samples: rust_misc_device: Expand the sample to support read()ing from userspace

A userland application can now operate on the char device with read() in
order to consume a locally held buffer.  Memory for the buffer is to be
provisioned and the buffer populated in its subsequently provided
write() counterpart.

Signed-off-by: Lee Jones <lee@kernel.org>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250822-iov-iter-v5-5-6ce4819c2977@google.com

1 files changed, 34 insertions, 2 deletions

diff --git a/samples/rust/rust_misc_device.rs b/samples/rust/rust_misc_device.rs
index e7ab77448f75..9e4005e33796 100644
--- a/samples/rust/rust_misc_device.rs
+++ b/samples/rust/rust_misc_device.rs
@@ -100,8 +100,9 @@ use core::pin::Pin;
 use kernel::{
     c_str,
     device::Device,
-    fs::File,
+    fs::{File, Kiocb},
     ioctl::{_IO, _IOC_SIZE, _IOR, _IOW},
+    iov::{IovIterDest, IovIterSource},
     miscdevice::{MiscDevice, MiscDeviceOptions, MiscDeviceRegistration},
     new_mutex,
     prelude::*,
@@ -144,6 +145,7 @@ impl kernel::InPlaceModule for RustMiscDeviceModule {
 
 struct Inner {
     value: i32,
+    buffer: KVVec<u8>,
 }
 
 #[pin_data(PinnedDrop)]
@@ -165,7 +167,10 @@ impl MiscDevice for RustMiscDevice {
         KBox::try_pin_init(
             try_pin_init! {
                 RustMiscDevice {
-                    inner <- new_mutex!( Inner{ value: 0_i32 } ),
+                    inner <- new_mutex!(Inner {
+                        value: 0_i32,
+                        buffer: KVVec::new(),
+                    }),
                     dev: dev,
                 }
             },
@@ -173,6 +178,33 @@ impl MiscDevice for RustMiscDevice {
         )
     }
 
+    fn read_iter(mut kiocb: Kiocb<'_, Self::Ptr>, iov: &mut IovIterDest<'_>) -> Result<usize> {
+        let me = kiocb.file();
+        dev_info!(me.dev, "Reading from Rust Misc Device Sample\n");
+
+        let inner = me.inner.lock();
+        // Read the buffer contents, taking the file position into account.
+        let read = iov.simple_read_from_buffer(kiocb.ki_pos_mut(), &inner.buffer)?;
+
+        Ok(read)
+    }
+
+    fn write_iter(mut kiocb: Kiocb<'_, Self::Ptr>, iov: &mut IovIterSource<'_>) -> Result<usize> {
+        let me = kiocb.file();
+        dev_info!(me.dev, "Writing to Rust Misc Device Sample\n");
+
+        let mut inner = me.inner.lock();
+
+        // Replace buffer contents.
+        inner.buffer.clear();
+        let len = iov.copy_from_iter_vec(&mut inner.buffer, GFP_KERNEL)?;
+
+        // Set position to zero so that future `read` calls will see the new contents.
+        *kiocb.ki_pos_mut() = 0;
+
+        Ok(len)
+    }
+
     fn ioctl(me: Pin<&RustMiscDevice>, _file: &File, cmd: u32, arg: usize) -> Result<isize> {
         dev_info!(me.dev, "IOCTLing Rust Misc Device Sample\n");