From: Boqun Feng <boqun.feng@gmail.com>
To: Alice Ryhl <aliceryhl@google.com>
Cc: "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>,
"Benno Lossin" <benno.lossin@proton.me>,
"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 11:52:56 -0700 [thread overview]
Message-ID: <ZiFsCLb-BZWbBHsu@boqun-archlinux> (raw)
In-Reply-To: <20240418-alice-mm-v6-4-cb8f3e5d688f@google.com>
On Thu, Apr 18, 2024 at 08:59:20AM +0000, Alice Ryhl wrote:
> Adds a new struct called `Page` that wraps a pointer to `struct page`.
> This struct is assumed to hold ownership over the page, so that Rust
> code can allocate and manage pages directly.
>
> The page type has various methods for reading and writing into the page.
> These methods will temporarily map the page to allow the operation. All
> of these methods use a helper that takes an offset and length, performs
> bounds checks, and returns a pointer to the given offset in the page.
>
> This patch only adds support for pages of order zero, as that is all
> Rust Binder needs. However, it is written to make it easy to add support
> for higher-order pages in the future. To do that, you would add a const
> generic parameter to `Page` that specifies the order. Most of the
> methods do not need to be adjusted, as the logic for dealing with
> mapping multiple pages at once can be isolated to just the
> `with_pointer_into_page` method.
>
Thank you for doing this, and breaking the chicken-and-egg problem chain
;-) For sure, the whole package of page API would need more time to
design, implement and review, but this patch looks good enough to me.
> Rust Binder needs to manage pages directly as that is how transactions
> are delivered: Each process has an mmap'd region for incoming
> transactions. When an incoming transaction arrives, the Binder driver
> will choose a region in the mmap, allocate and map the relevant pages
> manually, and copy the incoming transaction directly into the page. This
> architecture allows the driver to copy transactions directly from the
> address space of one process to another, without an intermediate copy
> to a kernel buffer.
>
> This code is based on Wedson's page abstractions from the old rust
> branch, but it has been modified by Alice by removing the incomplete
> support for higher-order pages, by introducing the `with_*` helpers
> to consolidate the bounds checking logic into a single place, and
> various other changes.
>
> Co-developed-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
> Reviewed-by: Trevor Gross <tmgross@umich.edu>
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Something I want to bring up for discussion below:
[...]
> + /// 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.
Regards,
Boqun
> + ) -> Result<T> {
> + let bounds_ok = off <= PAGE_SIZE && len <= PAGE_SIZE && (off + len) <= PAGE_SIZE;
> +
> + if bounds_ok {
> + self.with_page_mapped(move |page_addr| {
> + // SAFETY: The `off` integer is at most `PAGE_SIZE`, so this pointer offset will
> + // result in a pointer that is in bounds or one off the end of the page.
> + f(unsafe { page_addr.add(off) })
> + })
> + } else {
> + Err(EINVAL)
> + }
> + }
> +
[...]
>
> --
> 2.44.0.683.g7961c838ac-goog
>
next prev parent reply other threads:[~2024-04-18 18:53 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 [this message]
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
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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