Re: [PATCH] rust: lib: Add necessary unsafes for container_of

[Alice Ryhl <aliceryhl@google.com>]

On Fri, Nov 14, 2025 at 03:00:21PM +0100, Philipp Stanner wrote:
> When trying to use LinkedList in the kernel crate, build fails with an
> error message demanding unsafe blocks in the container_of macro:
> 
> error[E0133]: call to unsafe function `core::ptr::mut_ptr::<impl *mut T>::byte_sub`
> is unsafe and requires unsafe block
>    --> rust/kernel/lib.rs:252:29
>     |
> 252 |           let container_ptr = field_ptr.byte_sub(offset).cast::<$Container>();
>     |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
>     |
>    ::: rust/kernel/drm/jq.rs:98:1
>     |
> 98  | / impl_list_item! {
> 99  | |     impl ListItem<0> for BasicItem { using ListLinks { self.links }; }
> 100 | | }
>     | |_- in this macro invocation
>     |
> note: an unsafe function restricts its caller, but its body is safe by default
>    --> rust/kernel/list/impl_list_item_mod.rs:216:13
>     |
> 216 |               unsafe fn view_value(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
>     |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>     |
>    ::: rust/kernel/drm/jq.rs:98:1
>     |
> 98  | / impl_list_item! {
> 99  | |     impl ListItem<0> for BasicItem { using ListLinks { self.links }; }
> 100 | | }
>     | |_- in this macro invocation
>     = note: requested on the command line with `-D unsafe-op-in-unsafe-fn`
>     = note: this error originates in the macro `$crate::container_of` which comes
>     from the expansion of the macro `impl_list_item`
> 
> Add unsafe blocks to container_of to fix the issue.
> 
> Fixes: b20fbbc08a36 ("rust: check type of `$ptr` in `container_of!`")
> Suggested-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Philipp Stanner <phasta@kernel.org>
> ---
> I'm currently writing DrmJobqueue, a new piece of infrastructure. It
> uses LinkedList and resides in the kernel crate. When using LinkedList
> from within there, for reasons I don't fully understand the error above
> shows up.
> The other testing infrastructure doesn't seem to run into the error,
> though.
> 
> P.
> ---
>  rust/kernel/lib.rs | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index fef97f2a5098..a26b87015e7d 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -249,8 +249,11 @@ macro_rules! container_of {
>      ($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>();
> -        $crate::assert_same_type(field_ptr, (&raw const (*container_ptr).$($fields)*).cast_mut());
> +        // SAFETY: Offsetting the pointer to the container is correct because the offset was
> +        // calculated validly above.
> +        let container_ptr = unsafe { field_ptr.byte_sub(offset).cast::<$Container>() };
> +        // SAFETY: Safe because the container_ptr was validly created above.
> +        $crate::assert_same_type(field_ptr, unsafe { (&raw const (*container_ptr).$($fields)*) }.cast_mut());

The unsafe block goes in the impl_list_item! macro. This change makes
container_of! a safe operation, but is should not be a safe operation
because it uses byte_sub which promises the compiler that this pointer
offset operation stays within a single allocation.

Alice