linux-kernel - [PATCH 5/5] samples: rust_misc_device: Expand the sample to support read()ing from userspace

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Message-ID: <20250311-iov-iter-v1-5-f6c9134ea824@google.com>
Date: Tue, 11 Mar 2025 14:25:16 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Arnd Bergmann <arnd@...db.de>, Miguel Ojeda <ojeda@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, Matthew Maurer <mmaurer@...gle.com>, Lee Jones <lee@...nel.org>, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	Alice Ryhl <aliceryhl@...gle.com>
Subject: [PATCH 5/5] samples: rust_misc_device: Expand the sample to support
 read()ing from userspace

From: Lee Jones <lee@...nel.org>

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@...nel.org>
Co-developed-by: Alice Ryhl <aliceryhl@...gle.com>
Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
---
 samples/rust/rust_misc_device.rs | 37 +++++++++++++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

diff --git a/samples/rust/rust_misc_device.rs b/samples/rust/rust_misc_device.rs
index 40ad7266c2252e5c0b4e91e501ef9ada2eda3b16..6405713fc8ff69659799f30710074487ea3bb14e 100644
--- a/samples/rust/rust_misc_device.rs
+++ b/samples/rust/rust_misc_device.rs
@@ -101,7 +101,9 @@
     device::Device,
     fs::File,
     ioctl::{_IO, _IOC_SIZE, _IOR, _IOW},
-    miscdevice::{MiscDevice, MiscDeviceOptions, MiscDeviceRegistration},
+    iov::{IovIterDest, IovIterSource},
+    kvec,
+    miscdevice::{Kiocb, MiscDevice, MiscDeviceOptions, MiscDeviceRegistration},
     new_mutex,
     prelude::*,
     sync::Mutex,
@@ -143,6 +145,7 @@ fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
 
 struct Inner {
     value: i32,
+    buffer: KVec<u8>,
 }
 
 #[pin_data(PinnedDrop)]
@@ -164,7 +167,10 @@ fn open(_file: &File, misc: &MiscDeviceRegistration<Self>) -> Result<Pin<KBox<Se
         KBox::try_pin_init(
             try_pin_init! {
                 RustMiscDevice {
-                    inner <- new_mutex!( Inner{ value: 0_i32 } ),
+                    inner <- new_mutex!(Inner {
+                        value: 0_i32,
+                        buffer: kvec![],
+                    }),
                     dev: dev,
                 }
             },
@@ -172,6 +178,33 @@ fn open(_file: &File, misc: &MiscDeviceRegistration<Self>) -> Result<Pin<KBox<Se
         )
     }
 
+    fn read_iter(mut kiocb: Kiocb<'_, Self::Ptr>, iov: &mut IovIterDest<'_>) -> Result<usize> {
+        let me = kiocb.private_data();
+        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.private_data();
+        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");
 

-- 
2.49.0.rc0.332.g42c0ae87b1-goog