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

[Philipp Stanner <phasta@kernel.org>]

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());
         container_ptr
     }}
 }
-- 
2.49.0