[PATCH 3/3] rust: miscdevice: add abstraction for defining miscdevices

Alice Ryhl
2024-09-26T14:59:43Z

Provide a `MiscDevice` trait that lets you specify the file operations
that you wish to provide for your misc device. Most operations are
optional.

In the future, the file operations in the `MiscDevice` can be moved to a
general `FileOperations` trait so that it is useful for other file
systems too.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
rust/bindings/bindings_helper.h | 1 +
rust/kernel/lib.rs | 1 +
rust/kernel/miscdevice.rs | 401 ++++++++++++++++++++++++++++++++++++++++
3 files changed, 403 insertions(+)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index ca13659ded4c..ec2e28ef6568 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -18,6 +18,7 @@
#include <linux/fs.h>
#include <linux/jiffies.h>
#include <linux/mdio.h>
+#include <linux/miscdevice.h>
#include <linux/phy.h>
#include <linux/pid_namespace.h>
#include <linux/poll.h>
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 819126fc2d89..bf97817842c9 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -41,6 +41,7 @@
#[cfg(CONFIG_KUNIT)]
pub mod kunit;
pub mod list;
+pub mod miscdevice;
pub mod mm;
#[cfg(CONFIG_NET)]
pub mod net;
diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
new file mode 100644
index 000000000000..6c4e33d77c58
--- /dev/null
+++ b/rust/kernel/miscdevice.rs
@@ -0,0 +1,401 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Miscdevice support.
+//!
+//! C headers: [`include/linux/miscdevice.h`](srctree/include/linux/miscdevice.h).
+//!
+//! Reference: <https://www.kernel.org/doc/html/latest/driver-api/misc_devices.html>
+
+use crate::{
+ bindings,
+ error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR},
+ fs::{File, LocalFile},
+ mm::virt::VmArea,
+ prelude::*,
+ str::CStr,
+ types::{ForeignOwnable, Opaque},
+};
+use core::{
+ ffi::{c_int, c_long, c_uint, c_ulong},
+ marker::PhantomData,
+ mem::MaybeUninit,
+ pin::Pin,
+ ptr::NonNull,
+};
+
+/// The kernel `loff_t` type.
+#[allow(non_camel_case_types)]
+pub type loff_t = bindings::loff_t;
+
+/// Options for creating a misc device.
+#[derive(Copy, Clone)]
+pub struct MiscDeviceOptions {
+ /// The name of the miscdevice.
+ pub name: &'static CStr,
+}
+
+impl MiscDeviceOptions {
+ /// Create a raw `struct miscdev` ready for registration.
+ pub const fn into_raw<T: MiscDevice>(self) -> bindings::miscdevice {
+ // SAFETY: All zeros is valid for this C type.
+ let mut result: bindings::miscdevice = unsafe { MaybeUninit::zeroed().assume_init() };
+ result.minor = bindings::MISC_DYNAMIC_MINOR as _;
+ result.name = self.name.as_char_ptr();
+ result.fops = create_vtable::<T>();
+ result
+ }
+}
+
+/// A registration of a miscdevice.
+///
+/// # Invariants
+///
+/// `inner` is a registered misc device.
+#[repr(transparent)]
+#[pin_data(PinnedDrop)]
+pub struct MiscDeviceRegistration<T> {
+ #[pin]
+ inner: Opaque<bindings::miscdevice>,
+ _t: PhantomData<T>,
+}
+
+unsafe impl<T> Send for MiscDeviceRegistration<T> {}
+unsafe impl<T> Sync for MiscDeviceRegistration<T> {}
+
+impl<T: MiscDevice> MiscDeviceRegistration<T> {
+ /// Register a misc device.
+ pub fn register(opts: MiscDeviceOptions) -> impl PinInit<Self, Error> {
+ try_pin_init!(Self {
+ inner <- Opaque::try_ffi_init(move |slot: *mut bindings::miscdevice| {
+ // SAFETY: The initializer can write to the provided `slot`.
+ unsafe { slot.write(opts.into_raw::<T>()) };
+
+ // SAFETY: We just wrote the misc device options to the slot. The miscdevice will
+ // get unregistered before `slot` is deallocated because the memory is pinned and
+ // the destructor of this type deallocates the memory.
+ // INVARIANT: If this returns `Ok(())`, then the `slot` will contain a registered
+ // misc device.
