Re: [PATCH v2 3/4] rust: miscdevice: Provide additional abstractions for iov_iter and kiocb structures

[Andreas Hindborg <a.hindborg@kernel.org>]

"Alice Ryhl" <aliceryhl@google.com> writes:

> On Tue, Jul 08, 2025 at 04:53:23PM +0200, Andreas Hindborg wrote:
>> "Alice Ryhl" <aliceryhl@google.com> writes:
>>
>> > These will be used for the read_iter() and write_iter() callbacks, which
>> > are now the preferred back-ends for when a user operates on a char device
>> > with read() and write() respectively.
>> >
>> > Co-developed-by: Lee Jones <lee@kernel.org>
>> > Signed-off-by: Lee Jones <lee@kernel.org>
>> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
>> > ---
>> >  rust/kernel/miscdevice.rs | 97 ++++++++++++++++++++++++++++++++++++++++++++++-
>> >  1 file changed, 96 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
>> > index 22f291211636f66efca6b33b675833236332719e..a49954c9b0d14117645be8139db792f1fd22589d 100644
>> > --- a/rust/kernel/miscdevice.rs
>> > +++ b/rust/kernel/miscdevice.rs
>> > @@ -14,13 +14,14 @@
>> >      error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR},
>> >      ffi::{c_int, c_long, c_uint, c_ulong},
>> >      fs::File,
>> > +    iov::{IovIterDest, IovIterSource},
>> >      mm::virt::VmaNew,
>> >      prelude::*,
>> >      seq_file::SeqFile,
>> >      str::CStr,
>> >      types::{ForeignOwnable, Opaque},
>> >  };
>> > -use core::{marker::PhantomData, mem::MaybeUninit, pin::Pin};
>> > +use core::{marker::PhantomData, mem::MaybeUninit, pin::Pin, ptr::NonNull};
>> >
>> >  /// Options for creating a misc device.
>> >  #[derive(Copy, Clone)]
>> > @@ -136,6 +137,16 @@ fn mmap(
>> >          build_error!(VTABLE_DEFAULT_ERROR)
>> >      }
>> >
>> > +    /// Read from this miscdevice.
>> > +    fn read_iter(_kiocb: Kiocb<'_, Self::Ptr>, _iov: &mut IovIterDest<'_>) -> Result<usize> {
>> > +        build_error!(VTABLE_DEFAULT_ERROR)
>> > +    }
>> > +
>> > +    /// Write to this miscdevice.
>> > +    fn write_iter(_kiocb: Kiocb<'_, Self::Ptr>, _iov: &mut IovIterSource<'_>) -> Result<usize> {
>> > +        build_error!(VTABLE_DEFAULT_ERROR)
>> > +    }
>> > +
>> >      /// Handler for ioctls.
>> >      ///
>> >      /// The `cmd` argument is usually manipulated using the utilities in [`kernel::ioctl`].
>> > @@ -177,6 +188,36 @@ fn show_fdinfo(
>> >      }
>> >  }
>> >
>> > +/// Wrapper for the kernel's `struct kiocb`.
>> > +///
>> > +/// The type `T` represents the private data of the file.
>> > +pub struct Kiocb<'a, T> {
>> > +    inner: NonNull<bindings::kiocb>,
>> > +    _phantom: PhantomData<&'a T>,
>> > +}
>>
>> Also, `kiocb` is not miscdevice specific. It should probably not live here.
>
> I can place it in rust/kernel/fs.rs, but this is an instance of the more
> general fact that miscdevice defines many things from `file_operations`
> that we should probably generalize in the future.

I support moving `Kiocb` to `fs` now, rather than later.


Best regards,
Andreas Hindborg