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

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Message-ID: <CAH5fLggWi2yTx2EBVcC_SZ1HBTeu_SJ15NZFa57xws8Z5uEXYw@mail.gmail.com>
Date: Tue, 27 May 2025 17:22:18 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Arnd Bergmann <arnd@...db.de>, 
	Andrew Morton <akpm@...ux-foundation.org>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] uaccess: rust: use newtype for user pointers

On Tue, May 27, 2025 at 5:20 PM Boqun Feng <boqun.feng@...il.com> wrote:
>
> 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@...gle.com>
>
> Reviewed-by: Boqun Feng <boqun.feng@...il.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?

Oh, I just forgot to add it.

Alice