[PATCH v4 0/6] rust: add improved version of `ForeignOwnable::borrow_mut`

[PATCH v4 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 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: ae7851c29747fa3765ecb722fe722117a346f988
change-id: 20241030-borrow-mut-75f181feef4c

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

[PATCH v4 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>
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 db9da352d588f65348aa7a5204abbb165b70197f..2c9b7d4a2554278ce8608f4f4c7f9cfe87b21492 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -337,9 +337,9 @@ fn into_foreign(self) -> *const core::ffi::c_void {
     }
 
     unsafe fn borrow<'a>(ptr: *const core::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 core::ffi::c_void) -> ArcBorrow<'a, T> {
     }
 
     unsafe fn from_foreign(ptr: *const core::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 v4 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>
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 d69c32496b86a2315f81cafc8e6771ebb0cf10d1..03b7107c60344d4f03d49c51230e114d3ab38027 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 core::ffi::c_void {
-        Box::into_raw(self) as _
+        Box::into_raw(self).cast()
     }
 
     unsafe fn from_foreign(ptr: *const core::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 core::ffi::c_void) -> &'a T {
@@ -380,13 +380,13 @@ impl<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
 
     fn into_foreign(self) -> *const core::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 core::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 core::ffi::c_void) -> Pin<&'a T> {
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 2c9b7d4a2554278ce8608f4f4c7f9cfe87b21492..bbb6ca8ff7d0e079c7f0efa7702a91622759cbd7 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 core::ffi::c_void {
-        ManuallyDrop::new(self).ptr.as_ptr() as _
+        ManuallyDrop::new(self).ptr.as_ptr().cast()
     }
 
     unsafe fn borrow<'a>(ptr: *const core::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 core::ffi::c_void) -> ArcBorrow<'a, T> {
     unsafe fn from_foreign(ptr: *const core::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 fae80814fa1c5e0f11933f2f15e173f0e3a10fe0..364dd2dc438eb7d1c4d0a4525bf2305a42297b2b 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 v4 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>
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 bbb6ca8ff7d0e079c7f0efa7702a91622759cbd7..6f5fe24f0e1bad5ca141091e8945b8d2f94e16aa 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 v4 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 03b7107c60344d4f03d49c51230e114d3ab38027..99d0fc0148bb8779e5a769a6e74291ef8101bf77 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 core::ffi::c_void {
+    fn into_foreign(self) -> *mut core::ffi::c_void {
         Box::into_raw(self).cast()
     }
 
-    unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
+    unsafe fn from_foreign(ptr: *mut core::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 core::ffi::c_void) -> &'a T {
+    unsafe fn borrow<'a>(ptr: *mut core::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 core::ffi::c_void {
+    fn into_foreign(self) -> *mut core::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 core::ffi::c_void) -> Self {
+    unsafe fn from_foreign(ptr: *mut core::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 core::ffi::c_void) -> Pin<&'a T> {
+    unsafe fn borrow<'a>(ptr: *mut core::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 6f5fe24f0e1bad5ca141091e8945b8d2f94e16aa..10819dc28b64038b9abc55b01c069826d1e5befa 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 core::ffi::c_void {
+    fn into_foreign(self) -> *mut core::ffi::c_void {
         ManuallyDrop::new(self).ptr.as_ptr().cast()
     }
 
-    unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> ArcBorrow<'a, T> {
+    unsafe fn borrow<'a>(ptr: *mut core::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 core::ffi::c_void) -> Self {
+    unsafe fn from_foreign(ptr: *mut core::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 364dd2dc438eb7d1c4d0a4525bf2305a42297b2b..59e71bd158713bb8e12cac95e134f57a277c1b49 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 core::ffi::c_void;
+    fn into_foreign(self) -> *mut core::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 core::ffi::c_void) -> Self::Borrowed<'a>;
+    unsafe fn borrow<'a>(ptr: *mut core::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 core::ffi::c_void) -> Self;
+    unsafe fn from_foreign(ptr: *mut core::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 core::ffi::c_void) -> Option<Self> {
+    unsafe fn try_from_foreign(ptr: *mut core::ffi::c_void) -> Option<Self> {
         if ptr.is_null() {
             None
         } else {
@@ -72,13 +72,13 @@ unsafe fn try_from_foreign(ptr: *const core::ffi::c_void) -> Option<Self> {
 impl ForeignOwnable for () {
     type Borrowed<'a> = ();
 
-    fn into_foreign(self) -> *const core::ffi::c_void {
+    fn into_foreign(self) -> *mut core::ffi::c_void {
         core::ptr::NonNull::dangling().as_ptr()
     }
 
-    unsafe fn borrow<'a>(_: *const core::ffi::c_void) -> Self::Borrowed<'a> {}
+    unsafe fn borrow<'a>(_: *mut core::ffi::c_void) -> Self::Borrowed<'a> {}
 
-    unsafe fn from_foreign(_: *const core::ffi::c_void) -> Self {}
+    unsafe fn from_foreign(_: *mut core::ffi::c_void) -> Self {}
 }
 
 /// Runs a cleanup function/closure when dropped.

