Re: [PATCH 1/4] rust: debugfs: drop 'static bound from ScopedDir file creation methods

["Danilo Krummrich" <dakr@kernel.org>]

On Thu Sep 3, 2026 at 3:12 PM CEST, Gary Guo wrote:
> This can be better done by storing `&'static bindings::file_operations` in
> `FileOps<T>` instead of just by value. That is actually better than the current
> impl, IMO, because `mode` for example doesn't have to be in static storage. (You
> can also then make `FileOps<T>` `Copy`).

That's a great suggestion, thanks. It simplifies the patch to:

Author: Danilo Krummrich <dakr@kernel.org>
Date:   Sat Aug 29 14:42:54 2026 +0200

    rust: debugfs: drop 'static bound from ScopedDir file creation methods

    Drop the T: 'static bound from ScopedDir's file creation methods
    (read_binary_file(), read_only_file(), etc.) to support registering
    debugfs files backed by types that contain non-'static references, such
    as dma::Coherent<'a, T>.

    The previous 'static bound existed because ScopedDir::create_file() took
    &'static FileOps<T>, and &'static requires T: 'static for well-
    formedness. However, this was overly conservative: the file_operations
    pointer passed to the C debugfs API just needs to be 'static, not the
    entire FileOps<T>.

    Store &'static bindings::file_operations in FileOps<T> instead of the
    file_operations by value. In each trait impl, take a reference to the
    file_operations struct within the const block; since
    bindings::file_operations does not mention T, the reference is promoted
    to 'static regardless of T's lifetime parameters.

    Replace the Deref impl with an explicit fops() method that returns the
    stored &'static reference.

    Signed-off-by: Danilo Krummrich <dakr@kernel.org>

diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
index d7b8014a6474..2beb55d444ca 100644
--- a/rust/kernel/debugfs.rs
+++ b/rust/kernel/debugfs.rs
@@ -538,7 +538,7 @@ pub fn dir<'dir2>(&'dir2 self, name: &CStr) -> ScopedDir<'data, 'dir2> {
         }
     }

-    fn create_file<T: Sync>(&self, name: &CStr, data: &'data T, vtable: &'static FileOps<T>) {
+    fn create_file<T: Sync>(&self, name: &CStr, data: &'data T, vtable: &FileOps<T>) {
         #[cfg(CONFIG_DEBUG_FS)]
         core::mem::forget(Entry::file(name, &self.entry, data, vtable));
     }
@@ -550,7 +550,7 @@ fn create_file<T: Sync>(&self, name: &CStr, data: &'data T, vtable: &'static Fil
     /// This function does not produce an owning handle to the file. The created
     /// file is removed when the [`Scope`] that this directory belongs
     /// to is dropped.
-    pub fn read_only_file<T: Writer + Send + Sync + 'static>(&self, name: &CStr, data: &'data T) {
+    pub fn read_only_file<T: Writer + Send + Sync>(&self, name: &CStr, data: &'data T) {
         self.create_file(name, data, &T::FILE_OPS)
     }

@@ -560,11 +560,7 @@ pub fn read_only_file<T: Writer + Send + Sync + 'static>(&self, name: &CStr, dat
     ///
     /// This function does not produce an owning handle to the file. The created file is removed
     /// when the [`Scope`] that this directory belongs to is dropped.
-    pub fn read_binary_file<T: BinaryWriter + Send + Sync + 'static>(
-        &self,
-        name: &CStr,
-        data: &'data T,
-    ) {
+    pub fn read_binary_file<T: BinaryWriter + Send + Sync>(&self, name: &CStr, data: &'data T) {
         self.create_file(name, data, &T::FILE_OPS)
     }

