rust: debugfs: Add support for writable files

Extends the `debugfs` API to support creating writable files. This
is done via the `Dir::write_only_file` and `Dir::read_write_file`
methods, which take a data object that implements the `Reader`
trait.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250904-debugfs-rust-v11-3-7d12a165685a@google.com
[ Fix up Result<()> -> Result. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/debugfs.rs

@@ -16,10 +16,10 @@ use core::marker::PhantomPinned;
 use core::ops::Deref;
 
 mod traits;
-pub use traits::Writer;
+pub use traits::{Reader, Writer};
 
 mod file_ops;
-use file_ops::{FileOps, ReadFile};
+use file_ops::{FileOps, ReadFile, ReadWriteFile, WriteFile};
 #[cfg(CONFIG_DEBUG_FS)]
 mod entry;
 #[cfg(CONFIG_DEBUG_FS)]
@@ -142,6 +142,39 @@ impl Dir {
         let file_ops = &<T as ReadFile<_>>::FILE_OPS;
         self.create_file(name, data, file_ops)
     }
+
+    /// Creates a read-write file in this directory.
+    ///
+    /// Reading the file uses the [`Writer`] implementation.
+    /// Writing to the file uses the [`Reader`] implementation.
+    pub fn read_write_file<'a, T, E: 'a>(
+        &'a self,
+        name: &'a CStr,
+        data: impl PinInit<T, E> + 'a,
+    ) -> impl PinInit<File<T>, E> + 'a
+    where
+        T: Writer + Reader + Send + Sync + 'static,
+    {
+        let file_ops = &<T as ReadWriteFile<_>>::FILE_OPS;
+        self.create_file(name, data, file_ops)
+    }
+
+    /// Creates a write-only file in this directory.
+    ///
+    /// The file owns its backing data. Writing to the file uses the [`Reader`]
+    /// implementation.
+    ///
+    /// The file is removed when the returned [`File`] is dropped.
+    pub fn write_only_file<'a, T, E: 'a>(
+        &'a self,
+        name: &'a CStr,
+        data: impl PinInit<T, E> + 'a,
+    ) -> impl PinInit<File<T>, E> + 'a
+    where
+        T: Reader + Send + Sync + 'static,
+    {
+        self.create_file(name, data, &T::FILE_OPS)
+    }
 }
 
 #[pin_data]

rust/kernel/debugfs/file_ops.rs

@@ -1,10 +1,11 @@
 // SPDX-License-Identifier: GPL-2.0
 // Copyright (C) 2025 Google LLC.
 
-use super::Writer;
+use super::{Reader, Writer};
 use crate::prelude::*;
 use crate::seq_file::SeqFile;
 use crate::seq_print;
+use crate::uaccess::UserSlice;
 use core::fmt::{Display, Formatter, Result};
 use core::marker::PhantomData;
 
@@ -126,3 +127,113 @@ impl<T: Writer + Sync> ReadFile<T> for T {
         unsafe { FileOps::new(operations, 0o400) }
     };
 }
+
+fn read<T: Reader + Sync>(data: &T, buf: *const c_char, count: usize) -> isize {
+    let mut reader = UserSlice::new(UserPtr::from_ptr(buf as *mut c_void), count).reader();
+
+    if let Err(e) = data.read_from_slice(&mut reader) {
+        return e.to_errno() as isize;
+    }
+
+    count as isize
+}
+
+/// # Safety
+///
+/// `file` must be a valid pointer to a `file` struct.
+/// The `private_data` of the file must contain a valid pointer to a `seq_file` whose
+/// `private` data in turn points to a `T` that implements `Reader`.
+/// `buf` must be a valid user-space buffer.
+pub(crate) unsafe extern "C" fn write<T: Reader + Sync>(
+    file: *mut bindings::file,
+    buf: *const c_char,
+    count: usize,
+    _ppos: *mut bindings::loff_t,
+) -> isize {
+    // SAFETY: The file was opened with `single_open`, which sets `private_data` to a `seq_file`.
+    let seq = unsafe { &mut *((*file).private_data.cast::<bindings::seq_file>()) };
+    // SAFETY: By caller precondition, this pointer is live and points to a value of type `T`.
+    let data = unsafe { &*(seq.private as *const T) };
+    read(data, buf, count)
+}
+
+// A trait to get the file operations for a type.
+pub(crate) trait ReadWriteFile<T> {
+    const FILE_OPS: FileOps<T>;
+}
+
+impl<T: Writer + Reader + Sync> ReadWriteFile<T> for T {
+    const FILE_OPS: FileOps<T> = {
+        let operations = bindings::file_operations {
+            open: Some(writer_open::<T>),
+            read: Some(bindings::seq_read),
+            write: Some(write::<T>),
+            llseek: Some(bindings::seq_lseek),
+            release: Some(bindings::single_release),
+            // SAFETY: `file_operations` supports zeroes in all fields.
+            ..unsafe { core::mem::zeroed() }
+        };
+        // SAFETY: `operations` is all stock `seq_file` implementations except for `writer_open`
+        // and `write`.
+        // `writer_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 matches the
+        // `FileOps` requirements.
+        // `write` only requires that the file's private data pointer points to `seq_file`
+        // which points to a `T` that will outlive it, which matches what `writer_open`
+        // provides.
+        unsafe { FileOps::new(operations, 0o600) }
+    };
+}
+
+/// # Safety
+///
+/// `inode` must be a valid pointer to an `inode` struct.
+/// `file` must be a valid pointer to a `file` struct.
+unsafe extern "C" fn write_only_open(
+    inode: *mut bindings::inode,
+    file: *mut bindings::file,
+) -> c_int {
+    // SAFETY: The caller ensures that `inode` and `file` are valid pointers.
+    unsafe { (*file).private_data = (*inode).i_private };
+    0
+}
+
+/// # Safety
+///
+/// * `file` must be a valid pointer to a `file` struct.
+/// * The `private_data` of the file must contain a valid pointer to a `T` that implements
+///   `Reader`.
+/// * `buf` must be a valid user-space buffer.
+pub(crate) unsafe extern "C" fn write_only_write<T: Reader + Sync>(
+    file: *mut bindings::file,
+    buf: *const c_char,
+    count: usize,
+    _ppos: *mut bindings::loff_t,
+) -> isize {
+    // SAFETY: The caller ensures that `file` is a valid pointer and that `private_data` holds a
+    // valid pointer to `T`.
+    let data = unsafe { &*((*file).private_data as *const T) };
+    read(data, buf, count)
+}
+
+pub(crate) trait WriteFile<T> {
+    const FILE_OPS: FileOps<T>;
+}
+
+impl<T: Reader + Sync> WriteFile<T> for T {
+    const FILE_OPS: FileOps<T> = {
+        let operations = bindings::file_operations {
+            open: Some(write_only_open),
+            write: Some(write_only_write::<T>),
+            llseek: Some(bindings::noop_llseek),
+            // SAFETY: `file_operations` supports zeroes in all fields.
+            ..unsafe { core::mem::zeroed() }
+        };
+        // SAFETY:
+        // * `write_only_open` populates the file private data with the inode private data
+        // * `write_only_write`'s only requirement is that the private data of the file point to
+        //   a `T` and be legal to convert to a shared reference, which `write_only_open`
+        //   satisfies.
+        unsafe { FileOps::new(operations, 0o200) }
+    };
+}

