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

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Mar 11, 2024 at 10:47:15AM +0000, 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@gmail.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>

Regards,
Boqun

> ---
>  rust/kernel/types.rs   | 67 ++++++++++++++++++++++++++++++++++++++++++++
>  rust/kernel/uaccess.rs | 75 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 141 insertions(+), 1 deletion(-)
[...]