+ to_result(unsafe { bindings::misc_register(slot) })
+ }),
+ _t: PhantomData,
+ })
+ }
+
+ /// Returns the private data associated with the provided file.
+ ///
+ /// Returns `None` if the file is not associated with this misc device.
+ pub fn try_get_private_data<'a>(
+ &self,
+ file: &'a LocalFile,
+ ) -> Option<<T::Ptr as ForeignOwnable>::Borrowed<'a>> {
+ // SAFETY: `fops` of a miscdevice is immutable after initialization.
+ let fops_this = unsafe { (*self.as_raw()).fops };
+ // SAFETY: `f_op` of a file is immutable after initialization.
+ let fops_file = unsafe { (*file.as_ptr()).f_op };
+
+ if core::ptr::eq(fops_this, fops_file) {
+ // SAFETY: We know that `file` is associated with a `MiscDeviceRegistration<T>`, so
+ // `private_data` is immutable.
+ let private_data = unsafe { (*file.as_ptr()).private_data };
+ // SAFETY:
+ // * The fops match, so the file's private date has the right type.
+ // * The returned borrow cannot outlive the file.
+ Some(unsafe { <T::Ptr as ForeignOwnable>::borrow(private_data) })
+ } else {
+ None
+ }
+ }
+
+ /// Returns a raw pointer to the misc device.
+ pub fn as_raw(&self) -> *mut bindings::miscdevice {
+ self.inner.get()
+ }
+}
+
+#[pinned_drop]
+impl<T> PinnedDrop for MiscDeviceRegistration<T> {
+ fn drop(self: Pin<&mut Self>) {
+ // SAFETY: We know that the device is registered by the type invariants.
+ unsafe { bindings::misc_deregister(self.inner.get()) };
+ }
+}
+
+/// Trait implemented by the private data of an open misc device.
+#[vtable]
+pub trait MiscDevice {
+ /// What kind of pointer should `Self` be wrapped in.
+ type Ptr: ForeignOwnable + Send + Sync;
+
+ /// Called when the misc device is opened.
+ ///
+ /// The returned pointer will be stored as the private data for the file.
+ fn open(_file: &File) -> Result<Self::Ptr>;
+
+ /// Called when the misc device is released.
+ fn release(device: Self::Ptr, _file: &File) {
+ drop(device);
+ }
+
+ /// Handle for mmap.
+ fn mmap(
+ _device: <Self::Ptr as ForeignOwnable>::Borrowed<'_>,
+ _file: &File,
+ _vma: Pin<&mut VmArea>,
+ ) -> Result<()> {
+ kernel::build_error(VTABLE_DEFAULT_ERROR)
+ }
+
+ /// Seeks this miscdevice.
+ fn llseek(
+ _device: <Self::Ptr as ForeignOwnable>::Borrowed<'_>,
+ _file: &LocalFile,
+ _offset: loff_t,
+ _whence: c_int,
+ ) -> Result<loff_t> {
+ kernel::build_error(VTABLE_DEFAULT_ERROR)
+ }
+
+ /// Read from this miscdevice.
+ fn read_iter(_kiocb: Kiocb<'_, Self::Ptr>, _iov: &mut IovIter) -> Result<usize> {
+ kernel::build_error(VTABLE_DEFAULT_ERROR)
+ }
+
+ /// Handler for ioctls
+ ///
+ /// The `cmd` argument is usually manipulated using the utilties in [`kernel::ioctl`].
+ ///
+ /// [`kernel::ioctl`]: mod@crate::ioctl
+ fn ioctl(
+ _device: <Self::Ptr as ForeignOwnable>::Borrowed<'_>,
+ _file: &File,
+ _cmd: u32,
+ _arg: usize,
+ ) -> Result<c_long> {
+ kernel::build_error(VTABLE_DEFAULT_ERROR)
+ }
+
+ /// Handler for ioctls
+ ///
+ /// Used for 32-bit userspace on 64-bit platforms.
+ #[cfg(CONFIG_COMPAT)]
+ fn compat_ioctl(
+ device: <Self::Ptr as ForeignOwnable>::Borrowed<'_>,
+ file: &File,
+ cmd: u32,
+ arg: usize,
+ ) -> Result<c_long> {
+ Self::ioctl(device, file, cmd, arg)
+ }
+}
+
+/// Wrapper for the kernel's `struct kiocb`.
