Re: [PATCH 1/3] rust: page: replace the page pointer wrapper with Opaque

[Alice Ryhl]

On Mon, Oct 7, 2024 at 10:28 PM Abdiel Janulgue
<abdiel.janulgue@xxxxxxxxx> wrote:
>
> Replace NonNull with Opaque to make it possible to cast to a Page pointer
> from a raw struct page pointer.
>
> Signed-off-by: Abdiel Janulgue <abdiel.janulgue@xxxxxxxxx>
> ---
>  rust/kernel/page.rs | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
>
> diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
> index 208a006d587c..08ff09a25223 100644
> --- a/rust/kernel/page.rs
> +++ b/rust/kernel/page.rs
> @@ -8,8 +8,9 @@
>      error::code::*,
>      error::Result,
>      uaccess::UserSliceReader,
> +    types::Opaque,
>  };
> -use core::ptr::{self, NonNull};
> +use core::ptr::{self};
>
>  /// A bitwise shift for the page size.
>  pub const PAGE_SHIFT: usize = bindings::PAGE_SHIFT as usize;
> @@ -25,8 +26,9 @@
>  /// # Invariants
>  ///
>  /// The pointer is valid, and has ownership over the page.
> +#[repr(transparent)]
>  pub struct Page {
> -    page: NonNull<bindings::page>,
> +    page: Opaque<bindings::page>,
>  }
>
>  // SAFETY: Pages have no logic that relies on them staying on a given thread, so moving them across
> @@ -65,15 +67,20 @@ pub fn alloc_page(flags: Flags) -> Result<Self, AllocError> {
>          // SAFETY: Depending on the value of `gfp_flags`, this call may sleep. Other than that, it
>          // is always safe to call this method.
>          let page = unsafe { bindings::alloc_pages(flags.as_raw(), 0) };
> -        let page = NonNull::new(page).ok_or(AllocError)?;
> +        if page.is_null() {
> +            return Err(AllocError);
> +        }
> +        // CAST: Self` is a `repr(transparent)` wrapper around `bindings::page`.
> +        let ptr = page.cast::<Self>();
>          // INVARIANT: We just successfully allocated a page, so we now have ownership of the newly
>          // allocated page. We transfer that ownership to the new `Page` object.
> -        Ok(Self { page })
> +        // SAFETY: According to invariant above ptr is valid.
> +        Ok(unsafe { ptr::read(ptr) })

Using `ptr::read` on the page is definitely not okay. That duplicates
the contents of the `struct page`. You'll need some sort of pointer
type around `Page` instead.

Alice