[PATCH 1/1] rust: sync: Add Lock::from_raw() for ZST data types

[Lyude Paul <lyude@redhat.com>]

A few of the APIs I've been writing bindings for (KMS in particular) rely
on the user manually acquiring specific locks before calling certain
functions. At the moment though, the only way of acquiring these locks in
bindings is to simply call the C locking functions directly - since said
locks are not acquired on the rust side of things.

However - if we add `#[repr(C)]` to `Lock<T, B>`, then given `T` is a ZST -
`Lock<T, B>` becomes equivalent in data layout to its inner `B::State`
type. Since locks in C don't have data explicitly associated with them
anyway, we can take advantage of this to add a `Lock::from_raw()` function
that can translate a raw pointer to `B::State` into its proper `Lock<T, B>`
equivalent. This lets us simply acquire a reference to the lock in question
and work with it like it was initialized on the rust side of things,
allowing us to use less unsafe code to implement bindings with lock
requirements.

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 rust/kernel/sync/lock.rs | 32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
index f6c34ca4d819f..f77cb178840b2 100644
--- a/rust/kernel/sync/lock.rs
+++ b/rust/kernel/sync/lock.rs
@@ -6,8 +6,8 @@
 //! spinlocks, raw spinlocks) to be provided with minimal effort.
 
 use super::LockClassKey;
-use crate::{init::PinInit, pin_init, str::CStr, types::Opaque, types::ScopeGuard};
-use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
+use crate::{init::PinInit, pin_init, prelude::*, str::CStr, types::Opaque, types::ScopeGuard};
+use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned, mem};
 use macros::pin_data;
 
 pub mod mutex;
@@ -81,6 +81,7 @@ unsafe fn relock(ptr: *mut Self::State, guard_state: &mut Self::GuardState) {
 ///
 /// Exposes one of the kernel locking primitives. Which one is exposed depends on the lock
 /// [`Backend`] specified as the generic parameter `B`.
+#[repr(C)]
 #[pin_data]
 pub struct Lock<T: ?Sized, B: Backend> {
     /// The kernel lock object.
@@ -117,6 +118,33 @@ pub fn new(t: T, name: &'static CStr, key: &'static LockClassKey) -> impl PinIni
             }),
         })
     }
+
+    /// Constructs a [`Lock`] from a raw pointer.
+    ///
+    /// This can be useful for interacting with a lock which was initialised outside of rust. This
+    /// can only be used when `T` is a ZST type.
+    ///
+    /// # Safety
+    ///
+    /// - The caller promises that `ptr` points to a valid initialised instance of [`State`].
+    /// - The caller promises that `T` is a type that it is allowed to create (e.g. `!` would not be
+    ///   allowed)
+    ///
+    /// [`State`]: Backend::State
+    pub unsafe fn from_raw<'a>(ptr: *mut B::State) -> &'a Self {
+        build_assert!(
+            mem::size_of::<T>() == 0,
+            "Lock::<T, B>::from_raw() can only be used if T is a ZST"
+        );
+
+        // SAFETY:
+        // * By the safety contract `ptr` must point to a valid initialised instance of `B::State`
+        // * We just asserted that `T` is a ZST, making `state` the only non-ZST member of the
+        //   struct
+        // * Combined with `#[repr(C)]`, this guarantees `Self` has an equivalent data layout to
+        //   `B::State`.
+        unsafe { &*ptr.cast() }
+    }
 }
 
 impl<T: ?Sized, B: Backend> Lock<T, B> {
-- 
2.46.0