-- 
2.47.0

[PATCH v4 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>
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 10819dc28b64038b9abc55b01c069826d1e5befa..3c779b343aa8c396d2d4b7efdbc0f1ef524a0f1c 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -337,25 +337,25 @@ fn into_foreign(self) -> *mut core::ffi::c_void {
         ManuallyDrop::new(self).ptr.as_ptr().cast()
     }
 
-    unsafe fn borrow<'a>(ptr: *mut core::ffi::c_void) -> ArcBorrow<'a, T> {
+    unsafe fn from_foreign(ptr: *mut core::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 core::ffi::c_void) -> Self {
+    unsafe fn borrow<'a>(ptr: *mut core::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 59e71bd158713bb8e12cac95e134f57a277c1b49..b8f3594737401a3df841f30a20c4bd85743853ef 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 core::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 core::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 core::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 core::ffi::c_void) -> Self::Borrowed<'a>;
 }
 
 impl ForeignOwnable for () {
@@ -76,9 +76,9 @@ fn into_foreign(self) -> *mut core::ffi::c_void {
         core::ptr::NonNull::dangling().as_ptr()
     }
 
-    unsafe fn borrow<'a>(_: *mut core::ffi::c_void) -> Self::Borrowed<'a> {}
-
     unsafe fn from_foreign(_: *mut core::ffi::c_void) -> Self {}
+
+    unsafe fn borrow<'a>(_: *mut core::ffi::c_void) -> Self::Borrowed<'a> {}
 }
 
 /// Runs a cleanup function/closure when dropped.

-- 
2.47.0

[PATCH v4 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 99d0fc0148bb8779e5a769a6e74291ef8101bf77..c7edcd970fe6abe2afce5364a5f6c565452da85e 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 core::ffi::c_void {
         Box::into_raw(self).cast()
@@ -370,6 +371,13 @@ unsafe fn borrow<'a>(ptr: *mut core::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 core::ffi::c_void {
         // SAFETY: We are still treating the box as pinned.
@@ -399,6 +408,18 @@ unsafe fn borrow<'a>(ptr: *mut core::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 3c779b343aa8c396d2d4b7efdbc0f1ef524a0f1c..8a0f44da8f732afca6009a078e90bd7a14034240 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 core::ffi::c_void {
         ManuallyDrop::new(self).ptr.as_ptr().cast()
@@ -357,6 +358,12 @@ unsafe fn borrow<'a>(ptr: *mut core::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 b8f3594737401a3df841f30a20c4bd85743853ef..c74223579111fe36c7c7cd135ba95f25f0b33fab 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 core::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 core::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 core::ffi::c_void) -> Option<Self> {
         if ptr.is_null() {
             None
@@ -60,17 +68,53 @@ unsafe fn try_from_foreign(ptr: *mut core::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 core::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 core::ffi::c_void) -> Self::BorrowedMut<'a>;
 }
 
 impl ForeignOwnable for () {
     type Borrowed<'a> = ();
+    type BorrowedMut<'a> = ();
 
     fn into_foreign(self) -> *mut core::ffi::c_void {
         core::ptr::NonNull::dangling().as_ptr()
@@ -79,6 +123,7 @@ fn into_foreign(self) -> *mut core::ffi::c_void {
     unsafe fn from_foreign(_: *mut core::ffi::c_void) -> Self {}
 
     unsafe fn borrow<'a>(_: *mut core::ffi::c_void) -> Self::Borrowed<'a> {}
+    unsafe fn borrow_mut<'a>(_: *mut core::ffi::c_void) -> Self::BorrowedMut<'a> {}
 }
 
 /// Runs a cleanup function/closure when dropped.

-- 
2.47.0

Re: [PATCH v4 6/6] rust: add improved version of `ForeignOwnable::borrow_mut`

[Tamir Duberstein]

This series conflicts with Gary's FFI types series:

https://lore.kernel.org/all/20240913213041.395655-4-gary@garyguo.net/

I've rebased it locally. Shall I send a v5?

Re: [PATCH v4 6/6] rust: add improved version of `ForeignOwnable::borrow_mut`

[Alice Ryhl]

On Mon, Nov 11, 2024 at 4:32 PM Tamir Duberstein <tamird@gmail.com> wrote:
>
> This series conflicts with Gary's FFI types series:
>
> https://lore.kernel.org/all/20240913213041.395655-4-gary@garyguo.net/
>
> I've rebased it locally. Shall I send a v5?

