linux-kernel - Re: [PATCH v6 4/4] rust: add abstraction for `struct page`

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Date: Thu, 18 Apr 2024 15:56:11 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Alice Ryhl <aliceryhl@...gle.com>, Miguel Ojeda <ojeda@...nel.org>,
	Matthew Wilcox <willy@...radead.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.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>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	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>, Trevor Gross <tmgross@...ch.edu>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org,
	Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v6 4/4] rust: add abstraction for `struct page`

On Thu, Apr 18, 2024 at 10:08:40PM +0000, Benno Lossin wrote:
> On 18.04.24 20:52, Boqun Feng wrote:
> > On Thu, Apr 18, 2024 at 08:59:20AM +0000, Alice Ryhl wrote:
> >> +    /// Runs a piece of code with a raw pointer to a slice of this page, with bounds checking.
> >> +    ///
> >> +    /// If `f` is called, then it will be called with a pointer that points at `off` bytes into the
> >> +    /// page, and the pointer will be valid for at least `len` bytes. The pointer is only valid on
> >> +    /// this task, as this method uses a local mapping.
> >> +    ///
> >> +    /// If `off` and `len` refers to a region outside of this page, then this method returns
> >> +    /// `EINVAL` and does not call `f`.
> >> +    ///
> >> +    /// # Using the raw pointer
> >> +    ///
> >> +    /// It is up to the caller to use the provided raw pointer correctly. The pointer is valid for
> >> +    /// `len` bytes and for the duration in which the closure is called. The pointer might only be
> >> +    /// mapped on the current thread, and when that is the case, dereferencing it on other threads
> >> +    /// is UB. Other than that, the usual rules for dereferencing a raw pointer apply: don't cause
> >> +    /// data races, the memory may be uninitialized, and so on.
> >> +    ///
> >> +    /// If multiple threads map the same page at the same time, then they may reference with
> >> +    /// different addresses. However, even if the addresses are different, the underlying memory is
> >> +    /// still the same for these purposes (e.g., it's still a data race if they both write to the
> >> +    /// same underlying byte at the same time).
> >> +    fn with_pointer_into_page<T>(
> >> +        &self,
> >> +        off: usize,
> >> +        len: usize,
> >> +        f: impl FnOnce(*mut u8) -> Result<T>,
> > 
> > I wonder whether the way to go here is making this function signature:
> > 
> >      fn with_slice_in_page<T> (
> >          &self,
> > 	       off: usize,
> > 	       len: usize,
> > 	       f: iml FnOnce(&UnsafeCell<[u8]>) -> Result<T>
> >      ) -> Result<T>
> > 
> > , because in this way, it makes a bit more clear that what memory that
> > `f` can access, in other words, the users are less likely to use the
> > pointer in a wrong way.
> > 
> > But that depends on whether `&UnsafeCell<[u8]>` is the correct
> > abstraction and the ecosystem around it: for example, I feel like these
> > two functions:
> > 
> > 	    fn len(slice: &UnsafeCell<[u8]>) -> usize
> > 	    fn as_ptr(slice: &UnsafeCell<[u8]>) -> *mut u8
> > 
> > should be trivially safe, but I might be wrong. Again this is just for
> > future discussion.
> 
> I think the "better" type would be `&[UnsafeCell<u8>]`. Since there you
> can always access the length.
> 

Hmm.. here is the thing, having `&UnsafeCell<[u8]>` means having a `*mut
[u8]>`, and it should always be safe to get a "length" of `*mut [u8]`,
right? I haven't found any method doing that, but the length should be
just a part of fat pointer, so I think getting that is a defined
behavior. But maybe I'm missing something.

> Another question would be if page allows for uninitialized bits, in that
> case, we would need `&[Opaque<u8>]`.
> 

Yes, or `&Opaque<[u8>]`.

Regards,
Boqun

> But I don't remember how to get a valid raw pointer from
> `&[UnsafeCell<u8>]`.
> 
> -- 
> Cheers,
> Benno
>