[PATCH rust-next v2 2/3] rust: miscdevice: add llseek support

[Ryosuke Yasuoka <ryasuoka@redhat.com>]

Add the ability to write a file_operations->llseek hook in Rust when
using the miscdevice abstraction.

Signed-off-by: Ryosuke Yasuoka <ryasuoka@redhat.com>
---
 rust/kernel/miscdevice.rs | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
index d698cddcb4a5..2efd0847cde1 100644
--- a/rust/kernel/miscdevice.rs
+++ b/rust/kernel/miscdevice.rs
@@ -126,6 +126,16 @@ fn release(device: Self::Ptr, _file: &File) {
         drop(device);
     }
 
+    /// Handler for llseek.
+    fn llseek(
+        _device: <Self::Ptr as ForeignOwnable>::Borrowed<'_>,
+        _file: &mut File,
+        _offset: i64,
+        _whence: i32,
+    ) -> Result<isize> {
+        build_error!(VTABLE_DEFAULT_ERROR)
+    }
+
     /// Handle for mmap.
     ///
     /// This function is invoked when a user space process invokes the `mmap` system call on
@@ -296,6 +306,27 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
         }
     }
 
+    /// # Safety
+    ///
+    /// `file` must be a valid file that is associated with a `MiscDeviceRegistration<T>`.
+    unsafe extern "C" fn llseek(file: *mut bindings::file, offset: i64, whence: c_int) -> i64 {
+        // SAFETY: The llseek call of a file can access the private data.
+        let private = unsafe { (*file).private_data };
+        // SAFETY: This is a Rust Miscdevice, so we call `into_foreign` in `open` and
+        // `from_foreign` in `release`, and `fops_llseek` is guaranteed to be called between those
+        // two operations.
+        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_mut(file) };
+
+        match T::llseek(device, file, offset, whence) {
+            Ok(res) => res as i64,
+            Err(err) => i64::from(err.to_errno()),
+        }
+    }
+
     /// # Safety
     ///
     /// `file` must be a valid file that is associated with a `MiscDeviceRegistration<T>`.
@@ -391,6 +422,11 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
     const VTABLE: bindings::file_operations = bindings::file_operations {
         open: Some(Self::open),
         release: Some(Self::release),
+        llseek: if T::HAS_LLSEEK {
+            Some(Self::llseek)
+        } else {
+            None
+        },
         mmap: if T::HAS_MMAP { Some(Self::mmap) } else { None },
         read_iter: if T::HAS_READ_ITER {
             Some(Self::read_iter)
-- 
2.51.0