[PATCH v2 1/1] rust: sync: Add Lock::from_raw() for Lock<(), B>

[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<(), B>`, then given `()` is a ZST
- `Lock<(), 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<(),
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>

---

V2:
* Don't implement this for all ZST types, just implement it for ()
---
 rust/kernel/sync/lock.rs | 28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
index f6c34ca4d819f..8a74a1ea1ecaf 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 +118,29 @@ pub fn new(t: T, name: &'static CStr, key: &'static LockClassKey) -> impl PinIni
             }),
         })
     }
+
+}
+
+impl<B: Backend> Lock<(), B> {
+    /// Constructs a [`Lock`] from a raw pointer.
+    ///
+    /// This can be useful for interacting with a lock which was initialised outside of rust.
+    ///
+    /// # Safety
+    ///
+    /// The caller promises that `ptr` points to a valid initialised instance of [`State`].
+    ///
+    /// [`State`]: Backend::State
+    pub unsafe fn from_raw<'a>(ptr: *mut B::State) -> &'a Self
+    {
+        // SAFETY:
+        // * By the safety contract `ptr` must point to a valid initialised instance of `B::State`
+        // * Since the lock data type is `()` which is a ZST, `state` is 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