[PATCH 3/7] rust: debugfs: support for binary large objects

[Danilo Krummrich <dakr@kernel.org>]

Introduce support for read-only, write-only, and read-write binary files
in Rust debugfs. This adds:

- BinaryWriter and BinaryReader traits for writing to and reading from
  user slices in binary form.
- New Dir methods: read_binary_file(), write_binary_file(),
  `read_write_binary_file`.
- Corresponding FileOps implementations: BinaryReadFile,
  BinaryWriteFile, BinaryReadWriteFile.

This allows kernel modules to expose arbitrary binary data through
debugfs, with proper support for offsets and partial reads/writes.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/debugfs.rs          |  67 ++++++++++++++-
 rust/kernel/debugfs/file_ops.rs | 144 +++++++++++++++++++++++++++++++-
 rust/kernel/debugfs/traits.rs   |  45 +++++++++-
 3 files changed, 249 insertions(+), 7 deletions(-)

diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
index 381c23b3dd83..b1a3adca7fd4 100644
--- a/rust/kernel/debugfs.rs
+++ b/rust/kernel/debugfs.rs
@@ -21,12 +21,15 @@
 use core::ops::Deref;
 
 mod traits;
-pub use traits::{Reader, Writer};
+pub use traits::{BinaryReader, BinaryWriter, Reader, Writer};
 
 mod callback_adapters;
 use callback_adapters::{FormatAdapter, NoWriter, WritableAdapter};
 mod file_ops;
-use file_ops::{FileOps, ReadFile, ReadWriteFile, WriteFile};
+use file_ops::{
+    BinaryReadFile, BinaryReadWriteFile, BinaryWriteFile, FileOps, ReadFile, ReadWriteFile,
+    WriteFile,
+};
 #[cfg(CONFIG_DEBUG_FS)]
 mod entry;
 #[cfg(CONFIG_DEBUG_FS)]
@@ -150,6 +153,33 @@ pub fn read_only_file<'a, T, E: 'a>(
         self.create_file(name, data, file_ops)
     }
 
+    /// Creates a read-only binary file in this directory.
+    ///
+    /// The file's contents are produced by invoking [`BinaryWriter::write_to_slice`] on the value
+    /// initialized by `data`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::c_str;
+    /// # use kernel::debugfs::Dir;
+    /// # use kernel::prelude::*;
+    /// # let dir = Dir::new(c_str!("my_debugfs_dir"));
+    /// let file = KBox::pin_init(dir.read_binary_file(c_str!("foo"), [0x1, 0x2]), GFP_KERNEL)?;
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn read_binary_file<'a, T, E: 'a>(
+        &'a self,
+        name: &'a CStr,
+        data: impl PinInit<T, E> + 'a,
+    ) -> impl PinInit<File<T>, E> + 'a
+    where
+        T: BinaryWriter + Send + Sync + 'static,
+    {
+        let file_ops = &<T as BinaryReadFile<_>>::FILE_OPS;
+        self.create_file(name, data, file_ops)
+    }
+
     /// Creates a read-only file in this directory, with contents from a callback.
     ///
     /// `f` must be a function item or a non-capturing closure.
@@ -206,6 +236,22 @@ pub fn read_write_file<'a, T, E: 'a>(
         self.create_file(name, data, file_ops)
     }
 
+    /// Creates a read-write binary file in this directory.
+    ///
+    /// Reading the file uses the [`BinaryWriter`] implementation.
+    /// Writing to the file uses the [`BinaryReader`] implementation.
+    pub fn read_write_binary_file<'a, T, E: 'a>(
+        &'a self,
+        name: &'a CStr,
+        data: impl PinInit<T, E> + 'a,
+    ) -> impl PinInit<File<T>, E> + 'a
+    where
+        T: BinaryWriter + BinaryReader + Send + Sync + 'static,
+    {
+        let file_ops = &<T as BinaryReadWriteFile<_>>::FILE_OPS;
+        self.create_file(name, data, file_ops)
+    }
+
     /// Creates a read-write file in this directory, with logic from callbacks.
     ///
     /// Reading from the file is handled by `f`. Writing to the file is handled by `w`.
@@ -248,6 +294,23 @@ pub fn write_only_file<'a, T, E: 'a>(
         self.create_file(name, data, &T::FILE_OPS)
     }
 
+    /// Creates a write-only binary file in this directory.
+    ///
+    /// The file owns its backing data. Writing to the file uses the [`BinaryReader`]
+    /// implementation.
+    ///
+    /// The file is removed when the returned [`File`] is dropped.
+    pub fn write_binary_file<'a, T, E: 'a>(
+        &'a self,
+        name: &'a CStr,
+        data: impl PinInit<T, E> + 'a,
+    ) -> impl PinInit<File<T>, E> + 'a
+    where
+        T: BinaryReader + Send + Sync + 'static,
+    {
+        self.create_file(name, data, &T::FILE_OPS)
+    }
+
     /// Creates a write-only file in this directory, with write logic from a callback.
     ///
     /// `w` must be a function item or a non-capturing closure.
diff --git a/rust/kernel/debugfs/file_ops.rs b/rust/kernel/debugfs/file_ops.rs
index 50fead17b6f3..ef31cac4cd44 100644
--- a/rust/kernel/debugfs/file_ops.rs
+++ b/rust/kernel/debugfs/file_ops.rs
@@ -1,13 +1,13 @@
 // SPDX-License-Identifier: GPL-2.0
 // Copyright (C) 2025 Google LLC.
 
