From: Benno Lossin <benno.lossin@proton.me>
To: Boqun Feng <boqun.feng@gmail.com>
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: Fri, 19 Apr 2024 08:36:11 +0000 [thread overview]
Message-ID: <079c88af-2e6d-45fe-bf58-afebbf7583b4@proton.me> (raw)
In-Reply-To: <ZiGnFI7cz5v-cowt@boqun-archlinux>
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
next prev parent reply other threads:[~2024-04-19 8:36 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
2024-04-18 23:04 ` Boqun Feng
2024-04-19 8:36 ` Benno Lossin [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=079c88af-2e6d-45fe-bf58-afebbf7583b4@proton.me \
--to=benno.lossin@proton.me \
--cc=a.hindborg@samsung.com \
--cc=akpm@linux-foundation.org \
--cc=alex.gaynor@gmail.com \
--cc=aliceryhl@google.com \
--cc=arnd@arndb.de \
--cc=arve@android.com \
--cc=bjorn3_gh@protonmail.com \
--cc=boqun.feng@gmail.com \
--cc=brauner@kernel.org \
--cc=cmllamas@google.com \
--cc=gary@garyguo.net \
--cc=gregkh@linuxfoundation.org \
--cc=joel@joelfernandes.org \
--cc=keescook@chromium.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=maco@android.com \
--cc=ojeda@kernel.org \
--cc=rust-for-linux@vger.kernel.org \
--cc=surenb@google.com \
--cc=tkjos@android.com \
--cc=tmgross@umich.edu \
--cc=viro@zeniv.linux.org.uk \
--cc=wedsonaf@gmail.com \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).