[PATCH] rust: miscdevice: Export vtable testing

[Matthew Maurer <mmaurer@google.com>]

A common pattern in the kernel is to test whether a file belongs to a
particular driver by checking its `f_op` struct against an expected
value. This provides a safe way to perform that test for `MiscDevice`
implementations without needing to directly expose the vtable.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
---
 rust/kernel/miscdevice.rs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
index 939278bc7b03489a647b697012e09223871c90cd..5f59eda57c38be5f0d54fa9692fe5b2819e31480 100644
--- a/rust/kernel/miscdevice.rs
+++ b/rust/kernel/miscdevice.rs
@@ -177,6 +177,14 @@ fn show_fdinfo(
     }
 }
 
+/// Determines whether a given `File` is backed by the `T` `MiscDevice` based on vtable matching.
+pub fn is_miscdevice_file<T: MiscDevice>(file: &File) -> bool {
+    let vtable = core::ptr::from_ref(&MiscdeviceVTable::<T>::VTABLE);
+    // SAFETY: `f_op` is not mutated after file creation
+    let file_vtable = unsafe { (*file.as_ptr()).f_op };
+    vtable == file_vtable
+}
+
 /// A vtable for the file operations of a Rust miscdevice.
 struct MiscdeviceVTable<T: MiscDevice>(PhantomData<T>);
 

---
base-commit: 86731a2a651e58953fc949573895f2fa6d456841
change-id: 20250627-linux-miscident-7b67db234a5c

Best regards,
-- 
Matthew Maurer <mmaurer@google.com>