[PATCH 1/8] rust: debugfs: Bind DebugFS directory creation

[Matthew Maurer <mmaurer@google.com>]

The basic API relies on `dput` to prevent leaks. Use of `debugfs_remove`
is delayed until the more full-featured API, because we need to avoid
the user having an reference to a dir that is recursively removed.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
---
 MAINTAINERS                     |   1 +
 rust/bindings/bindings_helper.h |   1 +
 rust/helpers/dcache.c           |  12 +++++
 rust/helpers/helpers.c          |   1 +
 rust/kernel/debugfs.rs          | 100 ++++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs              |   1 +
 6 files changed, 116 insertions(+)

diff --git a/MAINTAINERS b/MAINTAINERS
index 906881b6c5cb6ff743e13b251873b89138c69a1c..a3b835e427b083a4ddd690d9e7739851f0af47ae 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -7271,6 +7271,7 @@ F:	include/linux/kobj*
 F:	include/linux/property.h
 F:	include/linux/sysfs.h
 F:	lib/kobj*
+F:	rust/kernel/debugfs.rs
 F:	rust/kernel/device.rs
 F:	rust/kernel/device_id.rs
 F:	rust/kernel/devres.rs
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 8a2add69e5d66d1c2ebed9d2c950380e61c48842..787f928467faabd02a7f3cf041378fac856c4f89 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -13,6 +13,7 @@
 #include <linux/blkdev.h>
 #include <linux/cpumask.h>
 #include <linux/cred.h>
+#include <linux/debugfs.h>
 #include <linux/device/faux.h>
 #include <linux/dma-mapping.h>
 #include <linux/errname.h>
diff --git a/rust/helpers/dcache.c b/rust/helpers/dcache.c
new file mode 100644
index 0000000000000000000000000000000000000000..2396cdaa89a95a2be69fd84ec205e0f5f1b63f0c
--- /dev/null
+++ b/rust/helpers/dcache.c
@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright (C) 2025 Google LLC.
+ */
+
+#include <linux/dcache.h>
+
+struct dentry *rust_helper_dget(struct dentry *d)
+{
+	return dget(d);
+}
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index f34320e6d1f2fb56cc151ee2ffe5d331713fd36a..95f486c1175191483297b7140b99f1aa364c081c 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -15,6 +15,7 @@
 #include "cpumask.c"
 #include "cred.c"
 #include "device.c"
+#include "dcache.c"
 #include "dma.c"
 #include "err.c"
 #include "fs.c"
diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
new file mode 100644
index 0000000000000000000000000000000000000000..4d06cce7099607f95b684bad329f791a815d3e86
--- /dev/null
+++ b/rust/kernel/debugfs.rs
@@ -0,0 +1,100 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2025 Google LLC.
+
+//! DebugFS Abstraction
+//!
+//! C header: [`include/linux/debugfs.h`](srctree/include/linux/debugfs.h)
+
+use crate::error::{from_err_ptr, Result};
+use crate::str::CStr;
+use crate::types::{ARef, AlwaysRefCounted, Opaque};
+use core::ptr::NonNull;
+
+/// Handle to a DebugFS directory.
+pub struct Dir {
+    inner: Opaque<bindings::dentry>,
+}
+
+// SAFETY: Dir is just a `dentry` under the hood, which the API promises can be transferred
+// between threads.
+unsafe impl Send for Dir {}
+
+// SAFETY: All the native functions we re-export use interior locking, and the contents of the
+// struct are opaque to Rust.
+unsafe impl Sync for Dir {}
+
+// SAFETY: Dir is actually `dentry`, and dget/dput are the reference counting functions
+// for it.
+unsafe impl AlwaysRefCounted for Dir {
+    #[inline]
+    fn inc_ref(&self) {
+        // SAFETY: Since we have a reference to the directory,
+        // it's live, so it's safe to call dget on it.
+        unsafe {
+            kernel::bindings::dget(self.as_ptr());
+        }
+    }
+    #[inline]
+    unsafe fn dec_ref(obj: NonNull<Self>) {
+        // SAFETY: By the caller precondition on the trait, we know that the caller has a reference
+        // count to the object.
+        unsafe {
+            kernel::bindings::dput(obj.cast().as_ptr());
+        }
+    }
+}
+
+impl Dir {
+    /// Create a new directory in DebugFS. If `parent` is [`None`], it will be created at the root.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::c_str;
+    /// # use kernel::debugfs::Dir;
+    /// {
+    ///     let dir = Dir::new(c_str!("my_debug_dir"), None)?;
+    ///     // The directory will exist in DebugFS here.
+    /// }
+    /// // The directory will no longer exist in DebugFS here.
+    /// # Ok::<(), Error>(())
+    /// ```
+    ///
+    /// ```
+    /// # use kernel::c_str;
+    /// # use kernel::debugfs::Dir;
+    /// let parent = Dir::new(c_str!("parent"), None)?;
+    /// let child = Dir::new(c_str!("child"), Some(&parent))?;
+    /// // parent/child exists in DebugFS here.
+    /// drop(parent);
+    /// // The child dentry is still valid here, but DebugFS will have neither directory.
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn new(name: &CStr, parent: Option<&Self>) -> Result<ARef<Self>> {
+        let parent_ptr = match parent {
+            Some(parent) => parent.as_ptr(),
+            None => core::ptr::null_mut(),
+        };
+        // SAFETY:
+        // * name argument points to a null terminated string that lives across the call, by
+        //   invariants of &CStr
+        // * If parent is None, parent accepts null pointers to mean create at root
+        // * If parent is Some, parent accepts live dentry debugfs pointers
+        // * `debugfs_create_dir` either returns an error code or a legal `dentry` pointer,
+        //   so we can call `NonNull::new_unchecked`.
+        let dir = unsafe {
+            NonNull::new_unchecked(from_err_ptr(kernel::bindings::debugfs_create_dir(
+                name.as_char_ptr(),
+                parent_ptr,
+            ))?)
+        };
+        // SAFETY: Dir is a transparent wrapper for an Opaque<dentry>, and we received a live
+        // owning dentry from `debugfs_create_dir`, so we can wrap it in an ARef
+        Ok(unsafe { ARef::from_raw(dir.cast()) })
+    }
+
+    fn as_ptr(&self) -> *mut bindings::dentry {
+        self.inner.get()
+    }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index c3762e80b314316b4b0cee3bfd9442f8f0510b91..86f6055b828d5f711578293d8916a517f2436977 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -45,6 +45,7 @@
 #[doc(hidden)]
 pub mod build_assert;
 pub mod cred;
+pub mod debugfs;
 pub mod device;
 pub mod device_id;
 pub mod devres;

-- 
2.49.0.901.g37484f566f-goog