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

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Apr 23, 2025 at 01:40:10PM -0400, 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: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>

Reviewed-by: Alice Ryhl <aliceryhl@google.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)*) => {{

It's rather unusual to use an uppercase C in the name of this parameter.

Alice

> +        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>
>