linux-kernel - Re: [PATCH v3 3/4] rust: uaccess: add typed accessors for userspace pointers

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Date: Sat, 16 Mar 2024 14:56:20 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: 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>
Cc: Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng <boqun.feng@...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>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v3 3/4] rust: uaccess: add typed accessors for userspace pointers

On 3/11/24 11:47, Alice Ryhl wrote:
> Add safe methods for reading and writing Rust values to and from
> userspace pointers.
> 
> 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.
> 
> 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.
> 
> 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.)
> 
> 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`.
> 
> Co-developed-by: Wedson Almeida Filho <wedsonaf@...il.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@...il.com>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>

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