Commit 74d6a606 authored by Tamir Duberstein's avatar Tamir Duberstein Committed by Miguel Ojeda
Browse files

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: default avatarBenno Lossin <benno.lossin@proton.me>
Reviewed-by: default avatarAlice Ryhl <aliceryhl@google.com>
Signed-off-by: default avatarTamir Duberstein <tamird@gmail.com>
Reviewed-by: default avatarAndreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250409-container-of-mutness-v1-1-64f472b94534@gmail.com


Signed-off-by: default avatarMiguel Ojeda <ojeda@kernel.org>
parent a3b23473

rust/kernel/lib.rs

+2 −3
Original line number Diff line number Diff line
@@ -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>()

    }}

}

rust/kernel/rbtree.rs

+10 −13
Original line number Diff line number Diff line
@@ -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 }

    }