Re: [RFC PATCH 1/4] rust: list: Add unsafe for container_of

["Gary Guo" <gary@garyguo.net>]

On Tue Feb 3, 2026 at 8:14 AM GMT, Philipp Stanner wrote:
> impl_list_item_mod.rs calls container_of() without unsafe blocks at a
> couple of places. Since container_of() is an unsafe macro / function,
> the blocks are strictly necessary.
> 
> For unknown reasons, that problem was so far not visible and only gets
> visible once one utilizes the list implementation from within the core
> crate:

The reason is that the error enabled via "unsafe-op-in-unsafe-fn" is a lint
rather than a hard compiler error, and Rust suppresses lints triggered inside
macro from another crate.

When the macro is used in kernel crate itself, it's no longer suppressed.

> 
> 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.
> 
> Cc: stable@vger.kernel.org # v6.17+
> Fixes: c77f85b347dd ("rust: list: remove OFFSET constants")
> Suggested-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Philipp Stanner <phasta@kernel.org>

Reviewed-by: Gary Guo <gary@garyguo.net>

Can you send it as a standalone patch so it's clear that this is intended to be
picked rather than part of the RFC series?

Best,
Gary

> ---
>  rust/kernel/list/impl_list_item_mod.rs | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)