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Message-ID: <079c88af-2e6d-45fe-bf58-afebbf7583b4@proton.me> Date: Fri, 19 Apr 2024 08:36:11 +0000 From: Benno Lossin <benno.lossin@...ton.me> To: Boqun Feng <boqun.feng@...il.com> 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 19.04.24 01:04, Boqun Feng wrote: > On Thu, Apr 18, 2024 at 03:56:11PM -0700, Boqun Feng wrote: >> 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. There is `to_raw_parts` [1], but that is unstable. (Note that `<[T] as Pointee>::Metadata = usize`, see [2]) >> > > Hmm... but I guess one of the problems of this approach, is how to > construct a `&UnsafeCell<[u8]>` from a pointer and length... We could use `from_raw_parts` [3]. But when making the slice the outer type, we can use a stable function to convert a pointer and a length to a slice [4]. > > Regards, > Boqun > >>> Another question would be if page allows for uninitialized bits, in that >>> case, we would need `&[Opaque<u8>]`. >>> >> >> Yes, or `&Opaque<[u8>]`. I don't think that putting the slice on the inside is what we want. Also note that `Opaque<T>` requires that `T: Sized` and that is not the case for `[u8]`. [1]: https://doc.rust-lang.org/nightly/core/primitive.pointer.html#method.to_raw_parts [2]: https://doc.rust-lang.org/nightly/core/ptr/trait.Pointee.html#pointer-metadata [3]: https://doc.rust-lang.org/nightly/core/ptr/fn.from_raw_parts.html [4]: https://doc.rust-lang.org/nightly/core/slice/fn.from_raw_parts.html -- Cheers, Benno
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