linux-kernel - Re: [PATCH v11 7/8] rust: miscdevice: add mmap support

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Message-ID: <87jzbzag3s.fsf@kernel.org>
Date: Mon, 16 Dec 2024 14:53:59 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: "Miguel Ojeda" <ojeda@...nel.org>,  "Matthew Wilcox"
 <willy@...radead.org>,  "Lorenzo Stoakes" <lorenzo.stoakes@...cle.com>,
  "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>,  "Jann Horn"
 <jannh@...gle.com>,  "Suren Baghdasaryan" <surenb@...gle.com>,  "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>,
  "Trevor Gross" <tmgross@...ch.edu>,  <linux-kernel@...r.kernel.org>,
  <linux-mm@...ck.org>,  <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v11 7/8] rust: miscdevice: add mmap support

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

> Add the ability to write a file_operations->mmap hook in Rust when using
> the miscdevice abstraction. The `vma` argument to the `mmap` hook uses
> the `VmAreaNew` type from the previous commit; this type provides the
> correct set of operations for a file_operations->mmap hook.
>
> Acked-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com> (for mm bits)
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/miscdevice.rs | 37 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 37 insertions(+)
>
> diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
> index 7e2a79b3ae26..e5366f9c6d7f 100644
> --- a/rust/kernel/miscdevice.rs
> +++ b/rust/kernel/miscdevice.rs
> @@ -11,6 +11,8 @@
>  use crate::{
>      bindings,
>      error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR},
> +    fs::File,
> +    mm::virt::VmAreaNew,
>      prelude::*,
>      str::CStr,
>      types::{ForeignOwnable, Opaque},
> @@ -110,6 +112,15 @@ fn release(device: Self::Ptr) {
>          drop(device);
>      }
>
> +    /// Handle for mmap.
> +    fn mmap(
> +        _device: <Self::Ptr as ForeignOwnable>::Borrowed<'_>,
> +        _file: &File,
> +        _vma: &VmAreaNew,
> +    ) -> Result {
> +        kernel::build_error!(VTABLE_DEFAULT_ERROR)
> +    }
> +
>      /// Handler for ioctls.
>      ///
>      /// The `cmd` argument is usually manipulated using the utilties in [`kernel::ioctl`].
> @@ -156,6 +167,7 @@ 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 +228,31 @@ 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.

This safety comment seems unrelated to the safety requirements of `ForeignOwnable::borrow`.

Best regards,
Andreas Hindborg