@@ -596,11 +592,7 @@ pub fn read_callback_file<T, F>(&self, name: &CStr, data: &'data T, _f: &'static
     /// This function does not produce an owning handle to the file. The created
     /// file is removed when the [`Scope`] that this directory belongs
     /// to is dropped.
-    pub fn read_write_file<T: Writer + Reader + Send + Sync + 'static>(
-        &self,
-        name: &CStr,
-        data: &'data T,
-    ) {
+    pub fn read_write_file<T: Writer + Reader + Send + Sync>(&self, name: &CStr, data: &'data T) {
         let vtable = &<T as ReadWriteFile<_>>::FILE_OPS;
         self.create_file(name, data, vtable)
     }
@@ -612,7 +604,7 @@ pub fn read_write_file<T: Writer + Reader + Send + Sync + 'static>(
     ///
     /// This function does not produce an owning handle to the file. The created file is removed
     /// when the [`Scope`] that this directory belongs to is dropped.
-    pub fn read_write_binary_file<T: BinaryWriter + BinaryReader + Send + Sync + 'static>(
+    pub fn read_write_binary_file<T: BinaryWriter + BinaryReader + Send + Sync>(
         &self,
         name: &CStr,
         data: &'data T,
@@ -655,7 +647,7 @@ pub fn read_write_callback_file<T, F, W>(
     /// This function does not produce an owning handle to the file. The created
     /// file is removed when the [`Scope`] that this directory belongs
     /// to is dropped.
-    pub fn write_only_file<T: Reader + Send + Sync + 'static>(&self, name: &CStr, data: &'data T) {
+    pub fn write_only_file<T: Reader + Send + Sync>(&self, name: &CStr, data: &'data T) {
         let vtable = &<T as WriteFile<_>>::FILE_OPS;
         self.create_file(name, data, vtable)
     }
@@ -666,11 +658,7 @@ pub fn write_only_file<T: Reader + Send + Sync + 'static>(&self, name: &CStr, da
     ///
     /// This function does not produce an owning handle to the file. The created file is removed
     /// when the [`Scope`] that this directory belongs to is dropped.
-    pub fn write_binary_file<T: BinaryReader + Send + Sync + 'static>(
-        &self,
-        name: &CStr,
-        data: &'data T,
-    ) {
+    pub fn write_binary_file<T: BinaryReader + Send + Sync>(&self, name: &CStr, data: &'data T) {
         self.create_file(name, data, &T::FILE_OPS)
     }

diff --git a/rust/kernel/debugfs/entry.rs b/rust/kernel/debugfs/entry.rs
index 46aad64896ec..88a870d8c295 100644
--- a/rust/kernel/debugfs/entry.rs
+++ b/rust/kernel/debugfs/entry.rs
@@ -74,7 +74,7 @@ pub(crate) unsafe fn dynamic_file<T>(
                 parent.as_ptr(),
                 core::ptr::from_ref(data) as *mut c_void,
                 core::ptr::null(),
-                &**file_ops,
+                file_ops.fops(),
             )
         };

@@ -127,7 +127,7 @@ pub(crate) fn file<T>(
                 parent.as_ptr(),
                 core::ptr::from_ref(data) as *mut c_void,
                 core::ptr::null(),
-                &**file_ops,
+                file_ops.fops(),
             )
         };

diff --git a/rust/kernel/debugfs/file_ops.rs b/rust/kernel/debugfs/file_ops.rs
index f15908f71c4a..5c16a3196ca2 100644
--- a/rust/kernel/debugfs/file_ops.rs
+++ b/rust/kernel/debugfs/file_ops.rs
@@ -20,9 +20,6 @@

 use core::marker::PhantomData;

