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

[Faith Ekstrand <faith.ekstrand@collabora.com>]

On Fri, 2023-03-10 at 07:16 +0900, Asahi Lina wrote:
> On 10/03/2023 06.16, Faith Ekstrand wrote:
> > 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/dr
> > > m_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.
> 
> So I think this is going to be a pretty common pattern in this kind
> of
> abstraction. The problem is that, of course, in C there is no
> distinction between an owned reference and a borrowed one. Here we
> have
> a borrowed reference to a struct drm_device, and we need to turn it
> into
> a &Device (which is the Rust equivalent type). But for &Device to
> exist
> we need a Device to exist in the first place, and Device normally
> implies ownership of the underlying drm_device.

Thanks! Putting it in terms of borrow really helps clear up the
difference.

> We could just acquire a reference here, but then we're needlessly
> grabbing a ref only to drop it at the end of the function, which is
> pointless when the caller is holding another reference for us while
> the
> callback runs. And of course Rust likes to claim to offer zero-cost
> abstractions, so it would be kind of sad to have to do that... ^^

Yeah, I agree we don't want to take extra references.

> Just doing drm::device::Device::from_raw(raw_dev) is a ticking time
> bomb, because we haven't acquired a reference (which would normally
> be
> required). If that Device ever gets dropped, we've messed up the
> refcounting and stolen the caller's reference. We could try to ensure
> it
> gets passed to core::mem::forget in all paths out, but that gets
> error-prone very quickly when trying to cover error paths. So
> instead,
> we put it into a ManuallyDrop. That takes care of neutering the ref
> drop, so we don't have to worry about messing that up. Then the only
> remaining safety requirement is that that the ManuallyDrop<Device>
> never
> escape the callback function, and that's easy to ensure: we only pass
> a
> &ref to the user (which via auto-deref ends up being a &Device), and
> then nothing bad can happen. If the user wants an owned reference to
> the
> device to keep around, they can call .clone() on it and that's when
> the
> incref happens.
> 
> Basically, ManuallyDrop<T> where T is a reference counted type
> represents a borrowed reference to a T coming from the C side. You
> can
> see another use of this pattern in gem::Object, which contains a
> ManuallyDrop<Device> that represents a borrowed reference to the
> device
> that owns that object. The DRM core (as far as I know!) guarantees
> that
> DRM devices outlive all of their GEM objects, so we can materialize a
> borrowed reference and as long as it never leaves the GEM object, it
> will be sound. Then we can take &Device references from it whenever
> we
> want, and the usual Rust borrow checker rules ensure we can't do
> something illegal.

Ok, that all matches my understanding of what I thought was going on. I
do wonder if it would be good to wrap this up in a

struct DeviceBorrow {
   dev: ManuallyDrop<Device>
}

impl DeviceBorrow {
   pub unsafe fn from_raw(*mut bindings::drm_device) -> DeviceBorrow;
}

impl Deref<Device> for DeviceBorrow {
   ...
}

with documentation, etc.  Seeing a ManuallyDrop which is never dropped
sets my rust senses tingling.  Maybe that's too much typing for each
object?  I don't want to add a bunch of extra work but this seems like
a pretty common pattern we're going to hit everywhere.

~Faith