rust/kernel/debugfs/traits.rs

@@ -3,8 +3,15 @@
 
 //! Traits for rendering or updating values exported to DebugFS.
 
+use crate::prelude::*;
 use crate::sync::Mutex;
+use crate::uaccess::UserSliceReader;
 use core::fmt::{self, Debug, Formatter};
+use core::str::FromStr;
+use core::sync::atomic::{
+    AtomicI16, AtomicI32, AtomicI64, AtomicI8, AtomicIsize, AtomicU16, AtomicU32, AtomicU64,
+    AtomicU8, AtomicUsize, Ordering,
+};
 
 /// A trait for types that can be written into a string.
 ///
@@ -31,3 +38,65 @@ impl<T: Debug> Writer for T {
         writeln!(f, "{self:?}")
     }
 }
+
+/// 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.
+///
+/// It is automatically implemented for all atomic integers, or any type that implements `FromStr`
+/// wrapped in a `Mutex`.
+pub trait Reader {
+    /// Updates the value from the given user slice.
+    fn read_from_slice(&self, reader: &mut UserSliceReader) -> Result;
+}
+
+impl<T: FromStr> Reader for Mutex<T> {
+    fn read_from_slice(&self, reader: &mut UserSliceReader) -> Result {
+        let mut buf = [0u8; 128];
+        if reader.len() > buf.len() {
+            return Err(EINVAL);
+        }
+        let n = reader.len();
+        reader.read_slice(&mut buf[..n])?;
+
+        let s = core::str::from_utf8(&buf[..n]).map_err(|_| EINVAL)?;
+        let val = s.trim().parse::<T>().map_err(|_| EINVAL)?;
+        *self.lock() = val;
+        Ok(())
+    }
+}
+
+macro_rules! impl_reader_for_atomic {
+    ($(($atomic_type:ty, $int_type:ty)),*) => {
+        $(
+            impl Reader for $atomic_type {
+                fn read_from_slice(&self, reader: &mut UserSliceReader) -> Result {
+                    let mut buf = [0u8; 21]; // Enough for a 64-bit number.
+                    if reader.len() > buf.len() {
+                        return Err(EINVAL);
+                    }
+                    let n = reader.len();
+                    reader.read_slice(&mut buf[..n])?;
+
+                    let s = core::str::from_utf8(&buf[..n]).map_err(|_| EINVAL)?;
+                    let val = s.trim().parse::<$int_type>().map_err(|_| EINVAL)?;
+                    self.store(val, Ordering::Relaxed);
+                    Ok(())
+                }
+            }
+        )*
+    };
+}
+
+impl_reader_for_atomic!(
+    (AtomicI16, i16),
+    (AtomicI32, i32),
+    (AtomicI64, i64),
+    (AtomicI8, i8),
+    (AtomicIsize, isize),
+    (AtomicU16, u16),
+    (AtomicU32, u32),
+    (AtomicU64, u64),
+    (AtomicU8, u8),
+    (AtomicUsize, usize)
+);