[PATCH v2 1/5] rust: retain pointer mut-ness in `container_of!`

Tamir Duberstein
2025-03-09T16:01:05Z

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.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
rust/kernel/lib.rs | 5 ++---
rust/kernel/pci.rs | 2 +-
rust/kernel/platform.rs | 2 +-
rust/kernel/rbtree.rs | 23 ++++++++++-------------
4 files changed, 14 insertions(+), 18 deletions(-)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 7697c60b2d1a..9cd6b6864739 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -187,7 +187,7 @@ 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) };
@@ -196,9 +196,8 @@ 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/pci.rs b/rust/kernel/pci.rs
index 4c98b5b9aa1e..c37476576f02 100644
--- a/rust/kernel/pci.rs
+++ b/rust/kernel/pci.rs
@@ -364,7 +364,7 @@ pub unsafe fn from_dev(dev: ARef<device::Device>) -> Self {
fn as_raw(&self) -> *mut bindings::pci_dev {
// SAFETY: By the type invariant `self.0.as_raw` is a pointer to the `struct device`
// embedded in `struct pci_dev`.
- unsafe { container_of!(self.0.as_raw(), bindings::pci_dev, dev) as _ }
+ unsafe { container_of!(self.0.as_raw(), bindings::pci_dev, dev) }
}

/// Returns the PCI vendor ID.
diff --git a/rust/kernel/platform.rs b/rust/kernel/platform.rs
index 50e6b0421813..c51617569c01 100644
--- a/rust/kernel/platform.rs
+++ b/rust/kernel/platform.rs
@@ -189,7 +189,7 @@ unsafe fn from_dev(dev: ARef<device::Device>) -> Self {
fn as_raw(&self) -> *mut bindings::platform_device {
// SAFETY: By the type invariant `self.0.as_raw` is a pointer to the `struct device`
// embedded in `struct platform_device`.
- unsafe { container_of!(self.0.as_raw(), bindings::platform_device, dev) }.cast_mut()
+ unsafe { container_of!(self.0.as_raw(), bindings::platform_device, dev) }
}
}

diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
index 0d1e75810664..1fdea2806cfa 100644
--- a/rust/kernel/rbtree.rs
+++ b/rust/kernel/rbtree.rs
@@ -424,7 +424,7 @@ pub fn cursor_lower_bound(&mut self, key: &K) -> Option<Cursor<'_, K, V>>
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 @@ fn drop(&mut self) {
// 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 @@ pub fn remove_current(self) -> (Option<Self>, RBTreeNode<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 @@ fn remove_neighbor(&mut self, direction: Direction) -> Option<RBTreeNode<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 @@ unsafe fn to_key_value_mut<'b>(node: NonNull<bindings::rb_node>) -> (&'b K, &'b
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 @@ fn next(&mut self) -> Option<Self::Item> {

// 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 @@ pub fn get_mut(&mut self) -> &mut 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 @@ pub fn into_mut(self) -> &'a mut 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 @@ pub fn remove_node(self) -> RBTreeNode<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 @@ fn replace(self, node: RBTreeNode<K, V>) -> RBTreeNode<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 }
}

--
2.48.1