Re: [PATCH] uaccess: rust: use newtype for user pointers

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, May 06, 2025 at 01:25:58PM +0000, Alice Ryhl wrote:
> In C code we use sparse with the __user annotation to detect cases where
> a user pointer is mixed up with other things. To replicate that, we
> introduce a new struct UserPtr that serves the same purpose using the
> newtype pattern.
> 
> The UserPtr type is not marked with #[derive(Debug)], which means that
> it's not possible to print values of this type. This avoids ASLR
> breakage.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/uaccess.rs           | 48 ++++++++++++++++++++++++++++++++++------
>  samples/rust/rust_misc_device.rs |  4 +++-
>  2 files changed, 44 insertions(+), 8 deletions(-)
> 
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index 978205289d297a4001a51fa40ac29039bff73672..2914a35de3dfbc7860ebf0d6e11cc65d409e9481 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -14,8 +14,38 @@
>  };
>  use core::mem::{size_of, MaybeUninit};
>  
> -/// The type used for userspace addresses.
> -pub type UserPtr = usize;
> +/// A pointer into userspace.
> +///
> +/// This is the Rust equivalent to C pointers tagged with `__user`.
> +#[repr(transparent)]
> +#[derive(Copy, Clone)]
> +pub struct UserPtr(pub usize);

Again, why can't this be:
	pub struct UserPtr(pub *const u8);

This is a pointer to a stream of u8 bytes, so let's treat it like that
as that is what we are copying from here.

thanks,

greg k-h