linux-kernel - Re: [PATCH v8 7/7] rust: miscdevice: add mmap support

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Message-ID: <bd95657f-0fcf-4c13-a02a-d7293aa5f5ea@lucifer.local>
Date: Wed, 20 Nov 2024 20:07:14 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, John Hubbard <jhubbard@...dia.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, Christian Brauner <brauner@...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>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, rust-for-linux@...r.kernel.org,
        Andreas Hindborg <a.hindborg@...nel.org>
Subject: Re: [PATCH v8 7/7] rust: miscdevice: add mmap support

On Wed, Nov 20, 2024 at 02:50:01PM +0000, Alice Ryhl wrote:
> Using the vma support introduced by the previous commit, introduce mmap
> support for miscdevices. The mmap call is given a vma that is undergoing
> initial setup, so the VmAreaNew type is used.

Again, I'd be explicit about the VMA being passed to a struct
file_operations->mmap() hook on mmap. Otherwise this seems super vague to
me!

>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/miscdevice.rs | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
>
> diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
> index 7e2a79b3ae26..4e4b9476e092 100644
> --- a/rust/kernel/miscdevice.rs
> +++ b/rust/kernel/miscdevice.rs
> @@ -11,6 +11,7 @@
>  use crate::{
>      bindings,
>      error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR},
> +    mm::virt::VmAreaNew,
>      prelude::*,
>      str::CStr,
>      types::{ForeignOwnable, Opaque},
> @@ -110,6 +111,11 @@ fn release(device: Self::Ptr) {
>          drop(device);
>      }
>
> +    /// Handle for mmap.
> +    fn mmap(_device: <Self::Ptr as ForeignOwnable>::Borrowed<'_>, _vma: &VmAreaNew) -> Result {

You will have to help me with this :) ForeignOwnable, Borrowed<'_>, _vma I'm a
bit lost here!

> +        kernel::build_error(VTABLE_DEFAULT_ERROR)

What is this? Is this not yet implemented or something, and this is a
placeholder or something?

> +    }

> +
>      /// Handler for ioctls.
>      ///
>      /// The `cmd` argument is usually manipulated using the utilties in [`kernel::ioctl`].
> @@ -156,6 +162,7 @@ impl<T: MiscDevice> VtableHelper<T> {
>          const VTABLE: bindings::file_operations = bindings::file_operations {
>              open: Some(fops_open::<T>),
>              release: Some(fops_release::<T>),
> +            mmap: maybe_fn(T::HAS_MMAP, fops_mmap::<T>),
>              unlocked_ioctl: maybe_fn(T::HAS_IOCTL, fops_ioctl::<T>),
>              #[cfg(CONFIG_COMPAT)]
>              compat_ioctl: if T::HAS_COMPAT_IOCTL {
> @@ -216,6 +223,27 @@ impl<T: MiscDevice> VtableHelper<T> {
>      0
>  }
>
> +/// # Safety
> +///
> +/// `file` must be a valid file that is associated with a `MiscDeviceRegistration<T>`.
> +/// `vma` must be a vma that is currently being mmap'ed with this file.
> +unsafe extern "C" fn fops_mmap<T: MiscDevice>(
> +    file: *mut bindings::file,
> +    vma: *mut bindings::vm_area_struct,
> +) -> c_int {
> +    // SAFETY: The mmap call of a file can access the private data.
> +    let private = unsafe { (*file).private_data };
> +    // SAFETY: Mmap calls can borrow the private data of the file.
> +    let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };
> +    // SAFETY: The caller provides a vma that is undergoing initial VMA setup.
> +    let area = unsafe { VmAreaNew::from_raw(vma) };
> +
> +    match T::mmap(device, area) {
> +        Ok(()) => 0,
> +        Err(err) => err.to_errno() as c_int,
> +    }
> +}
> +
>  /// # Safety
>  ///
>  /// `file` must be a valid file that is associated with a `MiscDeviceRegistration<T>`.
>
> --
> 2.47.0.371.ga323438b13-goog
>