[PATCH v6 3/5] rust: debugfs: Support arbitrary owned backing for File

[Matthew Maurer <mmaurer@google.com>]

This allows `File`s to be backed by `Deref<Target=T>` rather than just
`&'static T`. This means that dynamically allocated objects can be
attached to `File`s without needing to take extra steps to create a
pinned reference that's guaranteed to live long enough.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
---
 rust/kernel/debugfs.rs | 51 ++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 39 insertions(+), 12 deletions(-)

diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
index 6a89557d8cf49327d2984d15741ffb6640defd70..cd83f21cf2818f406575941ebbc6c426575643e4 100644
--- a/rust/kernel/debugfs.rs
+++ b/rust/kernel/debugfs.rs
@@ -5,12 +5,13 @@
 //!
 //! C header: [`include/linux/debugfs.h`](srctree/include/linux/debugfs.h)
 
-#[cfg(CONFIG_DEBUG_FS)]
+use crate::alloc::KBox;
 use crate::prelude::GFP_KERNEL;
 use crate::str::CStr;
 #[cfg(CONFIG_DEBUG_FS)]
 use crate::sync::Arc;
 use core::fmt::Display;
+use core::ops::Deref;
 
 #[cfg(CONFIG_DEBUG_FS)]
 mod display_file;
@@ -61,40 +62,59 @@ fn create(_name: &CStr, _parent: Option<&Dir>) -> Self {
     }
 
     #[cfg(CONFIG_DEBUG_FS)]
-    fn create_file<T: Display + Sized>(&self, name: &CStr, data: &'static T) -> File {
+    fn create_file<D: Deref<Target = T> + 'static + Send + Sync, T: Display>(
+        &self,
+        name: &CStr,
+        data: D,
+    ) -> File {
+        let mut file = File {
+            _entry: entry::Entry::empty(),
+            _data: None,
+        };
+        let Some(data) = KBox::new(data, GFP_KERNEL).ok() else {
+            return file;
+        };
+
         let Some(parent) = &self.0 else {
-            return File {
-                _entry: entry::Entry::empty(),
-            };
+            return file;
         };
+
         // SAFETY:
         // * `name` is a NUL-terminated C string, living across the call, by `CStr` invariant.
         // * `parent` is a live `dentry` since we have a reference to it.
         // * `vtable` is all stock `seq_file` implementations except for `open`.
         //   `open`'s only requirement beyond what is provided to all open functions is that the
         //   inode's data pointer must point to a `T` that will outlive it, which we know because
-        //   we have a static reference.
+        //   we have an owning `D` in the `File`, and we tear down the file during `Drop`.
         let ptr = unsafe {
             bindings::debugfs_create_file_full(
                 name.as_char_ptr(),
                 0o444,
                 parent.as_ptr(),
-                data as *const _ as *mut _,
+                data.deref() as *const _ as *mut _,
                 core::ptr::null(),
                 &<T as display_file::DisplayFile>::VTABLE,
             )
         };
 
+        file._data = Some(data);
+
         // SAFETY: `debugfs_create_file_full` either returns an error code or a legal
         // dentry pointer, so `Entry::new` is safe to call here.
-        let entry = unsafe { entry::Entry::new(ptr, Some(parent.clone())) };
+        file._entry = unsafe { entry::Entry::new(ptr, Some(parent.clone())) };
 
-        File { _entry: entry }
+        file
     }
 
     #[cfg(not(CONFIG_DEBUG_FS))]
-    fn create_file<T: Display + Sized>(&self, _name: &CStr, _data: &'static T) -> File {
-        File {}
+    fn create_file<D: Deref<Target = T> + 'static + Send + Sync, T: Display>(
+        &self,
+        _name: &CStr,
+        data: D,
+    ) -> File {
+        File {
+            _data: KBox::new(data, GFP_KERNEL).ok().map(|x| x as _),
+        }
     }
 
     /// Create a DebugFS subdirectory.
@@ -125,7 +145,11 @@ pub fn subdir(&self, name: &CStr) -> Self {
     /// dir.display_file(c_str!("foo"), &200);
     /// // "my_debugfs_dir/foo" now contains the number 200.
     /// ```
-    pub fn display_file<T: Display + Sized>(&self, name: &CStr, data: &'static T) -> File {
+    pub fn display_file<D: Deref<Target = T> + 'static + Send + Sync, T: Display>(
+        &self,
+        name: &CStr,
+        data: D,
+    ) -> File {
         self.create_file(name, data)
     }
 
@@ -147,4 +171,7 @@ pub fn new(name: &CStr) -> Self {
 pub struct File {
     #[cfg(CONFIG_DEBUG_FS)]
     _entry: entry::Entry,
+    // The data needs to be kept in a `Box` to prevent it from moving when the file does, as
+    // this might invalidate the pointer that's been passed to debugfs.
+    _data: Option<KBox<dyn Send + Sync>>,
 }

-- 
2.50.0.rc2.696.g1fc2a0284f-goog