rust: retain pointer mut-ness in `container_of!`

Avoid casting the input pointer to `*const _`, allowing the output
pointer to be `*mut` if the input is `*mut`. This allows a number of
`*const` to `*mut` conversions to be removed at the cost of slightly
worse ergonomics when the macro is used with a reference rather than a
pointer; the only example of this was in the macro's own doctest.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250409-container-of-mutness-v1-1-64f472b94534@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

2 files changed, 12 insertions, 16 deletions

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 6e9287136cac..909d305d0be8 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -205,7 +205,7 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
 /// }
 ///
 /// let test = Test { a: 10, b: 20 };
-/// let b_ptr = &test.b;
+/// let b_ptr: *const _ = &test.b;
 /// // SAFETY: The pointer points at the `b` field of a `Test`, so the resulting pointer will be
 /// // in-bounds of the same allocation as `b_ptr`.
 /// let test_alias = unsafe { container_of!(b_ptr, Test, b) };
@@ -214,9 +214,8 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
 #[macro_export]
 macro_rules! container_of {
     ($ptr:expr, $type:ty, $($f:tt)*) => {{
-        let ptr = $ptr as *const _ as *const u8;
         let offset: usize = ::core::mem::offset_of!($type, $($f)*);
-        ptr.sub(offset) as *const $type
+        $ptr.byte_sub(offset).cast::<$type>()
     }}
 }
 
diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
index 5246b2c8a4ff..8d978c896747 100644
--- a/rust/kernel/rbtree.rs
+++ b/rust/kernel/rbtree.rs
@@ -424,7 +424,7 @@ where
         while !node.is_null() {
             // SAFETY: By the type invariant of `Self`, all non-null `rb_node` pointers stored in `self`
             // point to the links field of `Node<K, V>` objects.
-            let this = unsafe { container_of!(node, Node<K, V>, links) }.cast_mut();
+            let this = unsafe { container_of!(node, Node<K, V>, links) };
             // SAFETY: `this` is a non-null node so it is valid by the type invariants.
             let this_key = unsafe { &(*this).key };
             // SAFETY: `node` is a non-null node so it is valid by the type invariants.
@@ -496,7 +496,7 @@ impl<K, V> Drop for RBTree<K, V> {
             // but it is not observable. The loop invariant is still maintained.
 
             // SAFETY: `this` is valid per the loop invariant.
-            unsafe { drop(KBox::from_raw(this.cast_mut())) };
+            unsafe { drop(KBox::from_raw(this)) };
         }
     }
 }
@@ -761,7 +761,7 @@ impl<'a, K, V> Cursor<'a, K, V> {
         let next = self.get_neighbor_raw(Direction::Next);
         // SAFETY: By the type invariant of `Self`, all non-null `rb_node` pointers stored in `self`
         // point to the links field of `Node<K, V>` objects.
-        let this = unsafe { container_of!(self.current.as_ptr(), Node<K, V>, links) }.cast_mut();
+        let this = unsafe { container_of!(self.current.as_ptr(), Node<K, V>, links) };
         // SAFETY: `this` is valid by the type invariants as described above.
         let node = unsafe { KBox::from_raw(this) };
         let node = RBTreeNode { node };
@@ -806,7 +806,7 @@ impl<'a, K, V> Cursor<'a, K, V> {
             unsafe { bindings::rb_erase(neighbor, addr_of_mut!(self.tree.root)) };
             // SAFETY: By the type invariant of `Self`, all non-null `rb_node` pointers stored in `self`
             // point to the links field of `Node<K, V>` objects.
-            let this = unsafe { container_of!(neighbor, Node<K, V>, links) }.cast_mut();
+            let this = unsafe { container_of!(neighbor, Node<K, V>, links) };
             // SAFETY: `this` is valid by the type invariants as described above.
             let node = unsafe { KBox::from_raw(this) };
             return Some(RBTreeNode { node });
@@ -912,7 +912,7 @@ impl<'a, K, V> Cursor<'a, K, V> {
     unsafe fn to_key_value_raw<'b>(node: NonNull<bindings::rb_node>) -> (&'b K, *mut V) {
         // SAFETY: By the type invariant of `Self`, all non-null `rb_node` pointers stored in `self`
         // point to the links field of `Node<K, V>` objects.
-        let this = unsafe { container_of!(node.as_ptr(), Node<K, V>, links) }.cast_mut();
+        let this = unsafe { container_of!(node.as_ptr(), Node<K, V>, links) };
         // SAFETY: The passed `node` is the current node or a non-null neighbor,
         // thus `this` is valid by the type invariants.
         let k = unsafe { &(*this).key };
@@ -1021,7 +1021,7 @@ impl<K, V> Iterator for IterRaw<K, V> {
 
         // SAFETY: By the type invariant of `IterRaw`, `self.next` is a valid node in an `RBTree`,
         // and by the type invariant of `RBTree`, all nodes point to the links field of `Node<K, V>` objects.
-        let cur = unsafe { container_of!(self.next, Node<K, V>, links) }.cast_mut();
+        let cur = unsafe { container_of!(self.next, Node<K, V>, links) };
 
         // SAFETY: `self.next` is a valid tree node by the type invariants.
         self.next = unsafe { bindings::rb_next(self.next) };
@@ -1216,7 +1216,7 @@ impl<'a, K, V> OccupiedEntry<'a, K, V> {
         // SAFETY:
         // - `self.node_links` is a valid pointer to a node in the tree.
         // - We have exclusive access to the underlying tree, and can thus give out a mutable reference.
-        unsafe { &mut (*(container_of!(self.node_links, Node<K, V>, links).cast_mut())).value }
+        unsafe { &mut (*(container_of!(self.node_links, Node<K, V>, links))).value }
     }
 
     /// Converts the entry into a mutable reference to its value.
@@ -1226,7 +1226,7 @@ impl<'a, K, V> OccupiedEntry<'a, K, V> {
         // SAFETY:
         // - `self.node_links` is a valid pointer to a node in the tree.
         // - This consumes the `&'a mut RBTree<K, V>`, therefore it can give out a mutable reference that lives for `'a`.
-        unsafe { &mut (*(container_of!(self.node_links, Node<K, V>, links).cast_mut())).value }
+        unsafe { &mut (*(container_of!(self.node_links, Node<K, V>, links))).value }
     }
 
     /// Remove this entry from the [`RBTree`].
@@ -1239,9 +1239,7 @@ impl<'a, K, V> OccupiedEntry<'a, K, V> {
         RBTreeNode {
             // SAFETY: The node was a node in the tree, but we removed it, so we can convert it
             // back into a box.
-            node: unsafe {
-                KBox::from_raw(container_of!(self.node_links, Node<K, V>, links).cast_mut())
-            },
+            node: unsafe { KBox::from_raw(container_of!(self.node_links, Node<K, V>, links)) },
         }
     }
 
@@ -1272,8 +1270,7 @@ impl<'a, K, V> OccupiedEntry<'a, K, V> {
         // SAFETY:
         // - `self.node_ptr` produces a valid pointer to a node in the tree.
         // - Now that we removed this entry from the tree, we can convert the node to a box.
-        let old_node =
-            unsafe { KBox::from_raw(container_of!(self.node_links, Node<K, V>, links).cast_mut()) };
+        let old_node = unsafe { KBox::from_raw(container_of!(self.node_links, Node<K, V>, links)) };
 
         RBTreeNode { node: old_node }
     }