_from_foreign`] can result in undefined behavior.
+    /// any way except for [`from_foreign`], [`try_from_foreign`], [`borrow`], or [`borrow_mut`] 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;
 
     /// 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`] for
-    /// this object must have been dropped.
+    /// The provided pointer must have been returned by a previous call to [`into_foreign`], and it
+    /// 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;
 
     /// Tries to convert a foreign-owned object back to a Rust-owned one.
@@ -48,8 +55,9 @@ pub trait ForeignOwnable: Sized {
     ///
     /// # Safety
     ///
-    /// `ptr` must either be null or satisfy the safety requirements for
-    /// [`ForeignOwnable::from_foreign`].
+    /// `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> {
         if ptr.is_null() {
             None
@@ -60,17 +68,53 @@ unsafe fn try_from_foreign(ptr: *mut crate::ffi::c_void) -> Option<Self> {
         }
     }
 
-    /// Borrows a foreign-owned object.
+    /// Borrows a foreign-owned object immutably.
+    ///
+    /// This method provides a way to access a foreign-owned value from Rust immutably. It provides
+    /// you with exactly the same abilities as an `&Self` when the value is Rust-owned.
     ///
     /// # 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.
+    /// The provided pointer must have been returned by a previous call to [`into_foreign`], and if
+    /// the pointer is ever passed to [`from_foreign`], then that call must happen after the end of
+    /// the lifetime 'a.
+    ///
+    /// [`into_foreign`]: Self::into_foreign
+    /// [`from_foreign`]: Self::from_foreign
     unsafe fn borrow<'a>(ptr: *mut crate::ffi::c_void) -> Self::Borrowed<'a>;
+
+    /// Borrows a foreign-owned object mutably.
+    ///
+    /// This method provides a way to access a foreign-owned value from Rust mutably. It provides
+    /// you with exactly the same abilities as an `&mut Self` when the value is Rust-owned, except
+    /// that the address of the object must not be changed.
+    ///
+    /// Note that for types like [`Arc`], an `&mut Arc<T>` only gives you immutable access to the
+    /// inner value, so this method also only provides immutable access in that case.
+    ///
+    /// In the case of `Box<T>`, this method gives you the ability to modify the inner `T`, but it
+    /// does not let you change the box itself. That is, you cannot change which allocation the box
+    /// points at.
+    ///
+    /// # Safety
+    ///
+    /// The provided pointer must have been returned by a previous call to [`into_foreign`], and if
+    /// the pointer is ever passed to [`from_foreign`], then that call must happen after the end of
+    /// the lifetime 'a.
+    ///
+    /// The lifetime 'a must not overlap with the lifetime of any other call to [`borrow`] or
+    /// `borrow_mut` on the same object.
+    ///
+    /// [`into_foreign`]: Self::into_foreign
+    /// [`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>;
 }
 
 impl ForeignOwnable for () {
     type Borrowed<'a> = ();
+    type BorrowedMut<'a> = ();
 
     fn into_foreign(self) -> *mut crate::ffi::c_void {
         core::ptr::NonNull::dangling().as_ptr()
@@ -79,6 +123,7 @@ fn into_foreign(self) -> *mut crate::ffi::c_void {
     unsafe fn from_foreign(_: *mut crate::ffi::c_void) -> 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> {}
 }
 
 /// Runs a cleanup function/closure when dropped.

-- 
2.47.0