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Date: Sat, 16 Mar 2024 14:56:20 +0000
To: 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>
From: Benno Lossin <benno.lossin@proton.me>
Cc: Alex Gaynor <alex.gaynor@gmail.com>, Wedson Almeida Filho <wedsonaf@gmail.com>, Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, Andreas Hindborg <a.hindborg@samsung.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, =?utf-8?Q?Arve_Hj=C3=B8nnev=C3=A5g?= <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>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, Christian Brauner <brauner@kernel.org>
Subject: Re: [PATCH v3 3/4] rust: uaccess: add typed accessors for userspace pointers
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References: <20240311-alice-mm-v3-0-cdf7b3a2049c@google.com> <20240311-alice-mm-v3-3-cdf7b3a2049c@google.com>
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On 3/11/24 11:47, Alice Ryhl wrote:
> Add safe methods for reading and writing Rust values to and from
> userspace pointers.
>=20
> The C methods for copying to/from userspace use a function called
> `check_object_size` to verify that the kernel pointer is not dangling.
> However, this check is skipped when the length is a compile-time
> constant, with the assumption that such cases trivially have a correct
> kernel pointer.
>=20
> In this patch, we apply the same optimization to the typed accessors.
> For both methods, the size of the operation is known at compile time to
> be size_of of the type being read or written. Since the C side doesn't
> provide a variant that skips only this check, we create custom helpers
> for this purpose.
>=20
> The majority of reads and writes to userspace pointers in the Rust
> Binder driver uses these accessor methods. Benchmarking has found that
> skipping the `check_object_size` check makes a big difference for the
> cases being skipped here. (And that the check doesn't make a difference
> for the cases that use the raw read/write methods.)
>=20
> This code is based on something that was originally written by Wedson on
> the old rust branch. It was modified by Alice to skip the
> `check_object_size` check, and to update various comments, including the
> notes about kernel pointers in `WritableToBytes`.
>=20
> Co-developed-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Reviewed-by: Benno Lossin <benno.lossin@proton.me>