rust: debugfs: Add initial support for directories

Adds a `debugfs::Dir` type that can be used to create and remove
DebugFS directories. The `Dir` handle automatically cleans up the
directory on `Drop`.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250904-debugfs-rust-v11-1-7d12a165685a@google.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/debugfs/entry.rs

@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (C) 2025 Google LLC.
+
+use crate::str::CStr;
+use crate::sync::Arc;
+
+/// Owning handle to a DebugFS entry.
+///
+/// # Invariants
+///
+/// The wrapped pointer will always be `NULL`, an error, or an owned DebugFS `dentry`.
+pub(crate) struct Entry {
+    entry: *mut bindings::dentry,
+    // If we were created with an owning parent, this is the keep-alive
+    _parent: Option<Arc<Entry>>,
+}
+
+// SAFETY: [`Entry`] is just a `dentry` under the hood, which the API promises can be transferred
+// between threads.
+unsafe impl Send for Entry {}
+
+// SAFETY: All the C functions we call on the `dentry` pointer are threadsafe.
+unsafe impl Sync for Entry {}
+
+impl Entry {
+    pub(crate) fn dynamic_dir(name: &CStr, parent: Option<Arc<Self>>) -> Self {
+        let parent_ptr = match &parent {
+            Some(entry) => entry.as_ptr(),
+            None => core::ptr::null_mut(),
+        };
+        // SAFETY: The invariants of this function's arguments ensure the safety of this call.
+        // * `name` is a valid C string by the invariants of `&CStr`.
+        // * `parent_ptr` is either `NULL` (if `parent` is `None`), or a pointer to a valid
+        //   `dentry` by our invariant. `debugfs_create_dir` handles `NULL` pointers correctly.
+        let entry = unsafe { bindings::debugfs_create_dir(name.as_char_ptr(), parent_ptr) };
+
+        Entry {
+            entry,
+            _parent: parent,
+        }
+    }
+
+    /// Returns the pointer representation of the DebugFS directory.
+    ///
+    /// # Guarantees
+    ///
+    /// Due to the type invariant, the value returned from this function will always be an error
+    /// code, NULL, or a live DebugFS directory. If it is live, it will remain live at least as
+    /// long as this entry lives.
+    pub(crate) fn as_ptr(&self) -> *mut bindings::dentry {
+        self.entry
+    }
+}
+
+impl Drop for Entry {
+    fn drop(&mut self) {
+        // SAFETY: `debugfs_remove` can take `NULL`, error values, and legal DebugFS dentries.
+        // `as_ptr` guarantees that the pointer is of this form.
+        unsafe { bindings::debugfs_remove(self.as_ptr()) }
+    }
+}