linux-kernel - [PATCH v2 2/2] rust: miscdevice: access the `struct miscdevice` from fops->open()

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Message-ID: <20241209-miscdevice-file-param-v2-2-83ece27e9ff6@google.com>
Date: Mon, 09 Dec 2024 07:27:47 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Arnd Bergmann <arnd@...db.de>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	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>, Lee Jones <lee@...nel.org>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, Alice Ryhl <aliceryhl@...gle.com>
Subject: [PATCH v2 2/2] rust: miscdevice: access the `struct miscdevice` from fops->open()

Providing access to the underlying `struct miscdevice` is useful for
various reasons. For example, this allows you access the miscdevice's
internal `struct device` for use with the `dev_*` printing macros.

Note that since the underlying `struct miscdevice` could get freed at
any point after the fops->open() call, only the open call is given
access to it. To print from other calls, they should take a refcount on
the device to keep it alive.

Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
---
 rust/kernel/miscdevice.rs | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
index 0cb79676c139..c5af1d5ec4be 100644
--- a/rust/kernel/miscdevice.rs
+++ b/rust/kernel/miscdevice.rs
@@ -104,7 +104,7 @@ pub trait MiscDevice {
     /// Called when the misc device is opened.
     ///
     /// The returned pointer will be stored as the private data for the file.
-    fn open(_file: &File) -> Result<Self::Ptr>;
+    fn open(_file: &File, _misc: &MiscDeviceRegistration<Self>) -> Result<Self::Ptr>;
 
     /// Called when the misc device is released.
     fn release(device: Self::Ptr, _file: &File) {
@@ -190,14 +190,27 @@ impl<T: MiscDevice> VtableHelper<T> {
         return ret;
     }
 
+    // SAFETY: The opwn call of a file can access the private data.
+    let misc_ptr = unsafe { (*file).private_data };
+    // SAFETY: This is a miscdevice, so `misc_open()` set the private data to a pointer to the
+    // associated `struct miscdevice` before calling into this method. Furthermore, `misc_open()`
+    // ensures that the miscdevice can't be unregistered and freed during this call to `fops_open`.
+    let misc = unsafe { &*misc_ptr.cast::<MiscDeviceRegistration<T>>() };
+
     // SAFETY:
-    // * The file is valid for the duration of this call.
+    // * The file is valid for the duration of the `T::open` call.
     // * There is no active fdget_pos region on the file on this thread.
-    let ptr = match T::open(unsafe { File::from_raw_file(file) }) {
+    let file = unsafe { File::from_raw_file(file) };
+
+    let ptr = match T::open(file, misc) {
         Ok(ptr) => ptr,
         Err(err) => return err.to_errno(),
     };
 
+    // This overwrites the private data from above. It makes sense to not hold on to the misc
+    // pointer since the `struct miscdevice` can get unregistered as soon as we return from this
+    // call, so the misc pointer might be dangling on future file operations.
+    //
     // SAFETY: The open call of a file owns the private data.
     unsafe { (*file).private_data = ptr.into_foreign().cast_mut() };
 

-- 
2.47.1.545.g3c1d2e2a6a-goog