Allows creation of files for references that live forever and lack
metadata through the `Display` implementation.

The reference must live forever because we do not have a maximum
lifetime for the file we are creating.

The `Display` implementation is used because `seq_printf` needs to route
through `%pA`, which in turn routes through Arguments. A more generic
API is provided later in the series, implemented in terms of this one.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
---
 rust/kernel/debugfs.rs | 139 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 139 insertions(+)

diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
index 41ac1711e9c0e66de1a434217c363176f806f434..21b116abad864d303f11cc515fe6f86ce5d51cbf 100644
--- a/rust/kernel/debugfs.rs
+++ b/rust/kernel/debugfs.rs
@@ -7,6 +7,7 @@
 //! C header: [`include/linux/debugfs.h`](srctree/include/linux/debugfs.h)
 
 use crate::str::CStr;
+use core::fmt::Display;
 use core::marker::PhantomData;
 
 /// Owning handle to a DebugFS directory.
@@ -108,6 +109,57 @@ fn as_ptr(&self) -> *mut bindings::dentry {
     pub fn subdir<'b>(&'b self, name: &CStr) -> Dir<'b, true> {
         Dir::create(name, Some(self))
     }
+
+    /// Create a file in a DebugFS directory with the provided name, and contents from invoking
+    /// [`Display::fmt`] on the provided reference.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::c_str;
+    /// # use kernel::debugfs::Dir;
+    /// let dir = Dir::new(c_str!("my_debugfs_dir"));
+    /// dir.display_file(c_str!("foo"), &200);
+    /// // "my_debugfs_dir/foo" now contains the number 200.
+    /// ```
+    pub fn display_file<'b, T: Display + Sized>(
+        &'a self,
+        name: &CStr,
+        data: &'static T,
+    ) -> File<'b> {
+        // 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.
+        #[cfg(CONFIG_DEBUG_FS)]
+        let ptr = unsafe {
+            bindings::debugfs_create_file_full(
+                name.as_char_ptr(),
+                0o444,
+                self.as_ptr(),
+                data as *const _ as *mut _,
+                core::ptr::null(),
+                &<T as DisplayFile>::VTABLE,
+            )
+        };
+
+        #[cfg(not(CONFIG_DEBUG_FS))]
+        let ptr = {
+            // Mark parameters used
+            let (_, _) = (name, data);
+            ERR_PTR(ENODEV)
+        };
+
+        // SAFETY: `debugfs_create_file_full` either returns an error code or a legal
+        // dentry pointer, and without `CONFIG_DEBUGFS` we return an error pointer, so
+        // `Dir::from_ptr` is safe to call here.
+        let dir = unsafe { Dir::from_ptr(ptr) };
+
+        File(dir)
+    }
 }
 
 impl<'a> Dir<'a, false> {
@@ -157,3 +209,90 @@ fn drop(&mut self) {
         }
     }
 }
+/// Handle to a DebugFS file.
+#[repr(transparent)]
+pub struct File<'a>(Dir<'a, true>);
+
+impl<'a> File<'a> {
+    /// Remove the file from DebugFS.
+    ///
+    /// # Examples
+    /// ```
+    /// # use kernel::c_str;
+    /// # use kernel::debugfs::Dir;
+    /// let dir = Dir::new(c_str!("foo"));
+    /// let file = dir.display_file(c_str!("bar"), &0);
+    /// // "foo/bar" is created.
+    /// file.remove()
+    /// // "foo/bar" is removed"
+    pub fn remove(self) {
+        drop(self.0.owning())
+    }
+}
+
+#[cfg(CONFIG_DEBUG_FS)]
+mod helpers {
+    use crate::seq_file::SeqFile;
+    use crate::seq_print;
+    use core::fmt::Display;
+    use core::ptr::addr_of;
+
+    /// Implements `open` for `file_operations` via `single_open` to fill out a `seq_file`.
+    ///
+    /// # Safety
+    ///
+    /// * `inode`'s private pointer must point to a value of type `T` which will outlive the `inode`
+    ///   and will not be mutated during this call.
+    /// * `file` must point to a live, not-yet-initialized file object.
+    pub(crate) unsafe extern "C" fn display_open<T: Display>(
+        inode: *mut bindings::inode,
+        file: *mut bindings::file,
+    ) -> i32 {
+        // SAFETY:
+        // * `file` is acceptable by caller precondition.
+        // * `print_act` will be called on a `seq_file` with private data set to the third argument,
+        //   so we meet its safety requirements.
+        // * The `data` pointer passed in the third argument is a valid `T` pointer that outlives
+        //   this call by caller preconditions.
+        unsafe { bindings::single_open(file, Some(display_act::<T>), (*inode).i_private) }
+    }
+
+    /// Prints private data stashed in a seq_file to that seq file.
+    ///
+    /// # Safety
+    ///
+    /// `seq` must point to a live `seq_file` whose private data is a live pointer to a `T` which is
+    /// not being mutated.
+    pub(crate) unsafe extern "C" fn display_act<T: Display>(
+        seq: *mut bindings::seq_file,
+        _: *mut core::ffi::c_void,
+    ) -> i32 {
+        // SAFETY: By caller precondition, seq points to a live seq_file.
+        let private_addr = unsafe { addr_of!((*seq).private) };
+        // SAFETY: By caller precondition, this pointer is live, points to a value of type `T`, and
+        // is not being mutated.
+        let data = unsafe { &*(*private_addr as *mut T) };
+        // SAFETY: By caller precondition, `seq_file` points to a live `seq_file`, so we can lift
+        // it.
+        let seq_file = unsafe { SeqFile::from_raw(seq) };
+        seq_print!(seq_file, "{}", data);
+        0
+    }
+
+    // Work around lack of generic const items.
+    pub(crate) trait DisplayFile: Display + Sized {
+        const VTABLE: bindings::file_operations = bindings::file_operations {
+            read: Some(bindings::seq_read),
+            llseek: Some(bindings::seq_lseek),
+            release: Some(bindings::single_release),
+            open: Some(display_open::<Self> as _),
+            // SAFETY: `file_operations` supports zeroes in all fields.
+            ..unsafe { core::mem::zeroed() }
+        };
+    }
+
+    impl<T: Display + Sized> DisplayFile for T {}
+}
+
+#[cfg(CONFIG_DEBUG_FS)]
+use helpers::*;

-- 
2.49.0.906.g1f30a19c02-goog

On Fri, May 02, 2025 at 07:49:31PM +0000, Matthew Maurer wrote:
> +/// Handle to a DebugFS file.
> +#[repr(transparent)]
> +pub struct File<'a>(Dir<'a, true>);

As mentioned in [1], please create a base type Entry. While it's an improvement
to not expose things like subdir() from File directly, it's still odd to base
File on a type that has invalid methods its purpose.

[1] https://lore.kernel.org/lkml/aBRrniLfCzWX7nbR@pollux/

> +impl<'a> File<'a> {
> +    /// Remove the file from DebugFS.
> +    ///
> +    /// # Examples
> +    /// ```
> +    /// # use kernel::c_str;
> +    /// # use kernel::debugfs::Dir;
> +    /// let dir = Dir::new(c_str!("foo"));
> +    /// let file = dir.display_file(c_str!("bar"), &0);
> +    /// // "foo/bar" is created.
> +    /// file.remove()
> +    /// // "foo/bar" is removed"
> +    pub fn remove(self) {
> +        drop(self.0.owning())
> +    }

Same as with Dir, please make it the other way around.