Re: [PATCH RFC 03/18] rust: drm: file: Add File abstraction

Faith Ekstrand
2023-03-09T21:16:35Z

On Tue, 2023-03-07 at 23:25 +0900, Asahi Lina wrote:
> A DRM File is the DRM counterpart to a kernel file structure,
> representing an open DRM file descriptor. Add a Rust abstraction to
> allow drivers to implement their own File types that implement the
> DriverFile trait.
>
> Signed-off-by: Asahi Lina <lina@asahilina.net>
> ---
>  rust/bindings/bindings_helper.h |   1 +
>  rust/kernel/drm/drv.rs          |   7 ++-
>  rust/kernel/drm/file.rs         | 113
> ++++++++++++++++++++++++++++++++++++++++
>  rust/kernel/drm/mod.rs          |   1 +
>  4 files changed, 120 insertions(+), 2 deletions(-)
>
> diff --git a/rust/bindings/bindings_helper.h
> b/rust/bindings/bindings_helper.h
> index 2a999138c4ae..7d7828faf89c 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -8,6 +8,7 @@
>
>  #include <drm/drm_device.h>
>  #include <drm/drm_drv.h>
> +#include <drm/drm_file.h>
>  #include <drm/drm_ioctl.h>
>  #include <linux/delay.h>
>  #include <linux/device.h>
> diff --git a/rust/kernel/drm/drv.rs b/rust/kernel/drm/drv.rs
> index 29a465515dc9..1dcb651e1417 100644
> --- a/rust/kernel/drm/drv.rs
> +++ b/rust/kernel/drm/drv.rs
> @@ -144,6 +144,9 @@ pub trait Driver {
>      /// Should be either `drm::gem::Object<T>` or
> `drm::gem::shmem::Object<T>`.
>      type Object: AllocImpl;
>
> +    /// The type used to represent a DRM File (client)
> +    type File: drm::file::DriverFile;
> +
>      /// Driver metadata
>      const INFO: DriverInfo;
>
> @@ -213,8 +216,8 @@ macro_rules! drm_device_register {
>  impl<T: Driver> Registration<T> {
>      const VTABLE: bindings::drm_driver = drm_legacy_fields! {
>          load: None,
> -        open: None, // TODO: File abstraction
> -        postclose: None, // TODO: File abstraction
> +        open: Some(drm::file::open_callback::<T::File>),
> +        postclose: Some(drm::file::postclose_callback::<T::File>),
>          lastclose: None,
>          unload: None,
>          release: None,
> diff --git a/rust/kernel/drm/file.rs b/rust/kernel/drm/file.rs
> new file mode 100644
> index 000000000000..48751e93c38a
> --- /dev/null
> +++ b/rust/kernel/drm/file.rs
> @@ -0,0 +1,113 @@
> +// SPDX-License-Identifier: GPL-2.0 OR MIT
> +
> +//! DRM File objects.
> +//!
> +//! C header:
> [`include/linux/drm/drm_file.h`](../../../../include/linux/drm/drm_fi
> le.h)
> +
> +use crate::{bindings, drm, error::Result};
> +use alloc::boxed::Box;
> +use core::marker::PhantomData;
> +use core::ops::Deref;
> +
> +/// Trait that must be implemented by DRM drivers to represent a DRM
> File (a client instance).
> +pub trait DriverFile {
> +    /// The parent `Driver` implementation for this `DriverFile`.
> +    type Driver: drm::drv::Driver;
> +
> +    /// Open a new file (called when a client opens the DRM device).
> +    fn open(device: &drm::device::Device<Self::Driver>) ->
> Result<Box<Self>>;
> +}
> +
> +/// An open DRM File.
> +///
> +/// # Invariants
> +/// `raw` is a valid pointer to a `drm_file` struct.
> +#[repr(transparent)]
> +pub struct File<T: DriverFile> {
> +    raw: *mut bindings::drm_file,
> +    _p: PhantomData<T>,
> +}
> +
> +pub(super) unsafe extern "C" fn open_callback<T: DriverFile>(
> +    raw_dev: *mut bindings::drm_device,
> +    raw_file: *mut bindings::drm_file,
> +) -> core::ffi::c_int {
> +    let drm = core::mem::ManuallyDrop::new(unsafe {
> drm::device::Device::from_raw(raw_dev) });

