From: Boqun Feng <boqun.feng@gmail.com>
To: Benno Lossin <benno.lossin@proton.me>
Cc: "Alice Ryhl" <aliceryhl@google.com>,
"Miguel Ojeda" <ojeda@kernel.org>,
"Matthew Wilcox" <willy@infradead.org>,
"Al Viro" <viro@zeniv.linux.org.uk>,
"Andrew Morton" <akpm@linux-foundation.org>,
"Kees Cook" <keescook@chromium.org>,
"Alex Gaynor" <alex.gaynor@gmail.com>,
"Wedson Almeida Filho" <wedsonaf@gmail.com>,
"Gary Guo" <gary@garyguo.net>,
"Björn Roy Baron" <bjorn3_gh@protonmail.com>,
"Andreas Hindborg" <a.hindborg@samsung.com>,
"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
"Arve Hjønnevåg" <arve@android.com>,
"Todd Kjos" <tkjos@android.com>,
"Martijn Coenen" <maco@android.com>,
"Joel Fernandes" <joel@joelfernandes.org>,
"Carlos Llamas" <cmllamas@google.com>,
"Suren Baghdasaryan" <surenb@google.com>,
"Arnd Bergmann" <arnd@arndb.de>,
"Trevor Gross" <tmgross@umich.edu>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
rust-for-linux@vger.kernel.org,
"Christian Brauner" <brauner@kernel.org>
Subject: Re: [PATCH v6 4/4] rust: add abstraction for `struct page`
Date: Thu, 18 Apr 2024 15:56:11 -0700 [thread overview]
Message-ID: <ZiGlC5AtRRikE1AI@boqun-archlinux> (raw)
In-Reply-To: <87dc4cdf-ccf6-4b08-8915-313aad313f93@proton.me>
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
>
next prev parent reply other threads:[~2024-04-18 22:56 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-18 8:59 [PATCH v6 0/4] Memory management patches needed by Rust Binder Alice Ryhl
2024-04-18 8:59 ` [PATCH v6 1/4] rust: uaccess: add userspace pointers Alice Ryhl
2024-04-19 17:11 ` Boqun Feng
2024-04-19 18:12 ` Alice Ryhl
2024-04-18 8:59 ` [PATCH v6 2/4] uaccess: always export _copy_[from|to]_user with CONFIG_RUST Alice Ryhl
2024-05-14 17:27 ` Andrew Morton
2024-05-15 7:34 ` Arnd Bergmann
2024-04-18 8:59 ` [PATCH v6 3/4] rust: uaccess: add typed accessors for userspace pointers Alice Ryhl
2024-04-18 13:01 ` Benno Lossin
2024-04-18 13:17 ` Alice Ryhl
2024-04-18 16:23 ` Benno Lossin
2024-04-18 17:19 ` Boqun Feng
2024-04-18 19:35 ` Alice Ryhl
2024-04-18 20:01 ` Boqun Feng
2024-04-18 17:52 ` Trevor Gross
2024-04-25 16:13 ` Gary Guo
2024-04-26 7:13 ` Alice Ryhl
2024-04-18 8:59 ` [PATCH v6 4/4] rust: add abstraction for `struct page` Alice Ryhl
2024-04-18 18:52 ` Boqun Feng
2024-04-18 22:08 ` Benno Lossin
2024-04-18 22:56 ` Boqun Feng [this message]
2024-04-18 23:04 ` Boqun Feng
2024-04-19 8:36 ` Benno Lossin
2024-04-19 17:23 ` Boqun Feng
2024-04-19 19:24 ` Benno Lossin
2024-04-19 19:35 ` Boqun Feng
2024-04-25 16:20 ` Gary Guo
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