[PATCH 1/6] rust: add `container_of!` macro

[mattgilbride]

From: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>

This macro is used to obtain a pointer to an entire struct
when given a pointer to a field in that struct.

Signed-off-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
Signed-off-by: Matt Gilbride <mattgilbride@xxxxxxxxxx>
---
 rust/kernel/lib.rs | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 7ac39874aeac..c7963efd1318 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -102,3 +102,35 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
     // SAFETY: FFI call.
     unsafe { bindings::BUG() };
 }
+
+/// Produces a pointer to an object from a pointer to one of its fields.
+///
+/// # Safety
+///
+/// The pointer passed to this macro, and the pointer returned by this macro, must both be in
+/// bounds of the same allocation.
+///
+/// # Examples
+///
+/// ```
+/// # use kernel::container_of;
+/// struct Test {
+///     a: u64,
+///     b: u32,
+/// }
+///
+/// let test = Test { a: 10, b: 20 };
+/// let b_ptr = &test.b;
+/// // SAFETY: The pointer points at the `b` field of a `Test`, so the resulting pointer will be
+/// // in-bounds of the same allocation as `b_ptr`.
+/// let test_alias = unsafe { container_of!(b_ptr, Test, b) };
+/// assert!(core::ptr::eq(&test, test_alias));
+/// ```
+#[macro_export]
+macro_rules! container_of {
+    ($ptr:expr, $type:ty, $($f:tt)*) => {{
+        let ptr = $ptr as *const _ as *const u8;
+        let offset: usize = ::core::mem::offset_of!($type, $($f)*);
+        ptr.sub(offset) as *const $type
+    }}
+}

-- 
2.43.0.594.gd9cf4e227d-goog