You should probably wait for Miguel to decide what to do about
https://lore.kernel.org/all/20241111175842.550fc29d@canb.auug.org.au/

Alice

Re: [PATCH v4 6/6] rust: add improved version of `ForeignOwnable::borrow_mut`

[Tamir Duberstein]

> You should probably wait for Miguel to decide what to do about
> https://lore.kernel.org/all/20241111175842.550fc29d@canb.auug.org.au/
>
> Alice

Thanks for the pointer!

My reading of that discussion is that the new `crate::ffi` module
isn't going anywhere, it's just the later patches that diverge the
types that might be dropped for now and taken later.

In my case though I have a hard conflict as the patch introducing
`crate::ffi` and this series touch the same lines so I think a rebase
is unavoidable.


Tamir

Re: [PATCH v4 6/6] rust: add improved version of `ForeignOwnable::borrow_mut`

[Miguel Ojeda]

On Mon, Nov 11, 2024 at 5:22 PM Tamir Duberstein <tamird@gmail.com> wrote:
>
> My reading of that discussion is that the new `crate::ffi` module
> isn't going anywhere, it's just the later patches that diverge the
> types that might be dropped for now and taken later.

Yeah, that was my plan, and I ended up with that.

However, we still need to see how it actually goes -- we will see tomorrow.

> In my case though I have a hard conflict as the patch introducing
> `crate::ffi` and this series touch the same lines so I think a rebase
> is unavoidable.

If there are conflicts, then a new version is usually welcome, unless
the conflict is small or trivial (i.e. sometimes maintainers may do
the rebase for you and other adjustments -- a new version may not be
worth it, and it depends on the maintainer).

In this case, please wait a few days to see if you get more feedback,
and meanwhile the linux-next issue will be resolved one way or
another, so you can use whatever is in rust-next by then.

Thanks!

Cheers,
Miguel

Re: [PATCH v4 1/6] rust: arc: use `NonNull::new_unchecked`

[Andreas Hindborg]

"Tamir Duberstein" <tamird@gmail.com> writes:

> 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>
> Signed-off-by: Tamir Duberstein <tamird@gmail.com>
> ---

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>


Best regards,
Andreas Hindborg

Re: [PATCH v4 2/6] rust: types: avoid `as` casts

[Andreas Hindborg]

"Tamir Duberstein" <tamird@gmail.com> writes:

> 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>
> Signed-off-by: Tamir Duberstein <tamird@gmail.com>

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>


Best regards,
Andreas Hindborg

Re: [PATCH v4 3/6] rust: arc: split unsafe block, add missing comment

[Andreas Hindborg]

"Tamir Duberstein" <tamird@gmail.com> writes:

> The new SAFETY comment style is taken from existing comments in `deref`
> and `drop.
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Tamir Duberstein <tamird@gmail.com>

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>


Best regards,
Andreas Hindborg

Re: [PATCH v4 5/6] rust: reorder `ForeignOwnable` items

[Andreas Hindborg]

"Tamir Duberstein" <tamird@gmail.com> writes:

> `{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>
> Signed-off-by: Tamir Duberstein <tamird@gmail.com>

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>


Best regards,
Andreas Hindborg

Re: [PATCH v4 6/6] rust: add improved version of `ForeignOwnable::borrow_mut`

[Tamir Duberstein]

On Mon, Nov 11, 2024 at 7:02 PM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> On Mon, Nov 11, 2024 at 5:22 PM Tamir Duberstein <tamird@gmail.com> wrote:
> >
> > My reading of that discussion is that the new `crate::ffi` module
> > isn't going anywhere, it's just the later patches that diverge the
> > types that might be dropped for now and taken later.
>
> Yeah, that was my plan, and I ended up with that.
>
> However, we still need to see how it actually goes -- we will see tomorrow.
>
> > In my case though I have a hard conflict as the patch introducing
> > `crate::ffi` and this series touch the same lines so I think a rebase
> > is unavoidable.
>
> If there are conflicts, then a new version is usually welcome, unless
> the conflict is small or trivial (i.e. sometimes maintainers may do
> the rebase for you and other adjustments -- a new version may not be
> worth it, and it depends on the maintainer).

The conflict is trivial here, but I've already done the rebase 🤷🏼‍♂️.

> In this case, please wait a few days to see if you get more feedback,
> and meanwhile the linux-next issue will be resolved one way or
> another, so you can use whatever is in rust-next by then.

Will do! Thanks Miguel.