Re: [PATCH v4] rust: check type of `$ptr` in `container_of!`

["Benno Lossin" <lossin@kernel.org>]

On Thu May 29, 2025 at 3:11 PM CEST, Tamir Duberstein wrote:
> 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:220:45
>>     |
>> 220 |         $crate::assert_same_type(field_ptr, (&raw const (*container_ptr).$($fields)*).cast_mut());
>>     |         ------------------------ ---------  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `*mut rb_node`, found `*mut K`
>>     |         |                        |
>>     |         |                        expected all arguments to be this `*mut bindings::rb_node` type because they need to match the type of this parameter
>>     |         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 `*mut bindings::rb_node`
>>                found raw pointer `*mut K`
>> note: function defined here
>>    --> rust/kernel/lib.rs:227:8
>>     |
>> 227 | pub fn assert_same_type<T>(_: T, _: T) {}
>>     |        ^^^^^^^^^^^^^^^^ -  ----  ---- this parameter needs to match the `*mut bindings::rb_node` type of parameter #1
>>     |                         |  |
>>     |                         |  parameter #2 needs to match the `*mut bindings::rb_node` type of this parameter
>>     |                         parameter #1 and parameter #2 both reference this parameter `T`
>>     = note: this error originates in the macro `container_of` (in Nightly builds, run with -Z macro-backtrace for more info)

In the future we could make this a proc-macro and improve the error
message by creating the function inline and setting the spans for the
parameter of the function to the spans coming from the input.

> 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>

Reviewed-by: Benno Lossin <lossin@kernel.org>

---
Cheers,
Benno

> ---
> Changes in v4:
> - Revert back to v1 with assert_same_type extracted out of the macro. (Miguel Ojeda)
> - Drop Benno's RB since the implementation changed.
> - Rebase on rust-next.
> - Link to v3: https://lore.kernel.org/r/20250423-b4-container-of-type-check-v3-1-7994c56cf359@gmail.com
>
> 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 | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)