rust: pin-init: fix incorrect accessor reference lifetime

When a field has been initialized, `init!`/`pin_init!` create a reference
or pinned reference to the field so it can be accessed later during the
initialization of other fields. However, the reference it created is
incorrectly `&'static` rather than just the scope of the initializer.

This means that you can do

    init!(Foo {
        a: 1,
        _: {
            let b: &'static u32 = a;
        }
    })

which is unsound.

This is caused by `&mut (*#slot).#ident`, which actually allows arbitrary
lifetime, so this is effectively `'static`. Somewhat ironically, the safety
justification of creating the accessor is.. "SAFETY: TODO".

Fix it by adding `let_binding` method on `DropGuard` to shorten lifetime.
This results in exactly what we want for these accessors. The safety and
invariant comments of `DropGuard` have been reworked; instead of reasoning
about what caller can do with the guard, express it in a way that the
ownership is transferred to the guard and `forget` takes it back, so the
unsafe operations within the `DropGuard` can be more easily justified.

Fixes: 42415d163e5d ("rust: pin-init: add references to previously initialized fields")
Cc: stable@vger.kernel.org
Signed-off-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260427-pin-init-fix-v3-2-496a699674dd@garyguo.net
[ Reworded for missing word. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 19 insertions, 9 deletions

diff --git a/rust/pin-init/src/__internal.rs b/rust/pin-init/src/__internal.rs
index 90adbdc1893b..5720a621aed7 100644
--- a/rust/pin-init/src/__internal.rs
+++ b/rust/pin-init/src/__internal.rs
@@ -238,32 +238,42 @@ fn stack_init_reuse() {
 /// When a value of this type is dropped, it drops a `T`.
 ///
 /// Can be forgotten to prevent the drop.
+///
+/// # Invariants
+///
+/// - `ptr` is valid and properly aligned.
+/// - `*ptr` is initialized and owned by this guard.
 pub struct DropGuard<T: ?Sized> {
     ptr: *mut T,
 }
 
 impl<T: ?Sized> DropGuard<T> {
-    /// Creates a new [`DropGuard<T>`]. It will [`ptr::drop_in_place`] `ptr` when it gets dropped.
+    /// Creates a drop guard and transfer the ownership of the pointer content.
     ///
-    /// # Safety
+    /// The ownership is only relinguished if the guard is forgotten via [`core::mem::forget`].
     ///
-    /// `ptr` must be a valid pointer.
+    /// # Safety
     ///
-    /// It is the callers responsibility that `self` will only get dropped if the pointee of `ptr`:
-    /// - has not been dropped,
-    /// - is not accessible by any other means,
-    /// - will not be dropped by any other means.
+    /// - `ptr` is valid and properly aligned.
+    /// - `*ptr` is initialized, and the ownership is transferred to this guard.
     #[inline]
     pub unsafe fn new(ptr: *mut T) -> Self {
+        // INVARIANT: By safety requirement.
         Self { ptr }
     }
+
+    /// Create a let binding for accessor use.
+    #[inline]
+    pub fn let_binding(&mut self) -> &mut T {
+        // SAFETY: Per type invariant.
+        unsafe { &mut *self.ptr }
+    }
 }
 
 impl<T: ?Sized> Drop for DropGuard<T> {
     #[inline]
     fn drop(&mut self) {
-        // SAFETY: A `DropGuard` can only be constructed using the unsafe `new` function
-        // ensuring that this operation is safe.
+        // SAFETY: `self.ptr` is valid, properly aligned and `*self.ptr` is owned by this guard.
         unsafe { ptr::drop_in_place(self.ptr) }
     }
 }