2 files changed, 76 insertions, 7 deletions

diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
index 35fd5db35c46..cf06e73a6da0 100644
--- a/rust/kernel/fs/file.rs
+++ b/rust/kernel/fs/file.rs
@@ -10,8 +10,10 @@
 use crate::{
     bindings,
     cred::Credential,
-    error::{code::*, Error, Result},
-    types::{ARef, AlwaysRefCounted, NotThreadSafe, Opaque},
+    error::{code::*, to_result, Error, Result},
+    fmt,
+    sync::aref::{ARef, AlwaysRefCounted},
+    types::{NotThreadSafe, Opaque},
 };
 use core::ptr;
 
@@ -398,9 +400,8 @@ impl FileDescriptorReservation {
     pub fn get_unused_fd_flags(flags: u32) -> Result<Self> {
         // SAFETY: FFI call, there are no safety requirements on `flags`.
         let fd: i32 = unsafe { bindings::get_unused_fd_flags(flags) };
-        if fd < 0 {
-            return Err(Error::from_errno(fd));
-        }
+        to_result(fd)?;
+
         Ok(Self {
             fd: fd as u32,
             _not_send: NotThreadSafe,
@@ -460,8 +461,8 @@ impl From<BadFdError> for Error {
     }
 }
 
-impl core::fmt::Debug for BadFdError {
-    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+impl fmt::Debug for BadFdError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.pad("EBADF")
     }
 }
diff --git a/rust/kernel/fs/kiocb.rs b/rust/kernel/fs/kiocb.rs
new file mode 100644
index 000000000000..84c936cd69b0
--- /dev/null
+++ b/rust/kernel/fs/kiocb.rs
@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Kernel IO callbacks.
+//!
+//! C headers: [`include/linux/fs.h`](srctree/include/linux/fs.h)
+
+use core::marker::PhantomData;
+use core::ptr::NonNull;
+use kernel::types::ForeignOwnable;
+
+/// Wrapper for the kernel's `struct kiocb`.
+///
+/// Currently this abstractions is incomplete and is essentially just a tuple containing a
+/// reference to a file and a file position.
+///
+/// The type `T` represents the filesystem or driver specific data associated with the file.
+///
+/// # Invariants
+///
+/// `inner` points at a valid `struct kiocb` whose file has the type `T` as its private data.
+pub struct Kiocb<'a, T> {
+    inner: NonNull<bindings::kiocb>,
+    _phantom: PhantomData<&'a T>,
+}
+
+impl<'a, T: ForeignOwnable> Kiocb<'a, T> {
+    /// Create a `Kiocb` from a raw pointer.
+    ///
+    /// # Safety
+    ///
+    /// The pointer must reference a valid `struct kiocb` for the duration of `'a`. The private
+    /// data of the file must be `T`.
+    pub unsafe fn from_raw(kiocb: *mut bindings::kiocb) -> Self {
+        Self {
+            // SAFETY: If a pointer is valid it is not null.
+            inner: unsafe { NonNull::new_unchecked(kiocb) },
+            _phantom: PhantomData,
+        }
+    }
+
+    /// Access the underlying `struct kiocb` directly.
+    pub fn as_raw(&self) -> *mut bindings::kiocb {
+        self.inner.as_ptr()
+    }
+
+    /// Get the filesystem or driver specific data associated with the file.
+    pub fn file(&self) -> <T as ForeignOwnable>::Borrowed<'a> {
+        // SAFETY: We have shared access to this kiocb and hence the underlying file, so we can
+        // read the file's private data.
+        let private = unsafe { (*(*self.as_raw()).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: We have shared access to the kiocb, so we can read its `ki_pos` field.
+        unsafe { (*self.as_raw()).ki_pos }
+    }
+
+    /// Gets a mutable reference to the `ki_pos` field.
+    pub fn ki_pos_mut(&mut self) -> &mut i64 {
+        // SAFETY: We have exclusive access to the kiocb, so we can write to `ki_pos`.
+        unsafe { &mut (*self.as_raw()).ki_pos }
+    }
+}