[PATCH v3] rust: check type of `$ptr` in `container_of!`

[Tamir Duberstein <tamird@gmail.com>]

Add a compile-time check that `*$ptr` is of the type of `$type->$($f)*`.
Rename those placeholders for clarity.

Given the incorrect usage:

> diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
> index 8d978c896747..6a7089149878 100644
> --- a/rust/kernel/rbtree.rs
> +++ b/rust/kernel/rbtree.rs
> @@ -329,7 +329,7 @@ fn raw_entry(&mut self, key: &K) -> RawEntry<'_, K, V> {
>          while !(*child_field_of_parent).is_null() {
>              let curr = *child_field_of_parent;
>              // SAFETY: All links fields we create are in a `Node<K, V>`.
> -            let node = unsafe { container_of!(curr, Node<K, V>, links) };
> +            let node = unsafe { container_of!(curr, Node<K, V>, key) };
>
>              // SAFETY: `node` is a non-null node so it is valid by the type invariants.
>              match key.cmp(unsafe { &(*node).key }) {

this patch produces the compilation error:

> error[E0308]: mismatched types
>    --> rust/kernel/lib.rs:207:25
>     |
> 207 |             [field_ptr, container_field_ptr]; // typeof(`$field_ptr`) == typeof(`$Container.$($fields)*`)
>     |                         ^^^^^^^^^^^^^^^^^^^ expected `*mut rb_node`, found `*mut K`
>     |
>    ::: rust/kernel/rbtree.rs:270:6
>     |
> 270 | impl<K, V> RBTree<K, V>
>     |      - found this type parameter
> ...
> 332 |             let node = unsafe { container_of!(curr, Node<K, V>, key) };
>     |                                 ------------------------------------ in this macro invocation
>     |
>     = note: expected raw pointer `*mut bindings::rb_node`
>                found raw pointer `*mut K`
>     = note: this error originates in the macro `container_of` (in Nightly builds, run with -Z macro-backtrace for more info)
>
> error: aborting due to 1 previous error

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/all/CAH5fLgh6gmqGBhPMi2SKn7mCmMWfOSiS0WP5wBuGPYh9ZTAiww@mail.gmail.com/
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
I also considered an implementation using a function, but the resulting
compilation error was not as concise:

> error[E0308]: mismatched types
>    --> rust/kernel/lib.rs:207:42
>     |
> 207 |             assert_same_type($field_ptr, field_ptr);
>     |             ----------------             ^^^^^^^^^ expected `*const rb_node`, found `*mut K`
>     |             |
>     |             arguments to this function are incorrect
>     |
>    ::: rust/kernel/rbtree.rs:270:6
>     |
> 270 | impl<K, V> RBTree<K, V>
>     |      - found this type parameter
> ...
> 332 |             let node = unsafe { container_of!(curr, Node<K, V>, key) };
>     |                                 ------------------------------------ in this macro invocation
>     |
>     = note: expected raw pointer `*const bindings::rb_node`
>                found raw pointer `*mut K`
> note: function defined here
>    --> rust/kernel/lib.rs:205:16
>     |
> 205 |             fn assert_same_type<T>(_: *const T, _: *const T) {}
>     |                ^^^^^^^^^^^^^^^^                 -----------
>     |
>    ::: rust/kernel/rbtree.rs:332:33
>     |
> 332 |             let node = unsafe { container_of!(curr, Node<K, V>, key) };
>     |                                 ------------------------------------ in this macro invocation
>     = note: this error originates in the macro `container_of` (in Nightly builds, run with -Z macro-backtrace for more info)
> 
> error: aborting due to 1 previous error
---
Changes in v3:
- Fix comment typo.
- s/^;/   / in commit message and cover letter. (Miguel Ojeda)
- Evaluate $ptr only once. (Alice Ryhl)
- Link to v2: https://lore.kernel.org/r/20250412-b4-container-of-type-check-v2-1-f3cc9934c160@gmail.com

Changes in v2:
- Wrap in `if false` to improve unoptimized codegen. (Alice Ryhl)
- Shrink implementation using an array literal instead of a function.
- Link to v1: https://lore.kernel.org/r/20250411-b4-container-of-type-check-v1-1-08262ef67c95@gmail.com
---
 rust/kernel/lib.rs | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 1df11156302a..d14ed86efb68 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -198,9 +198,15 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
 /// ```
 #[macro_export]
 macro_rules! container_of {
-    ($ptr:expr, $type:ty, $($f:tt)*) => {{
-        let offset: usize = ::core::mem::offset_of!($type, $($f)*);
-        $ptr.byte_sub(offset).cast::<$type>()
+    ($field_ptr:expr, $Container:ty, $($fields:tt)*) => {{
+        let offset: usize = ::core::mem::offset_of!($Container, $($fields)*);
+        let field_ptr = $field_ptr;
+        let container_ptr = field_ptr.byte_sub(offset).cast::<$Container>();
+        if false {
+            let container_field_ptr = ::core::ptr::addr_of!((*container_ptr).$($fields)*).cast_mut();
+            [field_ptr, container_field_ptr]; // typeof(`$field_ptr`) == typeof(`$Container.$($fields)*`)
+        }
+        container_ptr
     }}
 }
 

---
base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8
change-id: 20250411-b4-container-of-type-check-06af1c204f59
prerequisite-change-id: 20250409-container-of-mutness-b153dab4388d:v1
prerequisite-patch-id: 53d5889db599267f87642bb0ae3063c29bc24863

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