Re: [PATCH v2] uaccess: rust: use newtype for user pointers

["Benno Lossin" <lossin@kernel.org>]

On Tue May 27, 2025 at 3:53 PM CEST, 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

Should we still implement it, not printing the address of course? So
something like `UserPtr(/* .. */)` or whatever people print instead of
showing the address?

> it's not possible to print values of this type. This avoids ASLR
> leakage.
>
> The type is added to the prelude as it is a fairly fundamental type
> similar to c_int. The wrapping_add() method is renamed to
> wrapping_byte_add() for consistency with the method name found on raw
> pointers.
>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

I left one question below, but it already looks good, so:

Reviewed-by: Benno Lossin <lossin@kernel.org>

> @@ -444,7 +484,11 @@ fn raw_strncpy_from_user(dst: &mut [MaybeUninit<u8>], src: UserPtr) -> Result<us
>  
>      // SAFETY: `dst` is valid for writing `dst.len()` bytes.
>      let res = unsafe {
> -        bindings::strncpy_from_user(dst.as_mut_ptr().cast::<c_char>(), src as *const c_char, len)
> +        bindings::strncpy_from_user(
> +            dst.as_mut_ptr().cast::<c_char>(),
> +            src.as_const_ptr().cast::<c_char>(),

Should we also have `as_{const,mut}_char_ptr` functions? After a quick
search, I found that `char __user *` is a common type for user pointers.

---
Cheers,
Benno

> +            len,
> +        )
>      };