+///
+/// The type `T` represents the private data of the file.
+pub struct Kiocb<'a, T> {
+ inner: NonNull<bindings::kiocb>,
+ _phantom: PhantomData<&'a T>,
+}
+
+impl<'a, T: ForeignOwnable> Kiocb<'a, T> {
+ /// Get the private data in this kiocb.
+ pub fn private_data(&self) -> <T as ForeignOwnable>::Borrowed<'a> {
+ // SAFETY: The `kiocb` lets us access the private data.
+ let private = unsafe { (*(*self.inner.as_ptr()).ki_filp).private_data };
+ // SAFETY: The kiocb has shared access to the private data.
+ unsafe { <T as ForeignOwnable>::borrow(private) }
+ }
+
+ /// Gets the current value of `ki_pos`.
+ pub fn ki_pos(&self) -> loff_t {
+ // SAFETY: The `kiocb` can access `ki_pos`.
+ unsafe { (*self.inner.as_ptr()).ki_pos }
+ }
+
+ /// Gets a mutable reference to the `ki_pos` field.
+ pub fn ki_pos_mut(&mut self) -> &mut loff_t {
+ // SAFETY: The `kiocb` can access `ki_pos`.
+ unsafe { &mut (*self.inner.as_ptr()).ki_pos }
+ }
+}
+
+/// Wrapper for the kernel's `struct iov_iter`.
+pub struct IovIter {
+ inner: Opaque<bindings::iov_iter>,
+}
+
+impl IovIter {
+ /// Gets a raw pointer to the contents.
+ pub fn as_raw(&self) -> *mut bindings::iov_iter {
+ self.inner.get()
+ }
+}
+
+const fn create_vtable<T: MiscDevice>() -> &'static bindings::file_operations {
+ const fn maybe_fn<T: Copy>(check: bool, func: T) -> Option<T> {
+ if check {
+ Some(func)
+ } else {
+ None
+ }
+ }
+
+ struct VtableHelper<T: MiscDevice> {
+ _t: PhantomData<T>,
+ }
+ impl<T: MiscDevice> VtableHelper<T> {
+ const VTABLE: bindings::file_operations = bindings::file_operations {
+ open: Some(fops_open::<T>),
+ release: Some(fops_release::<T>),
+ mmap: maybe_fn(T::HAS_MMAP, fops_mmap::<T>),
+ llseek: maybe_fn(T::HAS_LLSEEK, fops_llseek::<T>),
+ read_iter: maybe_fn(T::HAS_READ_ITER, fops_read_iter::<T>),
+ unlocked_ioctl: maybe_fn(T::HAS_IOCTL, fops_ioctl::<T>),
+ #[cfg(CONFIG_COMPAT)]
+ compat_ioctl: maybe_fn(T::HAS_IOCTL || T::HAS_COMPAT_IOCTL, fops_compat_ioctl::<T>),
+ ..unsafe { MaybeUninit::zeroed().assume_init() }
+ };
+ }
+
+ &VtableHelper::<T>::VTABLE
+}
+
+unsafe extern "C" fn fops_open<T: MiscDevice>(
+ inode: *mut bindings::inode,
+ file: *mut bindings::file,
+) -> c_int {
+ // SAFETY: The pointers are valid and for a file being opened.
+ let ret = unsafe { bindings::generic_file_open(inode, file) };
+ if ret != 0 {
+ return ret;
+ }
+
+ // SAFETY:
+ // * The file is valid for the duration of this call.
+ // * There is no active fdget_pos region on the file on this thread.
+ let ptr = match T::open(unsafe { File::from_raw_file(file) }) {
+ Ok(ptr) => ptr,
+ Err(err) => return err.to_errno(),
+ };
+
+ // SAFETY: The open call of a file owns the private data.
+ unsafe { (*file).private_data = ptr.into_foreign().cast_mut() };
+
+ 0
+}
+
+unsafe extern "C" fn fops_release<T: MiscDevice>(
+ _inode: *mut bindings::inode,
+ file: *mut bindings::file,
+) -> c_int {
+ // SAFETY: The release call of a file owns the private data.
+ let private = unsafe { (*file).private_data };
+ // SAFETY: We are taking ownership of the private data, so we can drop it.
