1 files changed, 247 insertions, 0 deletions
diff --git a/rust/kernel/debugfs/file_ops.rs b/rust/kernel/debugfs/file_ops.rs
new file mode 100644
index 000000000000..50fead17b6f3
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+++ b/rust/kernel/debugfs/file_ops.rs
@@ -0,0 +1,247 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (C) 2025 Google LLC.
+
+use super::{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::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
+/// off an inode which has a pointer to a `T` in its private data that is safe to convert
+/// into a reference.
+pub(super) struct FileOps<T> {
+    #[cfg(CONFIG_DEBUG_FS)]
+    operations: bindings::file_operations,
+    #[cfg(CONFIG_DEBUG_FS)]
+    mode: u16,
+    _phantom: PhantomData<T>,
+}
+
+impl<T> FileOps<T> {
+    /// # Safety
+    ///
+    /// 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 {
+        Self {
+            #[cfg(CONFIG_DEBUG_FS)]
+            operations,
+            #[cfg(CONFIG_DEBUG_FS)]
+            mode,
+            _phantom: PhantomData,
+        }
+    }
+
+    #[cfg(CONFIG_DEBUG_FS)]
+    pub(crate) const fn mode(&self) -> u16 {
+        self.mode
+    }
+}
+
+impl<T: Adapter> FileOps<T> {
+    pub(super) const fn adapt(&self) -> &FileOps<T::Inner> {
+        // SAFETY: `Adapter` asserts that `T` can be legally cast to `T::Inner`.
+        unsafe { core::mem::transmute(self) }
+    }
+}
+
+#[cfg(CONFIG_DEBUG_FS)]
+impl<T> Deref for FileOps<T> {
+    type Target = bindings::file_operations;
+
+    fn deref(&self) -> &Self::Target {
+        &self.operations
+    }
+}
+
+struct WriterAdapter<T>(T);
+
+impl<'a, T: Writer> Display for WriterAdapter<&'a T> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+        self.0.write(f)
+    }
+}
+
+/// 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 have any unique references alias it during the call.
+/// * `file` must point to a live, not-yet-initialized file object.
+unsafe extern "C" fn writer_open<T: Writer + Sync>(
+    inode: *mut bindings::inode,
+    file: *mut bindings::file,
+) -> c_int {
+    // SAFETY: The caller ensures that `inode` is a valid pointer.
+    let data = unsafe { (*inode).i_private };
+    // 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(writer_act::<T>), data) }
+}
+
+/// 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 valid pointer to a `T` which may
+/// not have any unique references alias it during the call.
+unsafe extern "C" fn writer_act<T: Writer + Sync>(
+    seq: *mut bindings::seq_file,
+    _: *mut c_void,
+) -> c_int {
+    // SAFETY: By caller precondition, this pointer is valid pointer to a `T`, and
+    // there are not and will not be any unique references until we are done.
+    let data = unsafe { &*((*seq).private.cast::<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, "{}", WriterAdapter(data));
+    0
+}
+
+// Work around lack of generic const items.
+pub(crate) trait ReadFile<T> {
+    const FILE_OPS: FileOps<T>;
+}
+
+impl<T: Writer + Sync> ReadFile<T> for T {
+    const FILE_OPS: FileOps<T> = {
+        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>),
+            // SAFETY: `file_operations` supports zeroes in all fields.
+            ..unsafe { core::mem::zeroed() }
+        };
+        // SAFETY: `operations` is all stock `seq_file` implementations except for `writer_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 matches the
+        // `FileOps` requirements.
+        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) }
+    };
+}