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

From: Alice Ryhl
Date: Mon Dec 09 2024 - 02:28:38 EST


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@xxxxxxxxxx>
---
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() };


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2.47.1.545.g3c1d2e2a6a-goog