-use super::{Reader, Writer};
+use super::{BinaryReader, BinaryWriter, Reader, Writer};
 use crate::debugfs::callback_adapters::Adapter;
 use crate::prelude::*;
 use crate::seq_file::SeqFile;
 use crate::seq_print;
 use crate::uaccess::UserSlice;
-use core::fmt::{Display, Formatter, Result};
+use core::fmt;
 use core::marker::PhantomData;
 
 #[cfg(CONFIG_DEBUG_FS)]
@@ -65,8 +65,8 @@ fn deref(&self) -> &Self::Target {
 
 struct WriterAdapter<T>(T);
 
-impl<'a, T: Writer> Display for WriterAdapter<&'a T> {
-    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+impl<'a, T: Writer> fmt::Display for WriterAdapter<&'a T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.0.write(f)
     }
 }
@@ -245,3 +245,139 @@ impl<T: Reader + Sync> WriteFile<T> for T {
         unsafe { FileOps::new(operations, 0o200) }
     };
 }
+
+extern "C" fn blob_read<T: BinaryWriter>(
+    file: *mut bindings::file,
+    buf: *mut c_char,
+    count: usize,
+    ppos: *mut bindings::loff_t,
+) -> isize {
+    // SAFETY:
+    // - `file` is a valid pointer to a `struct file`.
+    // - The type invariant of `FileOps` guarantees that `private_data` points to a valid `T`.
+    let this = unsafe { &*((*file).private_data.cast::<T>()) };
+
+    // SAFETY: `ppos` is a valid `loff_t` pointer.
+    let pos = unsafe { &mut *ppos };
+
+    let mut writer = UserSlice::new(UserPtr::from_ptr(buf.cast()), count).writer();
+
+    let ret = || -> Result<isize> {
+        let offset = (*pos).try_into()?;
+
+        let written = this.write_to_slice(&mut writer, offset)?;
+        *pos += bindings::loff_t::try_from(written)?;
+
+        Ok(written.try_into()?)
+    }();
+
+    match ret {
+        Ok(n) => n,
+        Err(e) => e.to_errno() as isize,
+    }
+}
+
+pub(crate) trait BinaryReadFile<T> {
+    const FILE_OPS: FileOps<T>;
+}
+
+impl<T: BinaryWriter + Sync> BinaryReadFile<T> for T {
+    const FILE_OPS: FileOps<T> = {
+        let operations = bindings::file_operations {
+            read: Some(blob_read::<T>),
+            llseek: Some(bindings::default_llseek),
+            open: Some(bindings::simple_open),
+            // SAFETY: `file_operations` supports zeroes in all fields.
+            ..unsafe { core::mem::zeroed() }
+        };
+
+        // SAFETY:
+        // - The private data of `struct inode` does always contain a pointer to a valid `T`.
+        // - `simple_open()` stores the `struct inode`'s private data in the private data of the
+        //   corresponding `struct file`.
+        // - `blob_read()` re-creates a reference to `T` from the `struct file`'s private data.
+        // - `default_llseek()` does not access the `struct file`'s private data.
+        unsafe { FileOps::new(operations, 0o400) }
+    };
+}
+
+extern "C" fn blob_write<T: BinaryReader>(
+    file: *mut bindings::file,
+    buf: *const c_char,
+    count: usize,
+    ppos: *mut bindings::loff_t,
+) -> isize {
+    // SAFETY:
+    // - `file` is a valid pointer to a `struct file`.
+    // - The type invariant of `FileOps` guarantees that `private_data` points to a valid `T`.
+    let this = unsafe { &*((*file).private_data.cast::<T>()) };
+
+    // SAFETY: `ppos` is a valid `loff_t` pointer.
+    let pos = unsafe { &mut *ppos };
+
+    let mut reader = UserSlice::new(UserPtr::from_ptr(buf.cast_mut().cast()), count).reader();
+
+    let ret = || -> Result<isize> {
+        let offset = (*pos).try_into()?;
+
+        let read = this.read_from_slice(&mut reader, offset)?;
+        *pos += bindings::loff_t::try_from(read)?;
+
+        Ok(read.try_into()?)
+    }();
+
+    match ret {
+        Ok(n) => n,
+        Err(e) => e.to_errno() as isize,
+    }
+}
+
+pub(crate) trait BinaryWriteFile<T> {
+    const FILE_OPS: FileOps<T>;
+}
+
+impl<T: BinaryReader + Sync> BinaryWriteFile<T> for T {
+    const FILE_OPS: FileOps<T> = {
+        let operations = bindings::file_operations {
+            write: Some(blob_write::<T>),
+            llseek: Some(bindings::default_llseek),
+            open: Some(bindings::simple_open),
+            // SAFETY: `file_operations` supports zeroes in all fields.
+            ..unsafe { core::mem::zeroed() }
+        };
+
+        // SAFETY:
+        // - The private data of `struct inode` does always contain a pointer to a valid `T`.
+        // - `simple_open()` stores the `struct inode`'s private data in the private data of the
+        //   corresponding `struct file`.
+        // - `blob_write()` re-creates a reference to `T` from the `struct file`'s private data.
+        // - `default_llseek()` does not access the `struct file`'s private data.
+        unsafe { FileOps::new(operations, 0o200) }
+    };
+}
+
+pub(crate) trait BinaryReadWriteFile<T> {
+    const FILE_OPS: FileOps<T>;
+}
+
+impl<T: BinaryWriter + BinaryReader + Sync> BinaryReadWriteFile<T> for T {
+    const FILE_OPS: FileOps<T> = {
+        let operations = bindings::file_operations {
+            read: Some(blob_read::<T>),
+            write: Some(blob_write::<T>),
+            llseek: Some(bindings::default_llseek),
+            open: Some(bindings::simple_open),
+            // SAFETY: `file_operations` supports zeroes in all fields.
+            ..unsafe { core::mem::zeroed() }
+        };
+
+        // SAFETY:
+        // - The private data of `struct inode` does always contain a pointer to a valid `T`.
+        // - `simple_open()` stores the `struct inode`'s private data in the private data of the
+        //   corresponding `struct file`.
+        // - `blob_read()` re-creates a reference to `T` from the `struct file`'s private data.
+        // - `blob_write()` re-creates a reference to `T` from the `struct file`'s private data.
+        // - `default_llseek()` does not access the `struct file`'s private data.
+        unsafe { FileOps::new(operations, 0o600) }
+    };
+}
diff --git a/rust/kernel/debugfs/traits.rs b/rust/kernel/debugfs/traits.rs
index ab009eb254b3..60a6ee6c6b58 100644
--- a/rust/kernel/debugfs/traits.rs
+++ b/rust/kernel/debugfs/traits.rs
@@ -5,7 +5,8 @@
 