+ let ptr = unsafe { <T::Ptr as ForeignOwnable>::from_foreign(private) };
+ // SAFETY:
+ // * The file is valid for the duration of this call.
+ // * There is no active fdget_pos region on the file on this thread.
+ T::release(ptr, unsafe { File::from_raw_file(file) });
+
+ 0
+}
+
+unsafe extern "C" fn fops_mmap<T: MiscDevice>(
+ file: *mut bindings::file,
+ vma: *mut bindings::vm_area_struct,
+) -> c_int {
+ // SAFETY: The release call of a file owns the private data.
+ let private = unsafe { (*file).private_data };
+ // SAFETY: Ioctl calls can borrow the private data of the file.
+ let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };
+ // SAFETY:
+ // * The file is valid for the duration of this call.
+ // * There is no active fdget_pos region on the file on this thread.
+ let file = unsafe { File::from_raw_file(file) };
+ // SAFETY: The caller ensures that the vma is valid.
+ let area = unsafe { kernel::mm::virt::VmArea::from_raw_mut(vma) };
+
+ match T::mmap(device, file, area) {
+ Ok(()) => 0,
+ Err(err) => err.to_errno() as c_int,
+ }
+}
+
+unsafe extern "C" fn fops_llseek<T: MiscDevice>(
+ file: *mut bindings::file,
+ offset: loff_t,
+ whence: c_int,
+) -> loff_t {
+ // SAFETY: The release call of a file owns the private data.
+ let private = unsafe { (*file).private_data };
+ // SAFETY: Ioctl calls can borrow the private data of the file.
+ let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };
+ // SAFETY:
+ // * The file is valid for the duration of this call.
+ // * We are inside an fdget_pos region, so there cannot be any active fdget_pos regions on
+ // other threads.
+ let file = unsafe { LocalFile::from_raw_file(file) };
+
+ match T::llseek(device, file, offset, whence) {
+ Ok(res) => res as loff_t,
+ Err(err) => err.to_errno() as loff_t,
+ }
+}
+
+unsafe extern "C" fn fops_read_iter<T: MiscDevice>(
+ kiocb: *mut bindings::kiocb,
+ iter: *mut bindings::iov_iter,
+) -> isize {
+ let kiocb = Kiocb {
+ inner: unsafe { NonNull::new_unchecked(kiocb) },
+ _phantom: PhantomData,
+ };
+ let iov = unsafe { &mut *iter.cast::<IovIter>() };
+
+ match T::read_iter(kiocb, iov) {
+ Ok(res) => res as isize,
+ Err(err) => err.to_errno() as isize,
+ }
+}
+
+unsafe extern "C" fn fops_ioctl<T: MiscDevice>(
+ file: *mut bindings::file,
+ cmd: c_uint,
+ arg: c_ulong,
+) -> c_long {
+ // SAFETY: The release call of a file owns the private data.
+ let private = unsafe { (*file).private_data };
+ // SAFETY: Ioctl calls can borrow the private data of the file.
+ let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };
+ // SAFETY:
+ // * The file is valid for the duration of this call.
+ // * There is no active fdget_pos region on the file on this thread.
+ let file = unsafe { File::from_raw_file(file) };
+
+ match T::ioctl(device, file, cmd as u32, arg as usize) {
+ Ok(ret) => ret,
+ Err(err) => err.to_errno() as c_long,
+ }
+}
+
+#[cfg(CONFIG_COMPAT)]
+unsafe extern "C" fn fops_compat_ioctl<T: MiscDevice>(
+ file: *mut bindings::file,
+ cmd: c_uint,
+ arg: c_ulong,
+) -> c_long {
+ // SAFETY: The release call of a file owns the private data.
+ let private = unsafe { (*file).private_data };
+ // SAFETY: Ioctl calls can borrow the private data of the file.
+ let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };
+ // SAFETY:
+ // * The file is valid for the duration of this call.
+ // * There is no active fdget_pos region on the file on this thread.
+ let file = unsafe { File::from_raw_file(file) };
+
+ match T::compat_ioctl(device, file, cmd as u32, arg as usize) {
+ Ok(ret) => ret,
+ Err(err) => err.to_errno() as c_long,
+ }
+}

--
2.46.0.792.g87dc391469-goog