-#[cfg(CONFIG_DEBUG_FS)]
-use core::ops::Deref;
-
 /// # Invariant
 ///
 /// `FileOps<T>` will always contain an `operations` which is safe to use for a file backed
@@ -30,7 +27,7 @@
 /// into a reference.
 pub(super) struct FileOps<T> {
     #[cfg(CONFIG_DEBUG_FS)]
-    operations: bindings::file_operations,
+    operations: &'static bindings::file_operations,
     #[cfg(CONFIG_DEBUG_FS)]
     mode: u16,
     _phantom: PhantomData<T>,
@@ -41,7 +38,7 @@ impl<T> FileOps<T> {
     ///
     /// The caller asserts that the provided `operations` is safe to use for a file whose
     /// inode has a pointer to `T` in its private data that is safe to convert into a reference.
-    const unsafe fn new(operations: bindings::file_operations, mode: u16) -> Self {
+    const unsafe fn new(operations: &'static bindings::file_operations, mode: u16) -> Self {
         Self {
             #[cfg(CONFIG_DEBUG_FS)]
             operations,
@@ -65,11 +62,11 @@ pub(super) const fn adapt(&self) -> &FileOps<T::Inner> {
 }

 #[cfg(CONFIG_DEBUG_FS)]
-impl<T> Deref for FileOps<T> {
-    type Target = bindings::file_operations;
-
-    fn deref(&self) -> &Self::Target {
-        &self.operations
+impl<T> FileOps<T> {
+    /// Returns a `'static` reference to the inner `file_operations`.
+    #[inline]
+    pub(crate) fn fops(&self) -> &'static bindings::file_operations {
+        self.operations
     }
 }

@@ -130,11 +127,11 @@ pub(crate) trait ReadFile<T> {

 impl<T: Writer + Sync> ReadFile<T> for T {
     const FILE_OPS: FileOps<T> = {
-        let operations = bindings::file_operations {
+        let operations = &bindings::file_operations {
             read: Some(bindings::seq_read),
             llseek: Some(bindings::seq_lseek),
             release: Some(bindings::single_release),
-            open: Some(writer_open::<Self>),
+            open: Some(writer_open::<T>),
             ..pin_init::zeroed()
         };
         // SAFETY: `operations` is all stock `seq_file` implementations except for `writer_open`.
@@ -181,7 +178,7 @@ pub(crate) trait ReadWriteFile<T> {

 impl<T: Writer + Reader + Sync> ReadWriteFile<T> for T {
     const FILE_OPS: FileOps<T> = {
-        let operations = bindings::file_operations {
+        let operations = &bindings::file_operations {
             open: Some(writer_open::<T>),
             read: Some(bindings::seq_read),
             write: Some(write::<T>),
@@ -238,7 +235,7 @@ pub(crate) trait WriteFile<T> {

 impl<T: Reader + Sync> WriteFile<T> for T {
     const FILE_OPS: FileOps<T> = {
-        let operations = bindings::file_operations {
+        let operations = &bindings::file_operations {
             open: Some(write_only_open),
             write: Some(write_only_write::<T>),
             llseek: Some(bindings::noop_llseek),
@@ -290,7 +287,7 @@ pub(crate) trait BinaryReadFile<T> {

 impl<T: BinaryWriter + Sync> BinaryReadFile<T> for T {
     const FILE_OPS: FileOps<T> = {
-        let operations = bindings::file_operations {
+        let operations = &bindings::file_operations {
             read: Some(blob_read::<T>),
             llseek: Some(bindings::default_llseek),
             open: Some(bindings::simple_open),
@@ -344,7 +341,7 @@ pub(crate) trait BinaryWriteFile<T> {

 impl<T: BinaryReader + Sync> BinaryWriteFile<T> for T {
     const FILE_OPS: FileOps<T> = {
-        let operations = bindings::file_operations {
+        let operations = &bindings::file_operations {
             write: Some(blob_write::<T>),
             llseek: Some(bindings::default_llseek),
             open: Some(bindings::simple_open),
@@ -368,7 +365,7 @@ pub(crate) trait BinaryReadWriteFile<T> {

 impl<T: BinaryWriter + BinaryReader + Sync> BinaryReadWriteFile<T> for T {
     const FILE_OPS: FileOps<T> = {
-        let operations = bindings::file_operations {
+        let operations = &bindings::file_operations {
             read: Some(blob_read::<T>),
             write: Some(blob_write::<T>),
             llseek: Some(bindings::default_llseek),