 use crate::prelude::*;
 use crate::sync::Mutex;
-use crate::uaccess::UserSliceReader;
+use crate::transmute::{AsBytes, FromBytes};
+use crate::uaccess::{UserSliceReader, UserSliceWriter};
 use core::fmt::{self, Debug, Formatter};
 use core::str::FromStr;
 use core::sync::atomic::{
@@ -39,6 +40,30 @@ fn write(&self, f: &mut Formatter<'_>) -> fmt::Result {
     }
 }
 
+/// Trait for types that can be written out as binary.
+pub trait BinaryWriter {
+    /// Writes the binary form of `self` into `writer`.
+    ///
+    /// `offset` is the requested offset into the binary representation of `self`.
+    ///
+    /// On success, returns the number of bytes written in to `writer`.
+    fn write_to_slice(&self, writer: &mut UserSliceWriter, offset: usize) -> Result<usize>;
+}
+
+impl<T: AsBytes> BinaryWriter for T {
+    fn write_to_slice(&self, writer: &mut UserSliceWriter, offset: usize) -> Result<usize> {
+        writer.write_slice_partial(self.as_bytes(), offset)
+    }
+}
+
+impl<T: BinaryWriter> BinaryWriter for Mutex<T> {
+    fn write_to_slice(&self, writer: &mut UserSliceWriter, offset: usize) -> Result<usize> {
+        let guard = self.lock();
+
+        guard.write_to_slice(writer, offset)
+    }
+}
+
 /// A trait for types that can be updated from a user slice.
 ///
 /// This works similarly to `FromStr`, but operates on a `UserSliceReader` rather than a &str.
@@ -66,6 +91,24 @@ fn read_from_slice(&self, reader: &mut UserSliceReader) -> Result {
     }
 }
 
+/// Trait for types that can be constructed from a binary representation.
+pub trait BinaryReader {
+    /// Reads the binary form of `self` from `reader`.
+    ///
+    /// `offset` is the requested offset into the binary representation of `self`.
+    ///
+    /// On success, returns the number of bytes read from `reader`.
+    fn read_from_slice(&self, reader: &mut UserSliceReader, offset: usize) -> Result<usize>;
+}
+
+impl<T: AsBytes + FromBytes> BinaryReader for Mutex<T> {
+    fn read_from_slice(&self, reader: &mut UserSliceReader, offset: usize) -> Result<usize> {
+        let mut this = self.lock();
+
+        reader.read_slice_partial(this.as_bytes_mut(), offset)
+    }
+}
+
 macro_rules! impl_reader_for_atomic {
     ($(($atomic_type:ty, $int_type:ty)),*) => {
         $(
-- 
2.51.0