Maybe you can help educate me a bit here... This feels like a really
sketchy pattern. We're creating a Device from a pointer, an operation
which inherently consumes a reference but then marking it ManuallyDrop
so drm_device_put() never gets called. It took me a while but I think
I figured out what you're trying to do: Make it so all the Rust stuff
works with Device, not drm_device but it still feels really wrong. It
works, it just feels like there's a lot of unsafe abstraction juggling
happening here and I expect this operation is going to be pretty common
in the Rust abstraction layer.

Am I missing something?

~Faith

> +    // SAFETY: This reference won't escape this function
> +    let file = unsafe { &mut *raw_file };
> +
> +    let inner = match T::open(&drm) {
> +        Err(e) => {
> +            return e.to_kernel_errno();
> +        }
> +        Ok(i) => i,
> +    };
> +
> +    file.driver_priv = Box::into_raw(inner) as *mut _;
> +
> +    0
> +}
> +
> +pub(super) unsafe extern "C" fn postclose_callback<T: DriverFile>(
> +    _dev: *mut bindings::drm_device,
> +    raw_file: *mut bindings::drm_file,
> +) {
> +    // SAFETY: This reference won't escape this function
> +    let file = unsafe { &*raw_file };
> +
> +    // Drop the DriverFile
> +    unsafe { Box::from_raw(file.driver_priv as *mut T) };
> +}
> +
> +impl<T: DriverFile> File<T> {
> +    // Not intended to be called externally, except via
> declare_drm_ioctls!()
> +    #[doc(hidden)]
> +    pub unsafe fn from_raw(raw_file: *mut bindings::drm_file) ->
> File<T> {
> +        File {
> +            raw: raw_file,
> +            _p: PhantomData,
> +        }
> +    }
> +
> +    #[allow(dead_code)]
> +    /// Return the raw pointer to the underlying `drm_file`.
> +    pub(super) fn raw(&self) -> *const bindings::drm_file {
> +        self.raw
> +    }
> +
> +    /// Return an immutable reference to the raw `drm_file`
> structure.
> +    pub(super) fn file(&self) -> &bindings::drm_file {
> +        unsafe { &*self.raw }
> +    }
> +}
> +
> +impl<T: DriverFile> Deref for File<T> {
> +    type Target = T;
> +
> +    fn deref(&self) -> &T {
> +        unsafe { &*(self.file().driver_priv as *const T) }
> +    }
> +}
> +
> +impl<T: DriverFile> crate::private::Sealed for File<T> {}
> +
> +/// Generic trait to allow users that don't care about driver
> specifics to accept any File<T>.
> +///
> +/// # Safety
> +/// Must only be implemented for File<T> and return the pointer,
> following the normal invariants
> +/// of that type.
> +pub unsafe trait GenericFile: crate::private::Sealed {
> +    /// Returns the raw const pointer to the `struct drm_file`
> +    fn raw(&self) -> *const bindings::drm_file;
> +    /// Returns the raw mut pointer to the `struct drm_file`
> +    fn raw_mut(&mut self) -> *mut bindings::drm_file;
> +}
> +
> +unsafe impl<T: DriverFile> GenericFile for File<T> {
> +    fn raw(&self) -> *const bindings::drm_file {
> +        self.raw
> +    }
> +    fn raw_mut(&mut self) -> *mut bindings::drm_file {
> +        self.raw
> +    }
> +}
> diff --git a/rust/kernel/drm/mod.rs b/rust/kernel/drm/mod.rs
> index 69376b3c6db9..a767942d0b52 100644
> --- a/rust/kernel/drm/mod.rs
> +++ b/rust/kernel/drm/mod.rs
> @@ -4,4 +4,5 @@
>
>  pub mod device;
>  pub mod drv;
> +pub mod file;
>  pub mod ioctl;
>