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

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

On Tue, May 27, 2025 at 01:53:12PM +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
> 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>

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

A question below:

> ---
> This is based on top of the strncpy_from_user for Rust patch.
> ---
> Changes in v2:
> - Change usize to raw pointer.
> - Make field private.
> - Rename wrapping_add to wrapping_byte_add.
> - Add to prelude.
> - Rebase on v4 of strncpy_from_user
> - Link to v1: https://lore.kernel.org/r/20250506-userptr-newtype-v1-1-a0f6f8ce9fc5@google.com
> ---
>  rust/kernel/prelude.rs           |  2 ++
>  rust/kernel/uaccess.rs           | 68 +++++++++++++++++++++++++++++++++-------
>  samples/rust/rust_misc_device.rs |  2 ++
>  3 files changed, 60 insertions(+), 12 deletions(-)
> 
> diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
> index baa774a351ceeb995a2a647f78a27b408d9f3834..081af5bc07b0bcefb1da16e5a81fc611b3178aea 100644
> --- a/rust/kernel/prelude.rs
> +++ b/rust/kernel/prelude.rs
> @@ -41,3 +41,5 @@
>  pub use super::init::InPlaceInit;
>  
>  pub use super::current;
> +
> +pub use super::uaccess::UserPtr;
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index e6534b52a1920254d61f8349426d4cdb38286089..02e0561eb1c6f4d813a4ab13a124bfac2d2a5c75 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -14,8 +14,48 @@
>  };
>  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(*mut c_void);
> +
> +impl UserPtr {
> +    /// Create a `UserPtr` from an integer representing the userspace address.
> +    pub fn from_addr(addr: usize) -> Self {
> +        Self(addr as *mut c_void)
> +    }
> +
> +    /// Create a `UserPtr` from a pointer representing the userspace address.
> +    pub fn from_ptr(addr: *mut c_void) -> Self {
> +        Self(addr)
> +    }
> +
> +    /// Cast this userspace pointer to a raw const void pointer.
> +    ///
> +    /// It is up to the caller to use the returned pointer correctly.
> +    #[inline]
> +    pub fn as_const_ptr(self) -> *const c_void {
> +        self.0
> +    }
> +
> +    /// Cast this userspace pointer to a raw mutable void pointer.
> +    ///
> +    /// It is up to the caller to use the returned pointer correctly.
> +    #[inline]
> +    pub fn as_mut_ptr(self) -> *mut c_void {
> +        self.0
> +    }
> +

why are these two inline but the rest not?

Regards,
Boqun

> +    /// Increment this user pointer by `add` bytes.
> +    ///
> +    /// This addition is wrapping, so wrapping around the address space does not result in a panic
> +    /// even if `CONFIG_RUST_OVERFLOW_CHECKS` is enabled.
> +    pub fn wrapping_byte_add(self, add: usize) -> UserPtr {
> +        UserPtr(self.0.wrapping_add(add))
> +    }
> +}
>  
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