linux-kernel - Re: [PATCH 3/3] rust: add abstraction for `struct page`

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Date: Fri, 26 Jan 2024 13:33:46 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Wedson Almeida Filho <wedsonaf@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>, 
	Kees Cook <keescook@...omium.org>, Al Viro <viro@...iv.linux.org.uk>, 
	Andrew Morton <akpm@...ux-foundation.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Arve Hjønnevåg <arve@...roid.com>, 
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, 
	Joel Fernandes <joel@...lfernandes.org>, Carlos Llamas <cmllamas@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Arnd Bergmann <arnd@...db.de>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH 3/3] rust: add abstraction for `struct page`

On Fri, Jan 26, 2024 at 1:47 AM Boqun Feng <boqun.feng@...il.com> wrote:
>
> On Wed, Jan 24, 2024 at 11:20:23AM +0000, Alice Ryhl wrote:
> > +    /// Maps the page and reads from it into the given buffer.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// Callers must ensure that `dest` is valid for writing `len` bytes.
> > +    pub unsafe fn read(&self, dest: *mut u8, offset: usize, len: usize) -> Result {
> > +        self.with_pointer_into_page(offset, len, move |from_ptr| {
> > +            // SAFETY: If `with_pointer_into_page` calls into this closure, then
> > +            // it has performed a bounds check and guarantees that `from_ptr` is
> > +            // valid for `len` bytes.
> > +            unsafe { ptr::copy(from_ptr, dest, len) };
> > +            Ok(())
> > +        })
> > +    }
> > +
> > +    /// Maps the page and writes into it from the given buffer.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// Callers must ensure that `src` is valid for reading `len` bytes.
> > +    pub unsafe fn write(&self, src: *const u8, offset: usize, len: usize) -> Result {
>
> Use a slice like type as `src` maybe? Then the function can be safe:
>
>         pub fn write<S: AsRef<[u8]>>(&self, src: S, offset: usize) -> Result
>
> Besides, since `Page` impl `Sync`, shouldn't this `write` and the
> `fill_zero` be a `&mut self` function? Or make them both `unsafe`
> because of potential race and add some safety requirement?

Ideally, we don't want data races with these methods to be UB. They
could be mapped into the address space of a userspace process.

Alice