[PATCH v2 3/4] rust: miscdevice: Provide additional abstractions for iov_iter and kiocb structures

[Alice Ryhl <aliceryhl@google.com>]

These will be used for the read_iter() and write_iter() callbacks, which
are now the preferred back-ends for when a user operates on a char device
with read() and write() respectively.

Co-developed-by: Lee Jones <lee@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/miscdevice.rs | 97 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 96 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
index 22f291211636f66efca6b33b675833236332719e..a49954c9b0d14117645be8139db792f1fd22589d 100644
--- a/rust/kernel/miscdevice.rs
+++ b/rust/kernel/miscdevice.rs
@@ -14,13 +14,14 @@
     error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR},
     ffi::{c_int, c_long, c_uint, c_ulong},
     fs::File,
+    iov::{IovIterDest, IovIterSource},
     mm::virt::VmaNew,
     prelude::*,
     seq_file::SeqFile,
     str::CStr,
     types::{ForeignOwnable, Opaque},
 };
-use core::{marker::PhantomData, mem::MaybeUninit, pin::Pin};
+use core::{marker::PhantomData, mem::MaybeUninit, pin::Pin, ptr::NonNull};
 
 /// Options for creating a misc device.
 #[derive(Copy, Clone)]
@@ -136,6 +137,16 @@ fn mmap(
         build_error!(VTABLE_DEFAULT_ERROR)
     }
 
+    /// Read from this miscdevice.
+    fn read_iter(_kiocb: Kiocb<'_, Self::Ptr>, _iov: &mut IovIterDest<'_>) -> Result<usize> {
+        build_error!(VTABLE_DEFAULT_ERROR)
+    }
+
+    /// Write to this miscdevice.
+    fn write_iter(_kiocb: Kiocb<'_, Self::Ptr>, _iov: &mut IovIterSource<'_>) -> Result<usize> {
+        build_error!(VTABLE_DEFAULT_ERROR)
+    }
+
     /// Handler for ioctls.
     ///
     /// The `cmd` argument is usually manipulated using the utilities in [`kernel::ioctl`].
@@ -177,6 +188,36 @@ fn show_fdinfo(
     }
 }
 
+/// 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 Rust data that represents this misc device.
+    pub fn device(&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) -> i64 {
+        // 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 i64 {
+        // SAFETY: The `kiocb` can access `ki_pos`.
+        unsafe { &mut (*self.inner.as_ptr()).ki_pos }
+    }
+}
+
 /// A vtable for the file operations of a Rust miscdevice.
 struct MiscdeviceVTable<T: MiscDevice>(PhantomData<T>);
 
@@ -240,6 +281,50 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
         0
     }
 
+    /// # Safety
+    ///
+    /// `kiocb` must be correspond to a valid file that is associated with a
+    /// `MiscDeviceRegistration<T>`. `iter` must be a valid `struct iov_iter` for writing.
+    unsafe extern "C" fn read_iter(
+        kiocb: *mut bindings::kiocb,
+        iter: *mut bindings::iov_iter,
+    ) -> isize {
+        let kiocb = Kiocb {
+            // SAFETY: The read_iter call of a file is given a kiocb for that file.
+            inner: unsafe { NonNull::new_unchecked(kiocb) },
+            _phantom: PhantomData,
+        };
+        // SAFETY: This is a valid `struct iov_iter` for writing.
+        let iov = unsafe { IovIterDest::from_raw(iter) };
+
+        match T::read_iter(kiocb, iov) {
+            Ok(res) => res as isize,
+            Err(err) => err.to_errno() as isize,
+        }
+    }
+
+    /// # Safety
+    ///
+    /// `kiocb` must be correspond to a valid file that is associated with a
+    /// `MiscDeviceRegistration<T>`. `iter` must be a valid `struct iov_iter` for writing.
+    unsafe extern "C" fn write_iter(
+        kiocb: *mut bindings::kiocb,
+        iter: *mut bindings::iov_iter,
+    ) -> isize {
+        let kiocb = Kiocb {
+            // SAFETY: The read_iter call of a file is given a kiocb for that file.
+            inner: unsafe { NonNull::new_unchecked(kiocb) },
+            _phantom: PhantomData,
+        };
+        // SAFETY: This is a valid `struct iov_iter` for reading.
+        let iov = unsafe { IovIterSource::from_raw(iter) };
+
+        match T::write_iter(kiocb, iov) {
+            Ok(res) => res as isize,
+            Err(err) => err.to_errno() as isize,
+        }
+    }
+
     /// # Safety
     ///
     /// `file` must be a valid file that is associated with a `MiscDeviceRegistration<T>`.
@@ -336,6 +421,16 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
         open: Some(Self::open),
         release: Some(Self::release),
         mmap: if T::HAS_MMAP { Some(Self::mmap) } else { None },
+        read_iter: if T::HAS_READ_ITER {
+            Some(Self::read_iter)
+        } else {
+            None
+        },
+        write_iter: if T::HAS_WRITE_ITER {
+            Some(Self::write_iter)
+        } else {
+            None
+        },
         unlocked_ioctl: if T::HAS_IOCTL {
             Some(Self::ioctl)
         } else {

-- 
2.50.0.727.gbf7dc18ff4-goog